Astrophysical and cosmological constraints on life
Paul A. Masona,∗and Peter L. Biermannb,c,d,e,†
July 15, 2017
aNew Mexico State University, Las Cruces NM, 88003, USA, E-mail: email@example.com
bMax Planck Institute for Radio Astronomy, Bonn, Germany
cKarlsruhe Inst. f¨ur Tech., Karlsruhe, Germany
dUniv. of Alabama, Tuscaloosa, AL, USA
eFachb. f¨ur Phys. & Astron., Univ. Bonn, Germany
∗P. A. Mason is a member of the Sloan Digital Sky Survey (SDSS) Collaboration
†P. L. Biermann is a member of the Pierre Auger Collaboration
Constraints imposed on the development of life over cosmic time are reviewed. As the
Universe aged, elements required for life and its protection were forged while threats such
as gamma-ray burst and supernova rates decreased. Supermassive black hole activity was
especially detrimental within host galaxies. The frequency of gamma-ray bursts (especially
harmful if metal-poor) and supernova explosions was too high to provide much chance for life
as we know it until well after the peak of star formation. In the past, cosmic ray particle
densities were 102−4times the present level and occasionally much higher. Ironically, while
being very near a supernova is disastrous, the cosmic ray particles from supernova remnants
drive the magnetic ﬁelds to full strength inside the disk of galaxies, thus protecting life on the
surface of planets from extragalactic cosmic rays at a galactic magnetopause. Stellar winds
may generate an astrosphere which partially protects life from cosmic rays accelerated within
those same supernova remnants. Planetary magnetism provides additional protection from
galactic cosmic rays and high energy particles coming from host stars. We consider two levels
of habitability, 1) minimal habitability conditions and 2) conditions compatible with complex
life as we know it. Planets with minimal habitability are probably more abundant, so microbial
life is likely much more common than complex multicellular life. Extremophiles may thrive on
planets whenever liquid water, or some other solvent, and organic compounds are available.
Conditions that support complex life can be created by the microbial life as happened on
Earth. The atmosphere on Earth has been modiﬁed by photo-synthesizing life and, as a
result, many new niches on land and in the air became available. Complex life as we know
it requires more restrictive conditions which apparently were rare or impossible in the local
Universe before 5-6 Gyr ago. After that time, planets that formed with suﬃcient elemental
abundances in galaxies without high supernovae rates or active galactic nuclei were more likely
to be habitable. We introduce the term Super-Galactic Habitable Zone (SGHZ) to describe
regions far enough away from the centers of dense galaxy groups to allow complex surface life
to exist. Planets with high habitability potential reside within the SGHZ, in galactic disks,
within protective astrospheres, possessing strong magnetic ﬁelds and thick atmospheres.
Keywords: Galactic Habitable Zone; Super-galactic Habitable Zone; supermassive black holes;
cosmic rays; planetary magnetism; supernovae; protecting role of atmosphere.
In this chapter, constraints placed on life in the ever evolving Universe are examined. A prerequisite
for life is the availability of materials that form planets, support biospheres, and protect life from
catastrophic threats. We focus especially on how those factors have changed over cosmic time.
Also, we investigate how the location within a galaxy and the location of galaxies within galaxy
clusters may aﬀect the development and maintenance of life on planets. In this section, we outline
the main factors aﬀecting habitability of the Universe through cosmic time. Subsequent sections
are focused on the constraints placed on life, as supported by recent observations and theoretical
Our analysis indicates that conditions for life on the surface of planets depends on the chance
of avoiding catastrophic events described in Sections 2 and 3, as well as on the protecting role of
planetary and circumstellar environment, as discussed in Sections 4 and 5. Certain types of host
stars, certain locations within galaxies, and certain galaxies within clusters, are more favorable for
the origin and evolution of life than others. Habitability factors include being in a galaxy with a
low, but non-zero, star formation rate (SFR) and no, or at least an inactive, central supermassive
black hole (SMBH).
While some galaxies have very massive SMBHs, many if not most galaxies do not have a
SMBH. The Milky Way has a low mass and nearly inactive SMBH (which certainly has been
active in the past). In this context, habitability - the probability of developing complex life, as
we know it, is severely compromised in the presence of ionizing radiation and high-energy particle
ﬂux. A decrease in the local SFR, the reduction of growth and activity of SMBHs, and the
expansion of the Universe all generally enhance the probability of habitable planet development
over cosmic time. However, habitability does not necessarily improve monotonically, as galaxy
mergers increase the SFR and thereby elevate the supernova (SN) and gamma-ray bursts (GRB)
rates. SMBH mergers create havoc on a galactic scale and reduce habitability for extended periods
of time in merging galaxies. In addition to the Milky Way, we discuss the potential for life in the
nearby galaxies M31, M32, M33, M81, M82, M87, M90, M94, and Cen A.
1.1 Formation of the elements of life
According to the standard cosmological model, the Universe in the ﬁrst few minutes after the Big
Bang went through phases having similar density and temperature as the center of the Sun. In
these conditions, the fusion of protons into more massive nuclei occurred. This phase of nucleosyn-
thesis brieﬂy persisted until the Universe had cooled below threshold temperatures and pressures
by expansion (Gamow 1948;Alpher et al. 1948). Between about 10 to 2000 seconds after the Big
Bang, roughly 25% of the hydrogen H was converted into helium He, mainly 4He. Some deu-
terium, lithium, beryllium, and possibly boron nuclei were also produced at this time. The main
contribution of lithium, beryllium, and boron, in the interstellar medium (ISM) today comes from
cosmic ray spallation, which also produces trace amounts of some more massive elements. However,
important elemental constituents for life were not yet available. It took quite some time for the
heavier elements to emerge by nuclear fusion in the ﬁrst stars. This ended a period known as the
cosmic dark ages - a star-less era lasting until of order 100 million years after the expansion began.
The primordial 1H and 4He nuclei became the seeds for the production of critical elements in the
cores of stars (Burbidge et al. 1957) especially those elements that are essential for the formation
of rocky planets and for life itself. See Longair (1994) for a general introduction to high-energy
The ﬁrst stars apparently formed from pure H/He clouds (Loeb and Barkana 2001). The
interstellar clouds formed H2molecules allowing them to cool and facilitate contraction. These
earliest stars likely formed before galaxies, in regions of collapsing gas possibly associated with
clumps of dark matter. The ﬁrst stars are thought to form primarily with high masses, consume
most available fuel quickly, and to explode as a core-collapse supernova (SN-Type II), or collapse
directly into black holes (collapsars) relatively soon after formation. Hence, they do not exist in
the Universe today or at least not in normal galaxies. However, they played a critical role in
synthesizing the ﬁrst heavy nuclei and spreading their remnants back into the ISM. This spreading
of heavier elements also facilitated cooling, and this positive feedback may have increased the rate
of star formation. In this way, the ISM was slowly populated with the elements required for life;
carbon, nitrogen, oxygen, phosphorus, and sulfur, which in cold environments combined with H to
form the many molecules observed in molecular clouds today. This group of life elements is often
The ﬁrst galaxies are assumed to form from these ﬁrst stars, their black hole remnants, and
massive gas clouds. Within the heart of these galaxies, active galactic nuclei (AGN) turned on,
as SMBHs formed and grew in an ubiquitous, yet poorly understood process. SMBHs exist in
the center of essentially all galaxies with a stellar bulge (Ferrarese and Merritt 2000;Gebhardt
et al. 2000). Within galaxies, H/He clouds slowly transformed into molecular clouds as newer
generations of stars expelled elements generated in their cores during the ﬁnal stages of stellar
evolution. Metals, deﬁned somewhat loosely as all elements with mass from carbon and up and
produced in stellar nucleosynthesis, were necessary for the formation of lower mass stars. So later
generations of short lived high mass stars as well as lower mass stars with a longer life span formed
from increasingly metal-enriched molecular clouds. Steadily, the stellar population within each
host galaxy increased in numbers and became enriched in elemental composition.
Over 100 years ago, Hess (1912) performed balloon experiments and discovered that electro-
scopes discharge more rapidly at high altitudes than on the ground. It became clear that this
increase of discharge rate is caused by radiation coming from beyond Earth. We now know that
these cosmic rays (CRs) are highly energetic atomic nuclei, electrons, anti-protons, and positrons
impacting the atmosphere. The most massive stars are rare, but they play an important role in
determining the habitability of their cosmic neighborhood. Stars with mass above ca. 40Mat the
start of H fusion, ultimately eject a lot of mass including carbon and oxygen. If by chance carbon
dominates, the outﬂows of these stars includes soot (Woosley et al. 2002). Thus, these massive
stars are the main suppliers of carbon, nitrogen and oxygen (CNO).
Many stars are formed as binary star systems which provide additional channels for ISM
enrichment as galaxies evolve. For example, a single low mass star a bit more massive than the
Sun produces a large amount of CNO locked in a white dwarf. However, in the right binary system,
mass accretion onto a white dwarf may lead to its destruction as a SN-Type Ia if the Chandrasekhar
limit is exceeded. SN Ia are driven by the explosive burning of the CNO to Ni, which gets ejected,
and then decays into Fe. So SNe Ia supply lots of Fe. SN-Type Ia are not visible in CRs as a known
distinct particle population, even though their frequency is relatively large. They may contribute
to the same CR-population as do the stars formed with 8 to about 25M, that explode into the
ISM and not into their winds.
SN-Type Ia events occur generally later in the age of a galaxy compared to the SN-Type II
events, which are explosions of high mass stars at the end of nuclear fusion, because SNe Ia involve
the formation of white dwarfs which is the end point of the evolution of longer lived lower mass
stars. Other important channels of element enrichment open as the Universe ages. For example,
neutron star mergers may supply most of the r-process elements; i.e. those elements formed by
rapid neutron capture during the explosion. Binary neutron stars or a neutron star plus black
hole binary mergers are probably associated with observable events called kilonovae and/or short
GRBs and cause gravitational wave emission (Abbott et al. 2016).
The formation of rocky/metal planets became possible only after the abundances of Si, Mg,
and Fe increased enough to allow for their condensation into grains within protoplanetary disks.
The formation of planets is a complex process. It involves circumstellar disk formation and sub-
sequent feeding of protoplanets as well as stochastic and chaotic interactions within both nascent
and aged planetary systems. Protoplanetary collisions and migration of planets ensure the wide
variety of planetary systems observable today, see e.g. Seager (2013). On some of these planets,
conditions are suitable for life. Earth as the singular known location of life in the Universe remains
the paragon of habitability, and indicates one of the things universal to all life as we know it, its
dependence on water.
1.2 Protection of life on planets
Habitability of a particular planet depends on the balance between the possibility of various threats
that may temporarily, or even permanently, sterilize the surface and on the availability of protective
mechanisms that reduce the impact and frequency of such events. Earth for example, has undergone
many mass extinctions with subsequent biosphere recovery. Just in the last 0.5 Gyr, the so called
big ﬁve extinctions each eliminated from 75 to 96% of species on the Earth, see e.g. Raup and
Sepkoski (1982). Some of these have been attributed to impacts, hyper-volcanism, and global
heating or cooling. Many lesser mass extinctions have also occurred. Such events are salient to
the history of biological evolution on Earth itself. Indeed, mass extinctions likely have allowed the
diverse array of biological changes to occur on Earth, eventually allowing one species of mammals
to investigate astrobiology.
