Xavier Dumusque1,2, Francesco Pepe1, Christophe Lovis1, Damien Se ´gransan1, Johannes Sahlmann1, Willy Benz3,
François Bouchy1,4, Michel Mayor1, Didier Queloz1, Nuno Santos2,5& Ste ´phane Udry1
starat which water, if present, would be liquid. There are planets in the habitable zone of stars cooler than our Sun, but
The detection of a habitable Earth-mass planet orbiting a star similar to our Sun is extremely difficult, because such a
signal is overwhelmed by stellar perturbations. Here we report the detection of an Earth-mass planet orbiting our
neighbour star a Centauri B, a member of the closest stellar system to the Sun. The planet has an orbital period of
3.236 days and is about 0.04 astronomical units from the star (one astronomical unit is the Earth–Sun distance).
Since the discovery of the first exoplanet orbiting a solar-type star in
19951, the number of known planets has not stopped growing: at
present, there are more than 750 confirmed planets2with minimum
mass estimates, and over 2,300 transiting planet candidates detected
with the Kepler satellite that are awaiting confirmation. Two main
detection techniques have led to this impressive number of discov-
eries: the radial-velocity technique, which measures the change in the
velocity of the central star due to the gravitational pull of an orbiting
are complementary; the former gives the minimum mass of a planet
(minimum because the orbital inclination of the planet is unknown),
whereas the latter gives a planet’s radius.
One of the major challenges in the search for exoplanets is the
detection of an Earth twin, that is, an Earth-mass planet orbiting in
interesting targets. At a distance of 1.3parsecs, it is a member of the
closest stellar system to the Sun, composed of itself, a Centauri A and
Proxima Centauri. It also exhibits low stellar activity, similar to the
solar activity level, usually associated with a small perturbing contri-
bution of intrinsic stellar activity to the measured radial velocities. a
Centauri Bis cooler thanthe Sun (effective temperature3–65,214633
K, spectral type K1V), and has a smaller mass that our parent star7
(0.93460.006 solar masses). These two conditions ease the detection
of a potentially habitable planet using radial velocities; the relative
coolness implies a habitable zone closer to the star, and the smaller
mass leads to a stronger radial-velocity variation for a similar-mass
planet. In addition, theoretical studies show that the formation of an
the star (visual magnitude 1.33) would allow for an efficient char-
acterization of the atmosphere of potentially orbiting planets.
An Earth twin induces a typical radial-velocity variation of a few
tenths of a metre per second on a star like a Centauri B. Such detec-
tions, technically possible with the most stable high-resolution spec-
trographs, are however challenging due to the presence of intrinsic
stellar signals inducing a radial-velocity ‘jitter’ at the level of a few
metres per second, even for quiet stars.
We report here the discovery of a planetary companion around a
K of 0.51ms21, a period P of 3.236d, and a semi-major axis a of
0.04astronomical units(AU). This planet, with aminimum mass sim-
ilar to that of Earth, is both the lightest orbiting a solar-type star and
the closest to the Solar System found to date. Being much closer to its
parent star than Earth is to the Sun, it is not an Earth twin. However,
the small amplitude of the signal shows that the radial-velocity tech-
nique is capable of reaching the precision needed to detect habitable
Earths around cooler stars (that is, M-dwarfs). In addition, statistical
studies of exoplanets suggest that low-mass planets are preferentially
formed in multi-planetary systems10–12. There is therefore a high
probability that other planets orbit a Centauri B, perhaps in its hab-
High-precision radial velocities
High-precision measurements were obtained for a Centauri B
between February 2008 andJuly 2011 using the HARPS spectrograph
(Supplementary Information section 1, and Supplementary Data).