The availability of liquid water is by now a classical concept of habitability and it can be
quantiﬁed by stellar luminosity and temperature. Its limits have been deﬁned for single stars and
extended to planets in binary star systems. However, being within the standard habitable zone does
not imply habitability of a particular planet. In many cases, habitability is strongly aﬀected by local
conditions, such as stellar X-ray and UV radiation, stellar winds, and other catastrophic events
(e.g., climate instability, volcanism, asteroid impacts, nearby supernovae, GRBs, microquasars,
and local AGN). The ability for newly formed planets to support life depends on the abundance
of heavier elements after generations of both high and lower mass stars. The sterilizing eﬀect of
accreting and exploding stars decreased as the star formation rate declined in the Milky Way and
in similar galaxies (Madau and Dickinson 2014). Over time, there has been a decrease in the
number and activity of ionizing radiation sources in the local Universe. Planets with suﬃciently
thick atmospheres and strong magnetic ﬁelds provided protection against radiation from the host
stars, background CRs, and some nearby radiation events. Formation of a protective atmosphere
requires a signiﬁcant enrichment of CNO elements. High abundance of Si, Ni, and Fe is required to
form rocky planets capable of maintaining a strong magnetic ﬁeld over astrobiologically signiﬁcant
(Gyr) time-scales. The geomagnetic ﬁeld periodically reverses its polarity; and thus the magnetic
protection may become weak during the transition period. If present, atmospheric ozone can
provide a signiﬁcant UV shield for the surface of planets. The interconnected features of planet
habitability, such as presence of liquid water, plate-tectonics, magnetic dynamo, and atmospheric
composition, are driven by the planet’s internal thermal evolution. Habitability of planets orbiting
long lived stars can be limited by the geophysical lifetime (Franck et al. 2000), rather than the
Astrospheres, akin to the Solar heliosphere, formed by single and binary main sequence star
winds provide additional protection for habitable zone planets from CRs. Stellar winds are magne-
tized plasma ﬂows that may remove atmospheric gas from inner planets, but then expand into the
local ISM providing a bubble of protection against the onslaught of CRs. If the stellar winds are
too dense and fast, atmospheric erosion may occur (Vidotto et al. 2014). If protection mechanisms
are not strong enough, then the habitability of the planet is compromised.
A magnetic ﬁeld also permeates the disk of the Milky Way and other disk galaxies. This
Galactic magnetic ﬁeld originates from a relativistic ﬂux of CRs accelerated by SN remnants mostly
in the central plane of the disk. This CR ﬂux drives a Galactic wind perpendicular to the disk
(Hanasz et al. 2013). The SN rate per area must be suﬃciently high to drive this wind (e.g Rossa
and Dettmar (2003)). The Galactic dynamo also needs the shear ﬂow of diﬀerential rotation of
the disk (Jokipii and Morﬁll 1987). If this dynamo operates as expected, a galaxy’s magnetic ﬁeld
may deﬂect many CRs from extragalactic sources (Axford 1981;Hanasz et al. 2009a). However,
when the SFR, and hence, the SN rate is too high then particles trapped by the Galactic magnetic
ﬁeld will be hazardous to life on planets. If the SFR is too low, then there would be no galactic
wind, and presumably CRs just diﬀuse out of the disk rather than being convected out at an
Alfv´enic surface transition layer. In general, considering the gradual build up of the abundances
of heavy elements, habitability has increased for planets formed after the bulk of star formation
has occurred and most threats have subsided.
To analyze astrophysical constraints on the occurrence of life in the evolving Universe we use the
term habitability and the phrase potentially habitable for planets, or regions in stellar systems,
galaxies, and galaxy clusters with a signiﬁcant probability that life could originate and/or be
sustained on suitable planets within those regions. Habitable planets may or may not be inhabited.
The key is that they have a non-zero probability of being inhabited. In this context, habitability is
not a binary concept. Planets throughout the Universe undoubtedly oﬀer a wide multi-dimensional
spectrum of both hospitable and inhospitable environments. However, some environments are
better than others and some planets are more habitable than others. Another contributing factor
is time. In many cases, simple or complex life may develop, but be extinguished by a catastrophic
event. One assumption made here is that life as we know it requires suitable conditions that remain
relatively stable over very long (Gyr) time-scales.
It is generally assumed that complex life is most probable on rocky/metal planets. Also,
planets need to be large enough to support an atmosphere with CNO at suﬃcient surface pressure
to maintain surface water (Kasting et al. 1993). Life ﬂourished in Earth’s oceans long before it
spread over land; another role of oceans is to support the hydrologic cycle. Oxygen, the third most
abundant element in the Universe and the major constituent of water is assumed to be critical for
life. The build up of O2in the atmosphere of planets as a result of activity of photo-synthetic life-
forms or by other mechanisms is assumed to be of great beneﬁt (if not essential) for the development
of complex life, both aquatic and on the land. Hence, we will regard a minimum oxygen abundance
to be a necessary condition for habitable planets. The probability of complex life is enhanced in
the case of a thick and oxygen-rich atmosphere. Thus, planets can be habitable for complex life
only if they can retain surface water and that depends on the radiation and high-energy particle
Extremophiles, by deﬁnition, are organisms that tolerate and even thrive in conditions that
are harsh or even lethal for most life-forms on Earth. See the review of extremophiles by Roth-
schild and Mancinelli (2001) which motivates this discussion. We assume that environments on
Earth and elsewhere in the Solar System may be studied and extrapolated to potentially habitable
exoplanets (Preston and Dartnell 2014). Consider hyperthermophiles living in the hot geyser pools
of Yellowstone Park. These geothermal habitats are rare on Earth. One can imagine a planet with
much more intense geothermal activity than Earth. These hyperthermophiles would be abundant
on such an imagined world. Similarly, salt-loving halophiles might become abundant on a planet
with a surface exposed to repeated evaporation. A lesson from extremophile studies on Earth
is that these life-forms protect themselves by isolation from external conditions and by removing
threats or repairing damage quickly in order to survive. Generalizing this example we thus con-
sider physiological capacity of extremophiles as one among several layers of biological protection
against environmental challenges. Rothschild and Mancinelli (2001) suggest that even humans are
extremophiles in some sense as they thrive in an oxygenated atmosphere and experience damaging
eﬀects of free oxygen radicals. Some organisms, anaerobes, ﬂourish in anoxic environments. How-
ever, life as we know it, transformed the environment, allowing oxygen consuming aerobes to thrive
on the surface, in the air, and in shallow waters on Earth. At the same time, we do not assume
that the Earth is the best of all possible worlds. For example, planets with reducing atmospheres
may harbor a rich biosphere.
For simplicity, two types of habitability can be distinguished. The ﬁrst is basic habitabil-
ity supported by the existence of elements required to construct planets, their atmospheres, and
organic compounds. Within this basic habitability condition, microbial life, extremophile and oth-
erwise, may have existed in the past and may even exist today over vast areas of the Universe. Even
within entire galaxies without complex life as we know it. Deep ocean life on planets may only
be limited by the availability of water and atmospheric pressure to maintain the ocean. Complex
life may arise on planets within lakes and oceans even if the environments above the water are not
habitable. Thus, the overall habitability may depend on local niches on the surface, in shallow
waters or in the air, and provide a diversity of environments comparable to or exceeding (Heller and
Armstrong 2014) that on Earth. Long-term environmental stability has likely been of great beneﬁt
to the life on Earth. Thus, the second more restrictive condition of habitability, is the capacity
to support complex life as we know it on Earth. In this case, biospheres can be challenged by
factors such as atmosphere erosion, ozone removal, and long-term temperature regulation. Finally,
we assume that the second condition of habitability includes atmospheric O2, which is a powerful
energy source for animals, and an ozone O3layer for UV protection of the surface of the planet
and shallow waters. These conditions seem to be important mechanisms or catalysts promoting
the emergence of complex life.
2 Hazardous radiation and particles
In this section we discuss CRs, which are energetic particles of astrophysical origin. Most CRs
are protons, but hey also include a mixture of He nuclei and heavier element ions. At the highest
energies, massive nuclei are much more common (Thoudam et al. 2016). These highest energy
particles collide with particles in the upper atmosphere of planets, producing many secondary
particles; especially muons, which are highly penetrating. Some secondary particles are absorbed
in the air whereas others reach the planetary surface and contribute to the surface radiation dose
of living organisms. Other particles penetrate the surface and may impact sub-surface organic
chemistry and life. CR particles may be classiﬁed into three types based on three diﬀerent source
categories, as follows from data on particles detected on Earth. This classiﬁcation is related to
protection mechanisms that prevent particles within certain energy from reaching Earth (see next
section for details).
The eﬀect of radiation and particles on life is reviewed by Dartnell (2011), also see Abrevaya
and Thomas (2017) in this volume. Here we address sources of high energy radiation and particles
that are potentially hazardous to the biota. A plot of the cosmic ray (CR) particle ﬂux as a
function of energy is shown in Figure 1. The lowest energy CRs (E up to a few GeV) include solar
energetic particles (SEPs), which can occasionally be quite strong. This is because most of the
lowest energy particles from non-solar sources do not reach the surface of Earth. SEPs represent
the high energy tail of the solar wind particle distribution and do not contribute to the CRs shown
in Figure 1. Higher energy particles (with E above a few GeV) have a diﬀerent origin and are called
Galactic CRs (GCRs) or extragalactic CRs (EGCRs), depending on their source. GCRs are the
dominant population of CRs up to the so called ankle seen in Figure 1, with EGCRs dominating
A signiﬁcant fraction of primary CRs likely originate from massive star SNe. However, this is
probably not their only source. AGN and stellar mass black holes, neutron stars, and white dwarfs
probably also produce some CRs detectable on Earth. Note that charged particles accelerated from
galactic sources supply a galactic magnetosphere which traps CRs with lower energy. Planets in
the disk of galaxies with ongoing star formation, like our galaxy, get bathed by CRs. Galactic CRs
stay diﬀusively in the disk until they convect out in a Galactic wind, which takes about ten million
years at the location of the Sun and somewhat less in the inner Galaxy (Biermann et al. 2010,
2015;Wiegert et al. 2015). Some particles encounter the atmosphere of a planet, and those with
the highest energy produce an air-shower. Information of their origin is usually lost as they follow
trajectories deﬂected by the galactic magnetic ﬁeld and thus are detected isotropically (Biermann
et al. 2013). While GCRs trapped in the galactic magnetic ﬁeld are a hazard to life imposed by
SNe, the galactic magnetic ﬁeld generated by GCRs accelerated in the remnants of SNe provides
a shield against all but the highest energy EGCRs.
2.1 Solar/stellar energetic Particles (SEPs)
The heliosphere extends out to a (time variable) distance of about 120 au, depending on direction,
from the Sun, where the solar wind and ISM pressures balance. The solar wind velocity falls
below the supersonic threshold and the density of particles increases. Magnetic ﬁelds within the
termination shock provide a barrier to lower energy particles, mostly GCRs, with original energies
less than 300 MeV. The strength of the solar wind is variable and there is an observed anti-
correlation between solar activity and cosmic ray ﬂux, conﬁrming this protection mechanism, see
e.g. Shaviv (2003).
SEPs are responsible for mass loss of planetary atmospheres. However, planets may be mag-
netically protected. For example, the magnetic ﬁeld of the Earth deﬂects or traps SEPs, enhancing
the habitability of Earth. Mars currently does not have a signiﬁcant global magnetic ﬁeld, and its
mass is about 10% of the mass of the Earth. Mars still experiences signiﬁcant atmospheric mass
loss due to solar activity as measured by instruments aboard the MAVEN spacecraft (Dong et al.
2015). In the early life of the Sun it was less luminous than it is now, however it was also much
Considering stars of other types, the more massive upper main sequence stars consume avail-
able fuel and die before life can develop on their planets. Short stellar lifetimes eﬀectively limit
habitable planets to stars on the lower main sequence, below about 1.1M. Stars are typically
formed spinning much more rapidly than the present day Sun, e.g. Soderblom et al. (1993). As
stars in this lower mass range have outer convection zones, a magnetic dynamo is generated and a
stellar corona forms. As they age they gradually loose their angular momentum, thereby slowing
their rotation as the result of winds driven by corona. This model is was introduced by Parker
(1958) and studied further by Weber and Davis (1967). Planets in the habitable zone of magneti-
cally active stars are subject to X-ray and UV radiation as well as stellar winds. Magnetic activity
declines as the rotation period increases. Today, the activity of the Sun is lower by two to three
orders of magnitude than it was during the formation of the Earth. Correspondingly, the SEP ﬂux
from the Sun has minimal eﬀect on Earth today. However, the lowest mass stars remain active
2.2 Galactic cosmic rays (GCRs)
GCRs have long been thought to originate from the gaseous remnants of SN explosions (Baade
and Zwicky 1934). Direct detection of pion-decay signatures in supernova remnants, IC 433 and
W44, were made using the Fermi gamma-ray telescope (Ackermann et al. 2013). Speciﬁcally, they
detected a cutoﬀ in the gamma ray spectrum due to the decay of neutral pions, produced when
accelerated CRs interacted with the ISM surrounding these SNe. Neutral pions quickly decay into
gamma-rays, while charged pions become electrons, positrons and neutrinos. SNe are probably not
the source of all GCRs, but this detection suggests that shocks within SN remnants do accelerate
particles. Benyamin et al. (2016) considered the spiral arms of galaxies as the main location of
GCR sources. Aartsen et al. (2013) found a large-scale anisotropy in PeV emission and suggested
that this result is consistent with a superposition of ﬂux from a just few nearby sources, likely
SN remnants. SN explosions of the most massive stars (above 25M) are more important in
many aspects, but the standard SNe (stars between 8 and 25M) are the major contributors to
CR-protons, and dominate the overall energetics.
Other sources of GCRs likely include white dwarf novae, high and low mass X-ray binaries
(LMXB and HMXBs) - a low or high mass star accreting onto a neutron star or a black hole, and
microquasars,. Microquasars are scaled-down versions of AGN where matter accretes onto a stellar
mass black hole, rather than a SMBH, and mildly relativistic collimated particle jets are ejected.
Very high energy, short duration, bursts from a galactic black hole binary were ﬁrst observed by
Mason et al. (1997) with CGRO BATSE and later conﬁrmed at even higher, 100 GeV, energies
using MAGIC (Albert et al. 2007) from the HMXB Cygnus X-1, the ﬁrst conﬁrmed Galactic black
hole. See Mirabel et al. (2011) for an illuminating discussion on ionization due to stellar mass
2.3 Extragalactic cosmic rays (EGCRs)
The galactic magnetic ﬁeld prevents many of the lower energy EGCRs from entering the Galactic
disk; however the highest energy particles penetrate the magnetic ﬁeld of the Galaxy and continue
unimpeded through the heliosphere and the Earth’s magnetic ﬁeld to impact the atmosphere.
The highest energy CRs detected, with energies up to 3 ×1020 eV are called ultra-high energy
CRs (UHECRs), see Figure 1. The Cosmic Microwave Background Radiation as well as the far-
IR radiation ﬁelds interact with the UHECRs, limiting their travel distance, an eﬀect known for
protons as the GZK-cutoﬀ (Greisen 1966;Zatsepin and Kuz’min 1966). At the upper end of the
UHECR spectrum, massive nuclei dominate, so that the cutoﬀ is modiﬁed, see the review by Olinto
However, the sources of UHECRs are naturally limited at the highest energies. Apparently,
acceleration mechanisms are not able to accelerate CRs to the highest energies beyond the detection
cutoﬀ shown in Figure 1. Theoretically, around 1021 eV (for protons, and without relativistic
boosting) there seems to be a general limit. This combination derives ﬁrstly from the spatial limit
(Lovelace 1976) given by the magnetic ﬁeld, which ties into the jet and is energetically limited
by the Eddington power. So the more power, or luminosity of the jet, the higher is the possible
particle energy, giving Emax ∼L1/2
jet . A second constraint comes from losses, such as synchrotron
and photon interaction losses, giving a maximum energy that runs inversely with Ljet, as discussed
in Hillas (1984) and Biermann and Strittmatter (1987).
The Pierre Auger Collaboration et al. (2011) argues that the energy dependence of the com-
position of UHECRs supports the ankle interpretation of the transition from GCRs to EGCRs,
see Figure 1. The determination of EGCR sources is diﬃcult mainly due to insuﬃcient statistics,
but also due to magnetic deﬂections of particle trajectories that are energy dependent. A hot
spot of 19 UHECRs detected over 5 years by the Telescope Array (TA) collaboration is consistent
with the starburst galaxy M82 (Abbasi et al. 2014). While the localization is consistent with
other sources including a blazar, M82 remains the best candidate. Taken at face value, UHECRs
from M82 suggest that planets in galaxies with rapid star formation are subject to high ﬂuxes of
UHECRs from local sources, like GRBs, microquasars, and relativistic SNe remnants. However,
the incoming directions of UHECRs, which are not strongly deﬂected by magnetic ﬁelds, indicate
that Milky Way sources do not signiﬁcantly contribute to the UHECR ﬂux currently observed
impacting Earth. While the threat of UHECRs seems low because the ﬂux is low, see Figure 1,
many planets throughout the local Universe are exposed to higher ﬂuxes of CRs at all energies,
especially before the time Earth was formed.
Accretion onto SMBHs, associated with AGN, are also probably responsible, aided by mag-
netic ﬁelds, for accelerating UHECRs, see Biermann et al. (2016) for a review of the origin of
UHECRs. An AGN likely inhibits life or even eliminates habitability within its host galaxy. AGN
might even adversely aﬀect other nearby galaxies as UHECRs are able to break free from the
magnetic conﬁnes of the host galaxy. Only some AGN or some brief episodes of SMBH activity
are signiﬁcant CR sources, while many more are sources of beamed high energy electromagnetic
radiation. As in the case of GRBs, the ionizing radiation from SMBHs is also a major threat to
Radio galaxies are eﬀective particle accelerators. The Milky Way, and its cluster of galaxies
known as the Local Group, are on the edge of the Virgo Supercluster of galaxies, which is centered
on the rich Virgo Cluster of galaxies. In particular, at the heart of this supercluster with a mass
1015Mlies the giant elliptical galaxy M87 and its radio source Vir A, with a SMBH powering an
impressive jet. Other relatively near radio galaxies are Cen A and Cyg A. Benford and Protheroe
(2008) suggested that the radio lobe structure in Cen A, shown in Figure 2, might be a source of
detected EGCRs, speciﬁcally the UHECRs (E ≥3 EeV: above the ’ankle’ in Figure 1). UHECRs
may also include a proton component from many radio galaxies integrated over vast distances,
visible already below 3 EeV (Biermann et al. 2015).
Becker and Biermann (2009) developed a detailed AGN model that suggests that neutrinos
are produced near the launching region of the AGN jet, due to high optical depths for proton-
photon interactions. Subsequently, protons escape from shocks where optically thin proton-photon
interactions may occur. This model predicts CRs from FR-I galaxies (Fanaroﬀ 1974) - massive
galaxies often with jets and associated with X-ray emitting gas as they move through a rich galaxy
cluster. Importantly, this eﬀect is independent of source orientation. Direction is important for
neutrino detection as they are ejected only along the jet direction. BL Lac objects and ﬂat spectrum
radio sources are accreting SMBHs with jets directed towards Earth and they are signiﬁcant particle
accelerators. However, some AGN may be just in a spin-down mode, not in an accretion mode.
The spin-down mode gives low luminosity, but for an extended duration, and thus it may also
2.4 The star formation rate (SFR)
The SFR history plays an important role in the habitability of galaxies. The most massive stars
do not last long and explode as SN-Type Ib/c, followed by the chemical enrichment of the ISM.
However, nearby SNe and GRBs probably have quite adverse eﬀects on life (Gehrels et al. 2003;
Thomas et al. 2005), causing mass extinctions, temporary sterilization, and even atmosphere re-
moval if they happen very close. Images of two nearby SNe remnants, Cas A and the Crab Nebula,
are shown in Figure 3. Over billions of years, the occurrence of nearby SNe is unavoidable, however
a lower SFR and associated lower SN frequency increases the probability that a planet will avoid
the most catastrophic events. In Figure 4, the SFR history of galaxies, (Madau and Dickinson
2014), is shown along with other trends discussed herein. The far-IR luminosity of galaxies is used
as a proxy for SFR. Also, far-IR luminosity can be used as a proxy for gravitational wave events as
essentially all massive stars are in binaries (Chini et al. 2012). However, gravitational wave events
are not dangerous at the distance of concern here.
Planck Observatory results (Planck Collaboration et al. 2016) suggest that the reionization
of the Universe occurred at redshift value about z = 8.8, directly or indirectly as the result of star
formation that began the process of heavy element fusion. Notably, a single epoch for ionization
is extremely unlikely; it is suggested by all ﬂat ΛCDM (dark energy + cold dark matter) models
with the latest Planck cosmology parameters that this epoch could be quite protracted. Kogut
et al. (2003) wrote that star formation started probably no later than z = 20, using a simple 2-step
A promising reionization source was a ﬂurry of star formation in galaxies that are similar to
the local Green Pea galaxies, discovered by a citizen science program, (Cardamone et al. 2009) of
the Sloan Digital Sky Survey (SDSS). These compact low mass galaxies appear small and green
because of a vast region of doubly ionized oxygen. They include galaxy scale star formation regions
capable of ionizing the intergalactic medium with escaping Lyman continuum radiation. One local
green pea, J0925+1403, at z = 0.3, is leaking ionizing radiation with an escape fraction of 8%
(Izotov et al. 2016) and is a shining example of the ionizing power of high mass star formation.
If the SFR can be used to trace the SN rate (i.e., due to the short lifetimes of stars that become
SN-Type II), then it also likely tracks the GCR levels over cosmic time.
3 Local astrophysical threats to life
Astrophysical sources of radiation often display dramatic variability over time. Occasionally, enor-
mous bursts occur, which may be associated with relativistic particle ejection. Sometimes these
are one-time events like SNe or GRBs. For accreting sources like microquasars and AGN, haz-
ardous events may occur repeatedly. Here we discuss such sources and their impact on planet
habitability. The danger posed by individual objects is a strong function of distance and depends
on the direction of beamed radiation source. Planetary habitability is especially inhibited when
high local SFRs are coupled with the growth of SMBHs.
The worst threats are those that cause complete planetary sterilization; these are most often
local threats as a planet’s location is a key to habitability. Life on Earth has avoided the worst
catastrophes, since evidence shows the continuous presence of life for 3.5 Gyr. As discussed in the
introduction, mass extinctions were quite common on Earth, yet there is no evidence of a complete
planetary sterilization. Mass extinction events on Earth, have paved the way for an increase of
evolutionary diversity as new niches became open. A key factor of habitability is the severity and
frequency of sterilization and mass extinction events and how readily life is able to recover from
them. The expansion of the Universe and the local reduction in the SFR improves the habitability
of planets protected by magnetic ﬁelds and thick atmospheres.