HARPS is a high-resolution (R5110,000) cross-dispersed echelle
spectrograph installed on the 3.6-m telescope at La Silla Obser-
vatory (ESO, Chile). This instrument has demonstrated a long-term
precision of 0.8ms21, thereby becoming the most powerful machine
with which to hunt for exoplanets using the radial-velocity tech-
nique12,13,14. a Centauri B was observed with HARPS following an
intensive observational strategy, optimized to sample high- and med-
ium-frequency intrinsic stellar signals15, which makes it possible to
model and consequently remove their perturbing contributions. The
star was observed every possible night three times, with exposure
times of ten minutes, and with measurements optimally separated
by two hours14.
The raw radial velocities of a Centauri B (Fig. 1a) exhibit several
contributing signals that we could identify. Their origin is associated
with instrumental noise, stellar oscillation modes, granulation at the
surface of the star, rotational activity, long-term activity induced by a
magnetic cycle, the orbital motion of the binary composed of a
cise stellar coordinates.
In the following, we will consider each of these contributions sepa-
rately, modelling and removing them one by one, from the largest to
the smallest amplitude. The model parameters estimated for each
contribution will then be used as initial conditions for a global fit that
1Observatoirede Gene `ve,Universite ´ deGene `ve, 51chemindesMaillettes,CH-1290Sauverny,Switzerland.2CentrodeAstrofı `sicada UniversidadedoPorto,Rua dasEstrelas,P-4150-762Porto,Portugal.
3PhysikalischesInstitutUniversitatBern,Sidlerstrasse5,CH-3012Bern,Switzerland.4Institutd’AstrophysiquedeParis,UMR7095CNRS,Universite ´ Pierre&MarieCurie,98bisBoulevardArago,F-75014
Paris, France.5Departamento de Fı `sica e Astronomia, Faculdade de Cie ˆncias, Universidade do Porto, Rua do Campo Alegre s/n, P-4169-007 Porto, Portugal.
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will remove all the identified radial-velocity signals. In the residuals,
we will be able to search for small-amplitude planetary signals.
Perturbing signals for planet searches
a Centauri B is a quiet star among the targets monitored in searches
for low-mass planets. However, the very high precision of HARPS
allows us to discern in the measurements different perturbing signals
at the metre-per-second level. Compared to the radial-velocity signal
induced by terrestrial planets, a few to a few tens of centimetres per
elled and mitigated before searching for small-mass planets.
Instrumental noise. Guiding noise and other possible instrumental
noise are not considered in the data error bars. Their global effect is
most stable stars of the HARPS high-precision programme14.
lation modes16,17, with typical periods of less than five minutes. An
a few centimetres per second, the signal due to oscillation modes.
Granulation. a Centauri B is a solar-type star and has therefore an
outer convection zone responsible for a granulation pattern on its
surface. Depending on temperature, granulation cells have positive
or negative radial velocities, resulting in a non-zero global radial-
the disk of the star, weighted by the luminosity of the cells. The
granulation effect introduces radial-velocity variations on timescales
ranging from15mintoseveral hours18,19.For aCentauri B,models of
granulation20suggest an r.m.s. radial velocity of 0.6ms21.
Rotational activity signal. Owing to stellar rotation and the Doppler
average, while the other side has a negative one. However, if a spot
(darker or brighter than the mean stellar surface) is present on one
one side of the stellardisk to the other, introducing periodic signals at
the stellar rotational period and the corresponding harmonics21. The
lifetime of spots on the stellar surface is typically a few rotational
periods22, so after several rotations, the configuration of spots will
be different, thus changing the phase and amplitude of the signal.
The radial velocities of a Centauri B show a clear signal at 38.7d
(Figs 1d and 2b), which corresponds to the rotational period of the
star23. An efficient way to model rotational activity effects is to select
radial-velocity measurements over time intervals of a few rotational
periods, and fit sine waves at the rotational period and the corres-
ponding harmonics21(Supplementary Information section 2). The
best fit for the rotational activity signal for each observational season
can be seen in Fig. 3.