Life depends on water, and therefore, a catastrophic or gradual loss of atmospheric water can
result in a loss of habitability. Here we propose that a planet suﬃciently supplied with a protective
atmosphere and magnetic ﬁeld in the mid-disk of the Milky Way is currently more habitable than
similar planets in many other galaxies in the contemporary Universe. We argue this because,
ﬁrst, the Sun stays relatively far away from the SMBH in the center of the Milky Way, a major
threat. Second, the Galactic SMBH has a relatively low mass and has been unusually inactive for
some time, since only minor mergers have taken place in the Milky Way over the last 10-12 Gyr
(Gilmore et al. 2002). Third, the galactic disk currently forms new stars at a rate which sustains
a global magnetic disk wind, providing an eﬃcient shield against EGCRs without supplying too
many GCRs. It is diﬃcult to evaluate the danger of radiation and particle dosage as we do not
know what complex life is capable of withstanding. However, we speculate that life, and especially
metazoan life on the surface, can be harmed by large doses of particles and radiation. Within
this context, we conclude that life as we know it was rare or impossible in the disk of the Milky
Way earlier than about 5-6 Gyr ago. An earlier limit, at about 8 Gyr ago, also exists for only
microbial life. These temporal limits are illustrated in Figure 4. Habitable conditions probably
exist somewhat earlier in some disk galaxies, i.e. those without recent mergers or SMBH activity,
like the Milky Way, compared to other more recently active galaxies.
3.1 Supernovae (SNe)
Single massive stars above about 8Mexplode as SN-Type Ib/c. Two examples of these massive
star explosions, showing a stem-shaped breakout or jet, are shown in Figure 3. Such SN remnants
produce powerful shock-waves, which in turn accelerate CR particles. There are two types of
precursors of these SNe: red super giant stars with slow dense winds, and blue super giant stars
with fast low-density winds. Very massive stars above about 25M, depending on their heavy
element abundance, commonly produce stellar mass BHs (Heger et al. 2003;Woosley and Heger
2015). These very high mass stars explode via an unknown mechanism. Models involving neutrinos
(Bethe 1990), or the magneto-rotational mechanism (Bisnovatyi-Kogan 1970) have been proposed.
All of them occur in binary star systems (Chini et al. 2012). A small fraction of them explode as
GRBs or as relativistic SNe.
As a dying star ejects its envelope into space it encounters gas, either the stellar wind of
the predecessor star, or the immediate environment of an OB-super-bubble, or directly the ISM,
forming a supersonic shock. A mechanism called diﬀusive shock acceleration proposed by Fermi
(1949,1954) and investigated by many others (Axford et al. 1977;Axford 1981;Krymskii 1977;
Bell 1978a,b;Blandford and Ostriker 1978;Drury 1983) is thought to accelerate particles to GCR
The existence of expanding SNe remnants in the disk of the Milky Way and other disk galaxies
provides a continuous supply of CRs. This occurs as long as the frequency of SNe events is high
enough to replenish the GCR medium, which escapes the Galaxy on a time scale of 107years in
the solar neighborhood. At higher energy, that containment time is lower by E−1/3. In the central
region of the Galaxy the containment time may be much shorter (Biermann et al. 2010). The
eﬃciency of the conversion of SNe kinetic energy to CRs is uncertain. However, Duric et al. (1995)
performed an analysis of optical and radio observation of SNe in the Local Group spiral galaxy
M33. They ﬁnd that most (90%) of the SNe remnants are not signiﬁcant CR accelerators, with
the brightest 40 SNe remnants in M33 producing the bulk of the GCRs.
High mass stars, O and B types, explode either into the ISM, or into their own wind. When
the shock goes through the wind, it continues in the OB-super-bubble (Binns et al. 2007;Rauch
et al. 2009;Murphy et al. 2016). It is only a statistical ﬂuke that the Milky Way has only slow
cases of ISM-SNe, and not the fast cases as seen in M82 (Kronberg 1998;Bartel et al. 1987), all
of which are by several powers of ten more luminous than any SN remnant in our galaxy. We
interpret these luminous SNe in M82 as explosions of stars into their winds. Hence, some SNe may
have more adverse eﬀects on habitability than has been previously considered, using Galactic SNe.
3.2 Gamma-ray bursts (GRBs)
Gamma-ray bursts (GRBs) are stellar explosions that generate enormous radiation beamed in two
opposing directions. Local GRBs may occasionally sterilize large portions of the land-based life
in the Galaxy. Annis (1999) argues that GRBs are so critical to astrobiology that the Universe
is currently undergoing a phase transition from no intelligent life to intelligence as a result of the
reduction of GRBs. While we generally agree that the reduction of the number and luminosity of
GRBs is critical to habitability, the timing of this transition is likely not universal. Galaxies with
active SMBHs, especially those resulting from mergers, will remain un-inhabitable for many Gyr
into the future.
Notwithstanding atmospheric removal by a close and beamed GRB, the greatest impact of
GRBs on surface or shallow marine life is the depletion of ozone and the subsequent exposure to
stellar UV radiation. Melott et al. (2005) pointed out that DNA damage in organisms on Earth
from GRBs is greatest at mid latitudes due to local ozone depletion due to enhanced UVB radiation
and the direct angle of GRB at these latitudes. Hence, in most cases (except the worst scenarios)
GRBs would result in a mass extinction over large surface areas but not planetary sterilization.
Galante and Horvath (2007) also examined the astrobiological impacts of direct GRBs and found
that at distances up to 100 kpc there may be signiﬁcant ozone depletion and thus more stellar
UVB reaching the ground. Compounding the situation there would be less 350-450 nm stellar
radiation reaching the ground resulting in a less eﬃcient photo-repair of DNA damage. Also, the
reduction of visible light, also known as photosynthetically active radiation (PAR), would slow
Direct CR ﬂux from GRBs would be sterilizing only for a very close GRB source, less than
about 10 pc. Since the integrated radiation is what drives their impact, GRBs are too brief to
do extensive damage, unless they are quite close and/or if they are beamed directly at the planet.
Using determinations of the luminosity functions and the rates of GRBs with consideration of host
galaxy properties, Piran and Jimenez (2014) estimated the probability of a lethal GRB within a
galaxy, which appeared to be much greater in the inner Milky Way, than locally. They also found
that GRBs have a 95% probability of sterilizing a planet within 4 kpc of the galactic center. They
further suggested that planets further out in the Galaxy play a game of ‘GRB Roulette’ as the
probability of mass extinction by GRB decreases to 50% beyond 10 kpc. Their work also suggests
that the Earth, at the distance of 8 kpc from the Galactic center, has likely been exposed to one
sterilizing GRB event.
There is an indication that at least one Galactic GRB has occurred in the last Gyr or so. The
argument is as follows. The highest energy particles accelerated from a GRB are mostly neutrons.
Protons remain trapped in the magnetic ﬁeld and thus lose energy adiabatically during escape. A
relativistic neutron travels through the Galactic disk and decays after some time into a proton, an
electron, and an anti-neutrino. The proton is then caught by the Galactic magnetic ﬁeld and with
a small probability (about 0.05) interacts with the ISM, again becoming a neutron traveling to us
undeﬂected. Biermann et al. (2004) estimated that one to a few GRBs occurring about a million
years ago within 3 kpc of the Galactic center can account for the excess of 1018 eV CRs by this
process, detected by one instrument, AGASA (Hayashida et al. 1999;Teshima et al. 2001), but
not conﬁrmed by IceTop (Aartsen et al. 2016).
3.3 Nearby super-massive black holes (SMBHs).
As we observe SMBHs in the local Universe, it is clear that many of them remain quiet for a long
time, but these inactive SMBH have been active in the past and the right kind of accretion event
will turn an inactive SMBH into an active one. AGN activity as a function of redshift is shown in
Figure 4. The peak around z = 2 indicates that AGN were most active 10 Gyr years ago. AGN
activity dropped by an order of magnitude by the time the Earth formed, about 4.6 Gyr ago,
corresponding to z = 0.45, and it has fallen by another order of magnitude since then.
The density of EGCRs was dramatically higher in the past not only because the density of
CRs sources was orders of magnitude higher, but also due to an important cosmological eﬀect.
The co-moving volume of the Universe was smaller by a factor of (1 + z)3in the past, exposing
planets to more direct UHECRs from SMBHs as well as from extragalactic SNe. This means that
at the peak of the cosmic SFR, the local GCR density was about 15 times higher than it is today,
recall again Figure 4. As mentioned, AGN activity peaked at roughly the same redshift, z = 2,
when the Universe was a factor of 27 smaller. Thus the EGCR ﬂux from AGN was about 2400
times greater than at present. The activity of SNRs counts with (1 + z)3only as regards other
galaxies in the ﬁeld, not SN remnants in our Galaxy nor inside of our own local co-moving group.
As groups and clusters slowly grow by accretion, there is a weak countervailing eﬀect. Comparing
with Figure 1, at z = 2 the entire CR spectrum was higher and the ‘ankle’ ﬂux was as high as
the ‘knee’ ﬂux! These eﬀects combine to suggest that the background level of CRs may strongly
compromise or even prevent surface or shallow water life until about the formation of the Earth.
Furthermore, short very energetic energy bursts have probably adversely aﬀected habitability in
many locations, and many of them happened recently.
Planets that reside in large clusters of galaxies have larger ionizing radiation ﬂuxes than
planets in the Milky Way because they are likely to be near SN remnants and accreting SMBHs.
In our cosmic neighborhood we have several SMBH. These are in M31, M32, M81, M94, NGC5128
(Cen A), and in our own Galaxy, all within about 5 Mpc today. Within about 20 Mpc there are
many other SMBHs. Having more local AGN would be a serious threat to life. M87 for example
has consumed roughly 100 Milky Way mass galaxies and has generated an enormous amount of
radiation and relativistic particles in the process of growing its SMBH (Andrade-Santos et al.
2016). Several other giant elliptical galaxies in the heart of the Virgo supercluster probably have
also accreted many galaxies, but are currently between merger events.
3.4 Galaxy mergers and SMBH mergers
Galaxy mergers amplify the SN and GRB rates and SMBH activity. During a merger, giant
molecular clouds collide, resulting in a considerable increase in the SFR and its associated SN-
Type II rate. Considering the starburst galaxy M82, Kronberg et al. (1981,1985) found that
the radio, infrared, and X-ray luminosities from a region that is 600 pc in diameter within M82
is the result of an extreme burst of massive O and B type star formation. They detected many
SN remnants and gave evidence for buried SNe exploding within giant molecular clouds and then
breaking through the dense cloud to become visible. The SFR in that region is about 10 times that
of a normal star forming galaxy like the Milky Way. The VERITAS Collaboration et al. (2009)
detected gamma radiation from M82 and estimated a CR density roughly 500 times greater than
that of the Milky Way. M82 is likely experiencing a temporary enhanced star formation period due
to a recent merger, as M82 did not have signiﬁcant star formation outside the star forming nucleus
in the last 300 million years. Based on observations of planetary nebulae, M82 has cannibalized a
galaxy that was probably a 6 ×109Mstar forming galaxy within the last Gyr (Longobardi et al.
2015). Mergers may also result in accretion onto SMBHs. For example, the nearby galaxy M51
has a companion galaxy, NGC 5194, and Chandra X-ray observations show huge particle ejection
events from the SMBH in the smaller companion (Schlegel et al. 2016).
Planets in rich clusters of galaxies were exposed to high energy radiation and particles each
time their host galaxy merged. The fraction of merging galaxies is approximated as function of
FM=FM0(1 + z)M
This equation is used by a number of investigators, see Khochfar and Burkert (2001) for a summary
of results. FM0is the current fraction of galaxies undergoing mergers, estimated to be about 1%.
The power m ranges from 6 in rich clusters of galaxies to 3-4 for ﬁeld galaxies (Le F`evre et al.
2000). At the conservative end, the fraction of merging ﬁeld galaxies is FM= 27%. In a rich
cluster of galaxies the majority of galaxies are merging at z = 1. Complex life as we know it is
highly unlikely to occur in galaxies undergoing mergers, or in those that have merged recently.
When two big galaxies merge and both contain SMBHs, then their SMBHs also merge.
Gergely and Biermann (2009) showed that the spin of the ﬁnal merged black hole will be aligned
with the direction of the angular momentum of the SMBH binary orbit, and so it is in a totally
diﬀerent direction than the original spins of the two SMBHs. This means that while this happens
the two jets sweep through the sky, cleaning out a large conical solid angle (Gopal-Krishna et al.
Kun et al. (2017) presented a model of the merger of two SMBHs driving the consecutive
emission of gravitational waves, high energy neutrinos, and UHECRs, followed by a luminous
radio afterglow. This means that SMBH binary mergers involve interaction with matter on a
galactic scale, so maximal injection and acceleration of UHECRs occurs. Also tremendous mutual
interaction of the SMBHs leading to powerful beams not just of neutrinos, but also TeV photons,
neutrons, and copious UHECRs. These beams could be devastating, even at great distances. In
fact, this mechanism may explain the neutrino events detected by the IceCube Collaboration et al.
(2016), showing that such beams are visible in neutrinos across the entire Universe.
The blazar PKS 0723-008 is a strong candidate with evidence for a spin-ﬂip, high energy
neutrino emission, and a recent increase in radio emission by a factor of about 5. Kun et al. (2017)
also discussed four ﬂat spectrum quasar identiﬁcations with IceCube track events, two of which
have a ﬂat spectrum to near THz. Radio interferometry evidence showed that seven others are
undergoing mergers right now (±a few million years) and that all these have a ﬂat spectrum
to near THz, suggesting that a recent merger may have caused such extreme emission. A ﬂat
spectrum of a compact source implies a jet pointing at us, and thus extreme relativistic boosting
may occur, and only about 10% of all GHz ﬂat spectrum radio quasars have a spectrum extending
ﬂat to near THz.
3.5 AGN, SMBHs, and ultra-luminous X-ray sources (ULXs)
An important quantity concerning accretion onto compact objects such as black holes is the Ed-
dington luminosity - the radiation luminosity for which momentum transfer via radiation balances
the momentum transfer from gravitational attraction, or in other words, accretion is stopped. In-
terestingly, van Paradijs (1981) showed how accretion can circumvent this eﬀect in some cases that
concern us here. Consider the ﬁlling of a bubble with relativistic gas by a radio galaxy, clearly
visible in many cases like M87, NGC1275, Cen A (Figure 2), Her A, etc. (e.g. Owen et al. (2000)
for M87). All these radio galaxies are in a cluster of galaxies with its associated hot gas, at a tem-
perature of about 108K, and central density of order 10−3cm−3, so a pressure of about 10−10.5dyn
cm−2. The radio galaxy ﬁlls up the bubble, of radius, say, 1 Mpc. This ﬁlling is highly non-steady.
Whenever the central SMBH merges with another one (e.g. (Gergely and Biermann 2009;Kun
et al. 2017), a gigantic burst of new relativistic energy is released (Kronberg et al. 2001), possibly
approaching a good fraction of MBH c2, say 1/2 (Hawking 1971). In a fresh reﬁlling, the energy
may also go high enough so that the equation of state might get close to relativistic. Consider a
fresh outburst of energy spread around by a freshly merged central SMBH of 1010M(Caramete
and Biermann 2010). This would add an energy of 1064 erg to the bubble, resulting in a pressure
that exceeds the cluster pressure discussed above. The bubble would then be capable of bursting,
as we have ‘light ﬂuid’ - the relativistic gas, being held in by a ‘heavy ﬂuid’ - the thermal gas.
Such outbursts are directly visible in the SNR Cas A (Hwang et al. 2004;Fesen and Milisavljevic
2016), the Crab SN remnant = M1 (Black and Fesen 2015), both showing stem-shaped jets in Fig-
ure 3, and in some radio galaxies like the giants DA240 (Peng et al. 2004) and 4C+73.08 (Strom
et al. 2013). Outburst ﬂow has been modeled by Kompaneets (1960) and Moellenhoﬀ (1976), and
these models predicted the rather characteristic stem-like morphology clearly visible in Figure 3.
Outbursts from an over-pressured gas result in ﬂow velocities far above the speed of sound, as
derived in Fluid Mechanics (Landau and Lifshitz 1959), which can approach the speed of light
often seen directly in relativistic ﬂows. This outburst can then lead to an energy outﬂow of 1064
erg in possibly only 1014 sec, to give a short-lived luminosity of 1050 erg/sec, clearly above the
Eddington limit for the central black hole. Therefore, it is quite plausible that temporarily particle
population ﬂows may signiﬁcantly exceed the Eddington limit luminosity. The Eddington ratio
deduced here can be written as:
fEdd =1/2∗MBH c2
1038.3∗MBH /M∗R/c (.1)
where MBH is the SMBH mass used (here 1010 M) to produce a bubble of CRs (both hadronic
and leptonic); 1038.3erg/sec is the Eddington luminosity of a SMBH per M, and R is the radius
of the bubble (here 1 Mpc is used). The precise mass of the SMBH actually drops out again in
formulating the Eddington ratio. This speciﬁc example, patterned after M87 in SMBH mass, gives
an Eddington ratio of 50. Even if we allow an eﬃciency far less than 1/2 to produce a relativistic
ﬂuid, the Eddington ratio might still exceed the value of unity, what is normally referred to as the
Eddington limit. As linear scales of radio bubbles have an observed range from below 30 kpc to 4
Mpc, the Eddington ratio could occasionally also be far higher. The cosmic ray luminosity is likely
to be more constrained, but clearly Eddington ratios far greater than unity are possible (Stollman
and van Paradijs 1985).
A relatively rare class of accreting objects, called Ultra-luminous X-ray Sources (ULXs),
are located, oﬀ-nuclear, in galaxies and are often found nearby star forming regions. Because of
their very high luminosity (1040ergs/s) these are powered by black holes or neutron stars that are
accreting at Eddington or super-Eddington rates. The detection of the spin period pulsations in
M82X-2 securely establishes it as an accreting neutron star with a luminosity LX∼100 LEdd
(Bachetti et al. 2014). In this case, a highly magnetized neutron star funnels accreting plasma
from a mass losing companion star with a binary period of 2.5 days. The current high luminosity
state of M82X-2 will be relatively short lived. Biological threats from neutron star or black hole
ULXs thus likely follow the SFR, see Figure 4.
Many ULXs are likely black holes with high masses that may have luminosities such that L =
LEdd , while others are neutron stars like M82X-2 (King and Lasota 2016). The most energetic ULXs
may involve BHs with MBH >40M. Cygnus X-3 and XTE J1118+480 are Galactic microquasars
with observed bipolar jets, likely to be similar to some ULXs. Accretion onto compact objects
occurs over a wide range of masses and observed properties generally scale with the mass of the
accretor, a white dwarf (WD), a neutron star (NS), or a black hole (BH). For much higher mass
BHs we need to consider the SMBHs powering the emission of AGN. As opposed to the oﬀ-center
ULXs, AGN have BH masses MBH = 106−11 M.
By comparison, the current CR output of M87 is about 100 times higher than that of Cen A
(see Figure 2), but M87 is about 4 times as far as Cen A. So, M87 brieﬂy emitting at LEdd would
also increase the ﬂux of GCRs at Earth by a factor of 100 over current levels. (Whysong and
Antonucci 2003) show that the total power output of these two radio galaxies have this scaling.
The claim that the CR output scales in the same manner is empirical, and not speciﬁc. We must
conclude that past AGN activity of SMBHs, like that in M87, compromised life on a Super-galactic
scale. When considering the eﬀects of nearby AGN, SMBH, or ULX activity, we must conclude
that habitability is not a phase transition happening everywhere in the Universe at once. This
point is illustrated in Figure 4.
3.6 The Galactic center SMBH
The nucleus of the Milky Way emits high energy radiation and particles, potentially harmful to
life. Extremely high energy gamma-rays have been detected using Cherenkov telescopes, including
CANGAROO (Tsuchiya et al. 2004), VERITAS (Kosack et al. 2004), H.E.S.S. (Aharonian et al.
2004,2006,2008), and MAGIC (Albert et al. 2006). An exponential cut-oﬀ above some 10s of TeV
is observed (Aharonian et al. 2009). Thus the maximum accelerated energy for a proton is 200
TeV (Guo et al. 2013). A diﬀuse gamma-ray component, with the same spectrum is observed from
the disk surrounding the Galactic center (Aharonian et al. 2006;HESS Collaboration et al. 2016).
The observed cutoﬀ around 10 TeV results from a combination of interactions in the space between
the source and us and interactions happening at the source. The maximum energy of protons in
their synchrotron emission is about 1012 GeV (Biermann and Strittmatter 1987;Biermann 2006).
The current radiation ﬂux from the Galactic center is low and it is not a signiﬁcant concern for
life. However, this certainly has not always been the case. High levels of mass accretion onto the
Galactic SMBH could have resulted in an X-ray ﬂux at Earth comparable to that of the Sun, since
the sun emits 10−7of its bolometric luminosity in X-rays.
UHECRs detected at Earth are probably from accreting SMBHs at the centers of galaxies
(see Section 2.3). The Galactic magnetic ﬁeld helps to protect us from these EGCRs. Now let
us consider CRs from the Galactic SMBH, which is a mere 8 kpc away. It is currently one of the
least luminous of SMBHs observed. The gas accretion and luminosity of Sgr A* has certainly been
much higher in the past. If it were to radiate at 10 LEdd even brieﬂy, the CR ﬂux would be 107.6
times the current CR ﬂux from Cen A and hence 105.6times the current GCR ﬂux at Earth. These
estimates are quite uncertain and very energy dependent. With the ‘ankle’ energies used here, we
see that the Galactic SMBH poses the greatest threat to global sterilization of planetary surfaces
in the Galaxy. By analogy, the greatest threat to planets in external galaxies might arise from the
presence of local SMBHs. Many if not most galaxies do not harbor SMBHs, so these may be more
suitable galaxies for complex life.
4 Planetary protection
Many astrophysical events have a temporary or permanent adverse eﬀects on planets. Long-term
habitability requires a signiﬁcant amount of luck. The circumstellar habitable zone is discussed in
Section 5.3. However, many additional factors also aﬀect so called habitable zone planets. Plan-
etary system dynamics, host star activity, asteroid and comet impacts are important habitability
factors. A planet’s geomagnetic, atmospheric, hydrological, and biological history shapes its ability
to support life. Shock waves from SNe, direct GRB exposure, and local AGN activity may stress
and even exterminate life on many planets.
The question of whether life is prevalent in the Universe now and at times before Earth re-
quires analysis of not only the threats to life on planets, but also on the development of planetary
protection. Life on planets have several lines of protection from high energy particles. First, planets
in the disk of a star forming galaxy are protected from all but the highest energy EGCRs because
of shielding by the galactic magnetic ﬁeld. Lower energy CRs accelerated from galactic sources,
especially SN remnants, become deﬂected by magnetic ﬁelds to be contained in the Galactic mag-
netosphere. Those, mostly very high energy, EGCRs that penetrate the galactic magnetosphere.