Long-term activity signal. During a solar-like magnetic cycle, the
from zero to several hundreds. Inside these spots, a strong magnetic
field is present, which freezes the convection24–28. For the Sun, as for
other stars similar to a Centauri B in spectral type29, convection
induces ablueshiftof thestellarspectra30–32.Therefore, noconvection
means no convective blueshift inside these regions, and so the spec-
trum of the integrated stellar surface will appear redshifted. Because a
redshift means a measured positive radial velocity, a positive correla-
tion between the magnetic cycle variation and the long-term radial-
velocity variation is then expected.
a Centauri B shows signs of weak but detectable chromospheric
activity, evidenced by the re-emission in the centre of the Ca II H and
K lines (the log(R9HK) activity index). a Centauri B exhibits a mag-
netic cycle with a minimum amplitude of AR9HK<0.11 dex (Fig. 2a).
To correct the radial-velocity effect due to the magnetic cycle (see
Fig. 1b and c), we assume a linear correlation between the log(R9HK)
activity index and the activity-related radial-velocity variation33(that
is, both variations have the same shape; Supplementary Information
Orbital motion. The orbital period of the binary composed of a
Centauri A and B is PAB579.91yr (ref. 7). The HARPS observations
of a Centauri B cover an interval of only four years. The orbit of the
system over such an interval can then adequately be approximated by
a second order polynomial (see Fig. 1a).
Light contamination. Owing to the close separation on the sky
between a Centauri A and B, the spectra of B can be contaminated
by light coming from A when the observing conditions are poor. The
resulting effect on the radial-velocity measurements was estimated
JD – 2400000 (d)
Figure 2 | Magnetic cycle ofa Centauri B.a, The grey curve representsa low
pass filter applied to the activity index measurements (data points). b, The
observations done in 2010 are zoomed in to show the variation induced by
rotational activity, which highlights the HARPS precision in determining
than 0.015 dex).
RV (km s–1)
JD – 2400000 (d)
RV – binary (m s–1)
Figure 1 | Radial velocities of a Centauri B and fitting the long-timescale
are visible. These signals, highlighted by grey arrows (‘long-term activity’) in
radial-velocity variation. When these low-frequency perturbations are
removed, signals induced by rotational activity, pointed out by grey arrows in
JD, Julian date. Error bars in c, 1s.
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and problematic observations discarded (Supplementary Infor-
mation section 4).
Imprecise stellar coordinates. When estimating stellar radial velo-
cities with regard to the barycentre of the Solar System, we need to
remove the component of the velocity of the Earth in the direction of
the star. Imprecise coordinates will then result in an imprecise cor-
rection and therefore in a residual signal in the radial velocities. This
when the times of arrival were varying periodically in time owing to
imprecise pulsar coordinates34. Owing to the circular orbit of the
Earth around the Sun, this signal will be a sinusoid with a one-year
period. a Centauri A and B are gravitationally bound, resulting in a
binary orbital motion, which has to be corrected to obtain precise
coordinates for a Centauri B (Supplementary Information section 5).
Removing the various signals
The approaches used to remove or mitigate the effects of the various
signals potentially masking the existence of a low-mass planet have
been described in the preceding paragraphs. For contamination com-
be easily modelled, the estimated radial-velocity contribution from
and the rotational activity effect, involves 23 free parameters
(Supplementary Information section 6).
A planet with a minimum mass that of the Earth
The generalized Lomb-Scargle periodogram35of the radial-velocity
residuals shows two peaks at 3.236 and 0.762d, with a false alarm
probability (FAP) lower than a conservative 1% limit (Fig. 4a). These
two periods are aliases of one another. A careful analysis of the struc-
ture infrequency of the periodogram suggests thatthe peak at 3.236d
is the true signal (Supplementary Information section 7).
timescales. One could thus worry whether the signal at 3.236d could
have been introduced during the process of eliminating the stellar
signals. We performed Monte Carlo simulations to determine if this
could be the case, and concluded that the signal is real and not an
artefact of the fitting process (Supplementary Information section 8).