Both magnetically conﬁned GCRs and the dangerous high energy GCRs with direct trajectories
impact the astrosphere generated by the magnetized stellar wind. The astrosphere protects planets
from the lower energy (E <300 MeV) GCRs. A global planetary magnetic ﬁeld provides the next
layer of protection, not only from GCRs but also from the SEPs emitted by the host star(s). The
last line of protection is the planetary atmosphere.
4.1 The rise of the elements
Before the formation of the ﬁrst stars in the Universe, hydrogen H and helium He were practically
the only existing nuclei as there were mere traces of deuterium 2H and lithium Li, beryllium Be,
and perhaps even smaller amounts of boron B. Today, life on Earth beneﬁts from an atmosphere
consisting mostly of N2,O2,H2O,CO2. The availability of CNO atoms plays an important role
in the potential for life on the surface, in the air, and water. For example, suﬃcient atmospheric
pressure is needed to keep surface water from boiling. Planet formation also requires suﬃcient
abundances of heavier elements including silicon Si, magnesium Mg, nickel Ni, iron Fe, and probably
enough radioactive elements to maintain enough internal heat to drive magnetic ﬁeld generation
(see Section 4.4).
If oxygen is present in the atmosphere at suﬃcient density and atmospheric pressure, an
ozone layer may be established. Atmospheric ozone O3provides a signiﬁcant UV shield against
CRs (see Section 4.5). It is thought that the complexity and diversity of life on Earth increased as
a result of the increase of oxygen in the atmosphere produced by photosynthetic life. This increase
of oxygen coincided with the Cambrian explosion in the geological record, 541 million years ago.
CRs indirectly cause harm to potentially habitable planets by removing ozone. With the removal
of ozone, XUV radiation from the host star may desiccate the planet’s atmosphere even if it is in
the traditional habitable zone by deﬁnition.
The high abundance of Fe is necessary to establish the planetary magnetic ﬁeld. Planets
without a suﬃcient Fe core probably cannot maintain a dynamo of suﬃcient strength to supply
magnetic protection against CRs from a variety of sources over biologically relevant timescales.
Planets with weak magnetic moments subject to high and intermittently intense ﬂuxes of CRs are
not expected to support life on the surface.
4.2 Galactic magnetic ﬁelds: protection from EGCRs
As discussed above, life on planets in the disks of star forming galaxies are protected from EGCRs
by a magnetic galactopause generated by the galactic wind. This wind is analogous to the solar wind
generating the heliosphere surrounding the solar system. This protective eﬀect of the magnetic ﬁeld
in galaxies is closely connected to the spiral structure. It is driven by SNe explosions within the disk
as well the shearing eﬀect from diﬀerential rotation of the disk (Jokipii and Morﬁll 1987;Hanasz
et al. 2009b), see L´opez-Cob´a et al. (2017) as well as other recent Galactic wind investigations.
Consider star formation within the young Milky Way disk and the central role of SN ex-
plosions. When the stellar population was small and CNO elements were rare, a weak Galactic
magnetic ﬁeld likely provided little protection from EGCRs. Later, the SFR increased and the
frequency of nearby SN became much higher as well. The resulting organization and strengthen-
ing of the Galactic magnetic ﬁeld improved the protection of life from EGCRs. Later after CNO
abundances increased locally, the SFR slowed, and the associated threats to life decreased.
The Galactic halo wind has a k−2(k wave-number: k = 2 π/λ, λ = wavelength), spectrum in
magnetic irregularities (Biermann et al. 2015), which is standard for super-sonic ﬂow (Federrath
2013). The short wavelength for this spectrum is about 20 pc, and that corresponds to about
1016.6Z eV in the disk. The energy scales linearly with charge Z. Now a k−2spectrum implies
that the scattering has no energy dependence and this means that the ﬂux may be reduced inside
the galaxy by forcing the CRs out.
The WMAP haze from the Galactic center region is due to massive star explosions. The
spectrum of magnetic irregularities is measured from radio scattering data, e.g. using LOFAR, and
direct solar wind data. In the Galactic center region these assumptions allow an interpretation of
the haze and the 511 keV annihilation line, as well as for the size scales of the bubble (Biermann
et al. 2010). The irregularity spectrum is found to be Kolmogorov (k−5/3), in this region which
means that the scattering is proportional to E1/3and the time-scale to scatter across any region
of a speciﬁc scale is E−1/3. This spectrum aﬀects only particles with energy below about 1017.5
Z eV, as far as we know. CR transport occurs by scattering alone, so incoming particles may get
moved around, stored for some time and then ejected again. This suggests that the Galactic halo
wind transports all or most of the particles below about 1016.6Z eV back out and protects life on
planets in the Galactic disk.
There is some analogy between the Solar wind, which also has a Kolmogorov spectrum at
small scales, and pushes out things below about GeV, and the Galactic wind, which pushes out
things below about 1016.6eV. However, the time scale of driving the wind is of the order of 10
kpc/(500 km/s) = 2 ×107yrs, not surprisingly of the same order of magnitude as the storage
time of CRs in the CR disk. The wind might go much further out, but the interaction of the wind
with the intra-cluster medium is not known.
Locally, we are just barely above the protective threshold of a strong galactic wind (Rossa
and Dettmar 2003;Biermann et al. 2015), and thus it is conceivable that potential life in a region
not too far from the Galactic plane might not have enough protection from CRs, or suﬀer from a
temporary weakening of that Galactic wind. The same holds true for a planet that is farther out in
the disk, than about 10 kpc; out there the conditions to drive a galactic wind may no longer hold.
We can see that eﬀect in radio observations of external galaxies. So a suﬃciently high level of local
star formation per area in a galactic disk drives a super-sonic wind which is required for protection
of life from EGCRs. Maybe today, Galactic magnetic shielding is not essential for Earth. However,
during a starburst period, especially with associated SMBH accretion, in a neighbor galaxy like
the Andromeda Galaxy (M31), it would be.
4.3 Astrospheres: protection from GCRs
The circumstellar environment is dominated by electromagnetic radiation from the host star(s) and
stellar winds launched by magnetic ﬁelds in the hot corona of cool main sequence stars (Biermann
1951;Parker 1958). As the wind escapes it ﬁlls an astrosphere - a bubble of outﬂowing plasma
originating as a stellar wind and terminating at a dense hydrogen wall just outside the astropause.
In the case of our Sun, the heliopause is about 120 au from the Sun. Protection from host star
winds is a prerequisite to planetary habitability, however these winds also protect life on planets
from GCRs. Interaction of high energy charged particles with the dense layer of neutral atoms
results in charge exchange interactions for CRs with energies below about 300 MeV. Dense stellar
winds eﬀectively reduce the ﬂux of GCRs on the magnetosphere and atmosphere of planets. The
protective value of the astrosphere against GCRs depends on the star type(s) and also on stellar
activity cycles. A strong stellar wind signiﬁcantly reduces the ﬂux of GCRs, but the higher energy
CRs penetrate the astrosphere. Smith and Scalo (2009) examined the eﬀect of astrosphere collapse
on planetary habitability during close encounters with nearby stars.
The Voyager spacecrafts provided the ﬁrst opportunity to directly measure the interstellar
GCR ﬂux. They detected a sharp increase in GCRs as they passed through the heliopause on
their trek from the heliosphere into interstellar space, where the plasma ﬂow changes from the one
dominated by the solar wind into one dominated by the GCR ﬂux (Schlickeiser et al. 2014). The
interstellar proton number ﬂux per kinetic energy is 15 times higher than it is at 1 au from the
Sun (Cummings et al. 2016).
Planets in the HZ of the lowest mass M-type main sequence stars are subject to intense ﬂares
and SEPs, due to the combination of long lasting magnetic activity and close proximity of the
HZ. However, in absolute terms, low-mass star winds are weaker and corresponding astrospheres
are smaller and less eﬀective at stopping GCRs than winds from higher mass stars. Having a
suﬃciently protective astrosphere is especially important during episodes of increased GCR ﬂux
and during times of planetary magnetic ﬁeld reversals. Atri et al. (2013) pointed out that if such
planets are rotating slowly, due to synchronization with its orbital motion, their habitability may
be compromised. On the other hand, a planet in the circumbinary HZ of a pair of solar like stars
and with a binary period of say 25 days, will be exposed to reduced X-ray and UV radiation due to
tidal deceleration of stellar rotation in binaries (Mason et al. 2013,2015;Zuluaga et al. 2016). In
addition, a circumbinary astrosphere is produced from a combined wind, thereby reducing GCRs
compared to the single star case.
4.4 Planetary magnetic ﬁelds: protection from GCRs and SEPs
To examine the importance of planetary magnetic ﬁelds in protecting potential life we have to
look no further than at the nearest Solar System planets. Venus is too close to the Sun to have
surface water, but may have previously been in the HZ when the Sun was younger and fainter.
However, Venus rotates very slowly, once every 243 days, and as a result it does not have a global
magnetic ﬁeld. Venus lost its habitability as the result of its proximity to the Sun and it suﬀered
a runaway greenhouse eﬀect. Mars previously had surface water, but its low gravity is unable to
prevent atmospheric mass loss. Mars lost its habitability for complex life once it cooled, lost its
dynamo, and its atmosphere lost its protection capacity from SEPs.
For our purposes consider the stellar wind ram pressure ρwv2
wp where ρwis the stellar wind
density and vwp is the velocity of the wind relative to the planet and the magnetic pressure B2/8π.
The balance of these opposing pressures
forms a magnetopause between the star and the planet located at a stand-oﬀ distance above
the planet surface. Larger planets of similar composition will generate stronger magnetic ﬁelds.
However, the strength of the magnetic ﬁeld depends on its rotational frequency and on its heat
content from sources such as gravitational contraction and radiogenic heating. Tidal forces provide
additional heating in some cases. Atmospheric erosion and its protection by the magnetic ﬁeld has
complicated eﬀects on habitability because a signiﬁcant amount of the original H-rich atmosphere
of Earth had to be removed. The loss of a signiﬁcant amount of light gasses while retaining a
relatively thick CNO atmosphere is critical for planetary habitability. See G¨udel et al. (2014) for
a review of H-rich proto-atmospheres.
4.5 The atmosphere: a strong last line of protection
Ruderman (1974) considered potential eﬀects of a nearby supernova on the life on Earth, and
found that hard X-ray pulses or increased CR ﬂux may temporarily remove much of the Earth’s
protective ozone. High energy electromagnetic radiation from the formation of a black hole and
heralded by a GRB can be devastating to life. Thomas et al. (2005) modeled several GRB eﬀects
including ozone depletion and associated UVB surface exposure resulting in DNA damage, as well
as nitrous oxide (NO2) production in the atmosphere resulting in acid rain and global cooling.
For a typical burst, local ozone depletions were found to be highly dependent on geography and
seasonal factors with up to 74% predicted drop in ozone. The resulting NO2opacity could provide
strong local cooling for up to several months. In extreme cases, lethal UVB exposure of life forms at
the base of the marine food chain, such as phytoplankton, are expected to be signiﬁcant (Thomas
et al. 2005).
SEP and GCR eﬀects on atmospheres of Earth-like planets in the HZ around M-dwarf stars
were studied by Grießmeier et al. (2005,2016) and (Grenfell et al. 2007). These studies showed that
ozone levels are reduced, but some ozone remains, except in the highest SEP ﬂux cases. Segura
et al. (2010) modeled atmospheric eﬀects of the UV and SEP ﬂux from a large ﬂare of the active
M dwarf AD Leonis. These eﬀects on an Earth-like planet without a magnetic ﬁeld in the HZ were
simulated. They found that the UV emission from such a ﬂare should not have a signiﬁcant impact
on the atmospheric ozone, however, NO2produced by ionization from SEP protons may result in
a greater than 90% ozone depletion about two years after the ﬂare, with a predicted recovery time
of 50 years.
A solar proton event in 1989 resulted in a 1-2% drop in the column averaged ozone levels
Jackman et al. (2000). The most powerful solar ﬂare ever observed is the Carrington event of
1859. Thomas et al. (2007) estimated that this high proton ﬂux event may have caused up to 14%
localized depletion of ozone for 4 yrs, causing an increase in nitrate deposition. The beryllium-10
isotope provides means to measure the CR history on Earth. 10Be is formed mainly by the CR
spallation of oxygen and is preserved in ice. Thus, it is a direct proxy for CR ﬂux, and ice core
measurements in Greenland and Antarctica provide a record of CRs events.
High energy CRs, especially UHECRs, hit the atmosphere and create an air-shower (Thoudam
et al. 2016), as the enormous energy is converted into potentially millions of secondary particles,
including muons, neutrinos, electrons, positrons, neutrons, and protons, with muons having the
greatest penetration capacity. If the secondary particles are energetic enough, and their ﬂux is
suﬃciently high, muons can impact even subsurface life. If the radiation dose is too high, the
chances of sustaining life as we know it on the planet are very low. Atri and Melott (2011)
examined the dependence of the GCR-induced radiation dose on the strength of the planetary
magnetic ﬁeld and its atmospheric depth, and found that the latter is the decisive factor for the
protection of a planetary biosphere. Thicker atmospheres provide longer path lengths for both
primary and secondary CRs, especially lowering the ﬂux of secondary particles at the surface. An
atmosphere should be thick enough to provide surface pressure that is suﬃcient to maintain liquid
water (or other solvent), which is required for surface life.
5 Habitability in space and in time
Complex life requires a rather strict set of essential ingredients as well as a protected environment.
In this section, we discuss limits on complex life based on our assumptions and the evidence
presented in this review. We introduce the concept of a super-galactic habitable zone and discuss
the proposed galactic habitable zone. Then we discuss the stellar, or radiative, habitable zone
and stress that all of these must include the eﬀects of high energy radiation and CRs. Finally, we
discuss the cosmological implications for life over time, what we call cosmobiology.
5.1 The super-galactic habitable zone (SGHZ)
We propose that there are super-galactic habitable zones (SGHZs). It is suggested that complex
life is severely compromised on the surface of planets within the central region of large clusters of
galaxies and superclusters. This hypothesis is based on several independent observations including
1) the ubiquity of galaxy mergers, 2) periods of intense star formation experienced by many galaxies
either during natural spiral arm formation or as the result of a merger, and 3) activity associated
with SMBH accretion and CR acceleration, especially as the result of a SMBH merger. The merger
and star formation history is an important factor in the habitability of a galaxy. The occurrence
of central black holes is tied to structural properties of galaxies. Many galaxies have no SMBH
according to Spitzer observations (Buta et al. 2015). We may ﬁnd, that only relatively small
galaxies are able to maintain habitable environments over long times.
Our own galaxy is probably on the borderline. On the other hand, the metal abundance is
correlated with galaxy size, and thus oxygen (e.g., in water) and other life-required elements may
not be available in suﬃcient quantities in small galaxies. So between these two requirements, a)
no central SMBH and no extreme central star density (prone to make GRBs), and b) suﬃcient
amount of heavy elements, there may be only limited time in the Universe, and only a few galaxies,
to allow life, especially complex life to develop. These scenarios are illustrated schematically on
the right side of Figure 4. The limit estimated for minimal or microbial life depends greatly on
local conditions, but is estimated here to be about 8 Gyr ago or when the Universe was about 6
Gyr old (see Figure 4). At this time, the elemental abundances were about 10% of the Solar value
and AGN and SN rates had only just started to decline. Complex life was possible somewhat later.
About 5-6 Gyr ago, when the Universe was 8-9 Gyr old, metal abundances were comparable to
present day in some locations and AGN and the SFR had decreased to about 10 times present day
levels. Where habitable conditions were met, life probably transitioned from simple to complex
about that time.
5.2 The galactic habitable zone (GHZ)
The galactic habitable zone (GHZ) introduced by (Gonzalez et al. 2001) was originally based on
the idea that habitability is rendered impossible unless heavy elements are widely available. While
threats are common in the inner region of the Galaxy where gas is metal rich, threats are few but
life supporting elements are sparse in the outer regions of the galactic disk. Thus, life exists only
(or at least predominately) in a thin, metal rich annulus around the galactic center, beyond the
GHZ modeling (Lineweaver et al. 2004;Prantzos 2008;Gowanlock et al. 2011) has focused on
the frequency of hazards and on the build-up of elements within the disk of the Milky Way. Where
stars formed most rapidly ﬁrst near the Galactic center and then continued more slowly further
outwards, constructing a radial metallicity gradient. Models apply observed metallicity gradients
in the our Galaxy and others including M31 (Carigi et al. 2013) and M33 (Forgan et al. 2017) and
consider building up of elements as well as sterilization by exposure of planets to SN and GRBs.
While predictions vary greatly, most argue that planets within 3-4 kpc of the Galactic center are
subject to either too frequent or too violent events, thus forming the inner boundary of the GHZ.
The GHZ is represented here, in Figure 5 (top view) and Figure 6 (side view), showing the
distribution of metallicity in the disk as derived from the SDSS APOGEE survey (Bovy et al. 2014;
Hayden et al. 2014). An approximate GHZ is shown bounded by the box in Figure 6. The inner
bound of 4 kpc is based on age and sterilization threats. The outer and thin disk borders are based
on a metallicity criterion for complex (metazoan) life of roughly 50% Solar values. Around Solar
distances (8 kpc) planets are expected to have a moderate chance of avoiding a GRB sterilization
event for at least 1-2 Gyr. However, evidence is increasing that GRB have a strong luminosity and
a dependence on metallicity. Low metallicity GRBs are a rare class of highly luminous GRBs. Thus
planets located nearby metal poor star forming regions will likely have a much higher probability
of direct impact by a devastating GRB. Lower metallicity star formation is currently taking place
in the outer spiral arms of the Milky Way, recall Figures 5 and 6. We emphasize that a planet at 8
kpc from the galactic center, like Earth, is exposed to a signiﬁcant risk from GRB sources located
in the lower metallicity star forming regions at 10-12 kpc from the galactic center.
It likely that the GHZ in the disk of a spiral galaxy generally moves from an initial inner edge
gradually outwards with time. For any localized region within a galaxy, the habitability increases
only after a signiﬁcant number of stars have formed, substantial metal enrichment has occurred,
and the SFR has declined. Only then the probability of sterilization events and CR density can
decline below the harmful threshold. Thus, many stars formed before complex life can take hold
on the planets.
If the argument made in the previous section for the existence of SGHZs is true, then the
GHZ concept must also take into account the activity of SMBH. Consider two examples: NGC4258,
where the central black hole is currently producing major outﬂows into the disk (Moran 2008), and
M33 = NGC 0598, where there is no known central black hole at all, and therefore no such activity.
Even if major outﬂows are not currently taking place, they may occur from time to time from any
SMBH. The inner radius of the GHZ in those examples may be quite diﬀerent and complex life
may not be possible galaxy wide.
5.3 The circumstellar habitable zone (HZ)
For more than a half a century, a cornerstone of astrobiology is that there exists a shell surrounding
a star called the habitable zone (HZ), characterized by stellar luminosity and temperature within
which an Earth-like planet could maintain surface water (Huang 1959;Dole 1964;Hart 1979). The
stability and habitability of planets in orbits around one or both stars in a close binary star system
has also been investigated (Huang 1960;Holman and Wiegert 1999;Dvorak et al. 1989;Eggl et al.
2012;Pilat-Lohinger et al. 2012;Mason et al. 2013,2015;Haghighipour and Kaltenegger 2013;
Zuluaga et al. 2016).
(Kasting et al. 1993) derived the boundaries of the HZ for main sequence stars as a function
of time. The boundaries are aﬀected by the changes in the luminosity of the star that reduces
the part of the HZ. Kopparapu et al. (2013,2014) updated those calculations. This classical
HZ concept is fundamental to assessment of the potential for liquid water on planetary surfaces.
Many additional habitability factors must be considered, see the reviews by Lammer et al. (2009),
Dartnell (2011), Forget (2013), France et al. (2014), G¨udel et al. (2014), and Gonzalez (2014). For
example, habitability within the habitable zone is strongly constrained by stellar UV radiation,
winds, and catastrophic events such as nearby SNe, asteroid impacts as well as other planetary
dynamical issues. Especially deleterious situations occur if the planet is caught in the beam of a
microquasar, ULX, GRB, or AGN (SMBH).
Main sequence stars O, B, and A types, with the largest mass have short lifetimes. So
considering, times required for planet formation and cooling, only stars with lifetimes of greater
than or about 2 Gyr are considered to have habitable planets. The chromospheric activity of low
mass stars is comparatively high, with X-ray luminosity about 0.1 Lbol . In addition, because they
lose angular momentum more slowly, the high activity of these stars lasts much longer than for solar
mass stars. Long lasting, high activity of low-mass stars suggests that planets orbiting these stars
cannot be habitable unless they were formed with enormous quantities of water (Zuluaga et al.
2013;Tian and Ida 2015). In order to fully address the habitability of planets over a wide range
of environments, an integrated system approach of geodynamical, geochemical, climatological, and
biochemical factors is needed, (Franck et al. 2000;Franck et al. 2007).
6 Life as we know it in the Universe
Here we consider cosmological eﬀects on life as we know it. The simplest cosmological model con-
sistent with the four main observations, namely the accelerating expansion of the Universe, cosmic
microwave background radiation, abundance of the primordial elements from big-bang nucleosyn-
thesis, and the large scale galaxy distribution is called ΛCDM cosmology. The ﬁrst Friedman-
Robertson-Walker equation gives the expansion rate H, also known as the Hubble parameter, as
a function of the distance scale a(t) and in terms of the constant of gravitation G, the total mat-
ter plus radiation density ρ, the Friedman curvature of space kF, and the Einstein cosmological
constant Λ as follows:
H2(a) = 8πG
where the scale factor a(t) and its time derivative are related by H(a) = ˙a/a. Presently, a = 1 and
H = H0= 67.74 ±0.46 km/s/Mpc (Planck Collaboration et al. 2016). The total radiation plus
matter density may be simpliﬁed by recognizing that the current radiation and neutrino densities
are much smaller than the baryon and dark matter densities. Assuming a zero curvature, the
time of transition from a dark matter dominated Universe to a dark energy Universe depends only
on the respective densities, ΩB, ΩDM , ΩΛ, as deﬁned by Frieman et al. (2008), and the Hubble
Constant H0. Let ΩM= ΩB+ ΩDM . The simpliﬁed Friedman-Robertson-Walker equation may
be solved analytically for the scale factor and then setting ¨a= 0. It follows that the transition to
a dark energy dominated Universe takes place when
Evaluating with measured values for the Universe today: ΩΛ= 0.6911 ±0.0062, ΩDM =
0.2603 ±0.0062, and ΩB= 0.04860 ±0.00031 from the Planck Collaboration et al. (2016) yields
a = 0.61 ±0.01, i.e. when the Universe was 61% of its current size. Along with a= (1 + z)−1we
ﬁnd that the transition occurred near z = 0.62, 6-7 Gyr ago, see Figure 4.
Let us consider a time-line of astrobiologically salient features of the evolving Universe. The
formation and accumulation of planet and life building elements was made possible by the large
quantity of dark matter in halos, binding galaxies and clusters together. In the Milky Way,
signiﬁcant abundances of C, N, O, Si, and Fe were not available much before 8 Gyr ago, recall
Figure 4. Planets formed after that time might develop oceans with deep sea life, but surface
life was often compromised. From then until the formation of the Earth, 4.5 Gyr ago, and until
today the GRBs possibly had a devastating eﬀect on surface habitability, although the average
luminosity of GRBs declined over time. The transition to a dark energy dominated Universe (and
thereby expanding with acceleration) about 6.5 Gyr ago means that some denser regions of the
Universe, like the Virgo and Perseus clusters with giant elliptical radio galaxies along with the
numerous spiral galaxies they consume, remained inhospitable to life even today, well after the rise
of elemental abundances. However, galaxies in other groups with less aggressive SMBHs and ﬁeld
galaxies are moving safely away owing to the transition to dark energy domination.
The Local Group galaxies M31 and M32 may have far fewer habitable planets that the Milky
Way has today. The mass of the SMBH in M31 is 60 times the mass of the SMBH in the Milky
Way. M32 appears to have been stripped of most of its stars, so the remaining stars are very
close to the SMBH. The local starburst galaxy M82, the merging Cen A, and M87 appear to
be inhospitable even towards neighboring galaxies, like M81 and M90 both of which also harbor
SMBHs. The Local Group spiral galaxy M33 appears to be well suited for life as it has a signiﬁcant
galactic magnetic ﬁeld and lacks a SMBH. To properly address the habitability over cosmic time
in the Local Group, we must consider that the Andromeda Galaxy and others containing SMBH
likely have been much closer to the Milky Way than they are today.
To consider now life after Earth, prospects for Milky Way planets to sustain complex life may
worsen as the Andromeda Galaxy = M31 and our Galaxy are approaching each other and a major
merger will take place in about 5 Gyr (Cox and Loeb 2008). During the merger, Milky Way stars
and their planets will become close to several SMBHs much more massive than our own. Especially
the SMBH in the Andromeda Galaxy and in its current companion M32. Its worth noting that
NGC185 and NGC205 are also close to Andromeda, but they do not have SMBHs.
Finally, our motion in space relative to the Virgo super-cluster is appreciably slowed down
from cosmic expansion and it will reverse at some point in the future. Then we will start our
descent into the neighborhood of the galaxy guzzling SMBH in M87 and others. In fact, the radial
scale of reversal of relative motion between any two galaxies or galaxy groups/clusters increases
with cosmic epoch, and so at times in the deep future life in the Milky Way will likely be close to
some monster black holes and will have to emigrate and/or use high level intelligence to deal with
7 Summary of Conclusions
Habitability of the Universe in general has improved due to progressively increasing protection
of life from high energy radiation and CRs. The materials to form planets, and to protect life
on their surfaces became possible as CHNOPS, Si, Mg, Ni, and Fe were forged by generations
of stars. These elements allow the formation of a vigorous magnetic dynamo, protecting planets
from atmospheric erosion and water loss by stellar winds. Long-term magnetic protection of life
on rocky planets requires either enough mass to trap heat to keep the core cooling very slowly,
or a signiﬁcant supply of radioactive elements: K, Th, and U, with long decay lifetimes, or as in
Earth’s case, both. Far in the Galaxy’s future, new habitable planets may form from recent SN Ia
ejecta. New life will need planets formed from freshly produced Fe and radioactive nuclei, so as to
restart the clock on the inner planet core heating necessary for the dynamo.
However, the presence of complex life is subject to conditions that vary dramatically, from
place to place and over time, from the relatively calm present day Galactic disk with modest
SFRs and an inactive SMBH to the ionizing environments of starburst galaxies or near accreting
SMBH. Life on Earth and probably elsewhere beneﬁts from four layers of protection: (1) the
galactic magnetic ﬁeld protects life from EGCRs, (2) the solar wind protects life from GCRs, (3)
Earth’s magnetic ﬁeld protects life from SEPs, and (4) a thick atmosphere is the best protection
against GCRs. However, UHECRs can reach the surface of Earth and even penetrate underground.
While low now, UHECR ﬂuxes have been orders of magnitude higher in the past. All protection
mechanisms are dynamic. During magnetic ﬁeld reversals, planetary magnetic ﬁelds are quite weak,
and reversals might last thousands of years. Ozone provides the best protection against biologically
damaging radiation, but it may be depleted by SNe, GRBs, or during outbursts of the local SMBH.
Mergers of galaxies, both major and minor, have a profound eﬀect on local habitability.
Many planets in the densest regions of the local Universe may remain un-inhabitable due
to intense particle ﬂuxes and sterilizing events. We introduce the concept of a Super-Galactic
Habitable Zone to address this possibility. For the future, all this means that if humanity survives
long enough, then our descendants may need to keep moving, ﬁrst into space or to another planet
with a diﬀerent star (as the Sun dies), and then certainly to another galaxy.
List of acronyms and terms
AGASA - The Akeno Giant Air Shower Array
AGN - Active Galactic Nucleus
APOGEE - The APO Galactic Evolution Experiment of SDSS
au - Astronomical unit, the average distance from Earth to Sun: 1 au = 1.496 ×1011
BH - Black Hole
Big Bang - The star of the expansion of the Universe
BL Lac objects - A type of radio quiet galaxy with an active galactic nucleus (AGN)
Blazar - Any of several classes of AGN with jets pointing towards Earth
CANGAROO - Coll. of Australia and Nippon for a Gamma Ray Observatory in the Outback
CGRO BATSE - Compton Gamma-Ray Observatory’s Burst And Transient Source Exp.
CHNOPS - Carbon, Hydrogen, Nitrogen, Oxygen, Phosphorous, Sulfur
CNO - Carbon, Nitrogen, Oxygen
Collapsar - A star that collapses becoming a black hole without a supernova explosion
CR - Cosmic-Ray - high energy particle
EGCR - Extragalactic Cosmic-Ray
eV - electron-Volt
far-IR - Long wavelength infrared radiation
Fermi - gamma-ray telescope
FR-I - massive galaxies often with jets and associated with X-ray emitting gas
GCR - Galactic Cosmic-Ray
GHZ - Galactic Habitable Zone
GRB - Gamma-Ray Burst
Gyr - Gigayear, 109yr
H.E.S.S. - High Energy Stereoscopic System, gamma-ray telescope
HZ - Habitable Zone - around stars
IceTop - The surface array of the IceCube Neutrino Telescope
ISM - Interstellar Medium, gas and dust between stars
kilonova - Supernova from the merger of two neutron stars or a neutron star and a black hole
kpc - Kiloparsec, 103pc: 1 kpc = 3.086 ×1016 m
ΛCDM - Dark Energy plus Cold Dark Matter cosmology
LOFAR - The Low-Frequency Array, radio telescope
M- The Mass of the Sun: 1 M= 1.989 ×1030 kg
MAGIC - Major Atmospheric Gamma Imaging Cherenkov Telescopes
MAVEN - Mars Atmosphere and Volatile EvolutioN Mission
Mpc - Megaparsec, 106pc: 1 Mpc = 3.086 ×1019 m
NS - Neutron Star
OB-super-bubble - Hot gas surrounding high mass O and B type star
PAR - Photosynthetically Active Radiation
pc - Parsec, the distance of an source with a 1 arc-second parallax angle: 1pc = 3.086 ×1013m
Planck - Cosmic Microwave Background telescope
SEP - Solar/Stellar Energetic Particle
SDSS - Sloan Digital Sky Survey
SFR - Star Formation Rate
SN - Supernova, a generic stellar explosion
SNe - Supernovae, plural of supernova
SN ia - White dwarf explosion after exceeding the Chandrasekhar mass limit, no star remains
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SN II - Core collapse supernova
SMBH - Supermassive Black Hole
SGHZ - Super-Galactic Habitable Zone, see Figure 4
UHECR - Ultra High Energy Cosmic-Ray
ULX - Ultra-Luminous X-ray source
UV - Ultraviolet radiation
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WD - White Dwarf
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Figure .1: The cosmic-ray (CR) spectrum as detected on Earth. Galactic cosmic rays (GCRs) dominate
at Earth from below 1 GeV all the way to about 1018.5eV, the ankle, at which point they are taken over
by extragalactic CRs, which reach the highest energies so far observed, the high-energy cutoﬀ, about 1020
eV. SEP (Solar energetic particles) can contribute signiﬁcantly, if you are outside Earth, because of their
burst-like character, but on average they are low in ﬂux. SEPs can dominate on another planet, but at
Earth, the spectrum shown includes no SEPs whatsoever.
Figure .2: Centaurus A located 4 Mpc away in a composite X-ray, submillimeter, and optical image. It is
by far the most violent nearby accreting supermassive black hole (SMBH) in observable display. The radio
emission spans 20 times the size of the full Moon in the sky. A merger took place just a few tens of millions
of years ago. Both old and new old jets are seen in pointing diﬀerent directions, indicating the merger a pair
of SMBHs. As discussed in the text, Ultra-high energy CRs (UHECRs) from Cen A may be among those
detected on Earth. Image credit: X-ray: NASA/CXC/CfA/.; Submillimeter: MPIfR/ESO/APEX/A.;
Figure .3: Supernova remnants accelerate Galactic cosmic rays. Left: A composite image of the roughly
300 year old supernova remnant Cas A, which is 3.4 kpc away. It has two oppositely opposed jets, the
prominent one seen pointing towards the upper left. Image credit: NASA/CXC/SAO. Right: The 960
year old Crab Nebula supernova remnant which is located at a distance of 2 kpc. This Subaru telescope
image shows remarkable details of its jet in this O III image that has been scaled logarithmically to show
jet details. Inset: More detailed view of the jet. From Rudie et al. (2008), used by permission.
Figure .4: Time evolution curves of star formation rate (SFR) (a proxy for supernovae (SNe) and gamma-
ray burst (GRB) rates), Active galactic nuclei (AGN) activity, metallicity, and volume of the Universe.
The elements of life slowly build up, while habitability threats decrease. On the right side is a schematic
view of the development of a Super-Galactic Habitable Zone (SGHZ). Life is compromised near the center
of rich superclusters and merging galaxies. Far from dense galaxy concentrations habitability is possible in
regions with suﬃcient metallicity. The shaded region around 8 Gyr and extending to later times in certain
places corresponds to extremophile habitability without complex life on land as we know it.
Figure .5: The median Iron abundance [Fe/H] distribution near the mid-plane of the Milky-Way. The
square brackets indicate dex units, meaning decimal exponent deﬁned here with respect to the Sun, such
that 0 equals the Solar Fe/H value and -1 is one-tenth of the Solar abundance. The plot includes 4,330
stars in the SDSS APOGEE DR11 Red Clump sample, such that Z≤250 pc, where Z is the vertical
distance from Galactic disk. Figure is from Bovy et al. (2014), used by permission.
Figure .6: SDSS APOGEE metallicity [M/H] distribution as a function of Galactocentric and mid-plane
distances. The Galactic Habitable Zone (GHZ), estimated by the box, is the most probable region of the
Galaxy for complex life as we know it. The vertical distance from the Galactic plane is the absolute value
of the coordinate Z, as in Figure 5. The square brackets indicate dex units, meaning decimal exponent
deﬁned here with respect to the Sun, such that 0 equals the Solar M/H value and -1 is one-tenth of Solar
abundance. Metallicity refers to the sum of all of the elements other than hydrogen and helium. Figure is
adapted from Hayden et al. (2014), used by permission.