The peak at 3.236d in the radial-velocity residuals is significant
with a FAP of 0.02%. Using a Markov chain Monte Carlo algorithm
coupled to a genetic algorithm to characterize the Keplerian solution,
we obtained a signal with a well-constrained period and amplitude.
The eccentricity is poorly constrained but is compatible with zero
within a 1.4s uncertainty (Supplementary Information section 9).
To fit this planetary signal simultaneously with the other contribu-
the circular planet orbit to the global fit (Supplementary Information
section 6). The observed dispersion of the residuals around the final
solution is 1.20ms21and the reduced x2value is 1.51 (with 26 para-
meters for 459 radial-velocity points). The semi-amplitude of the
planetary signal is K50.5160.04ms21, which corresponds to a
a stellar mass of 0.934 solar masses and with an orbital period of
P53.235760.0008d. The orbital and planet parameters are given
in Table 1. In the residuals of the global fit, a signal with an FAP of
0.3% is present; however, it could have multiple origins (Supplemen-
tary Information section 6).
In Fig. 5, we show the radial-velocity measurements corrected for
stellar and binary effects, folded in phase with the 3.236-d period,
superimposed on the derived solution for the planetary signal. In
JD – 2400000 (d)
RV (m s–1)
Figure 3 | Fit of the rotational activity. a–d, Radial velocities (RV) of a
Centauri A), magnetic-cycle and coordinates effects, for the years 2008
The grey curve represents for each plot the fit of the rotational activity signal,
adjusting sinusoids at the stellar rotational period and the corresponding
harmonics. The rotational period estimated from the stellar activity model
decreases from the second season of observation to the last, with estimated
periods (in days) of 39.76 (b), 37.80 (c) and 36.71 (d) (Supplementary
Information section 6). This can be explained if the star exhibits differential
130u or 230u at the start of a magnetic cycle (like in 2008) and then migrate
towards the equator during the cycle. Owing to differential rotation, observed
for the Sun, the rotational period estimated by activity modelling should
decrease from the start to the end of a magnetic cycle. A similar effect is seen
detected here for this slow rotator41(X.D. et al., manuscript in preparation).
Figure 4 | Periodograms of the radial-velocity residuals after removing the
stellar, imprecise coordinates and binary effects, with continuous, dashed and
dotted lines indicating the 0.1%, 1% and 10% FAP, respectively. The highest
the periodogram around the planet signal is represented. The periodogram for
all seasons is shown in black, and the yearly periodograms for each
observational period (2008, 2009, 2010 and 2011) are shown in different
colours. The amplitudes of the yearly periodograms are normalized so that the
gives the phase between 0u and 360u. For each year of observation, the peak at
3.236d conserves the same phase, which is expected for a planetary signal. On
thecontrary,thepeak at2.8d anditsaliasat 3.35d donotkeepthesame phase
and are therefore associated with noise (these peaks appear only in 2009 and
their FAPs are higher than 10%).
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Fig. 4b we show that the 3.236-d signal conserves its phase for each
observational year, which is expected for a planetary signal.
An important piece of information about the inner composition of
an exoplanet is obtained when the planet is transiting its parent star,
allowing its radius to be measured. Combined with the real mass
estimate, the radius leads to the average density of the planet. In the
probability is estimated at 10%, with a transit depth of 1024. The
detection of a planet transit, only possible from space, would allow
a Centauri B.
The r.m.s. radial velocity induced by the stellar rotational activity
amounts to 1.5ms21on average. The detection of the tiny planetary
signal, with a semi-amplitude K50.51ms21,thus demonstrates that
of small-mass planets. Using an optimized observational strategy and
the present knowledge about activity-induced radial-velocity effects,
it is possible to model precisely and mitigate activity signals, and
therefore improve considerably the planet detection limits.
With a separation from its parent star of only 0.04 AU, the planet is
orbiting very close to a Centauri B compared to the location of the
habitable zone. However, the observed radial-velocity semi-ampli-
tude is equivalent to that induced by a planet of minimum mass four
The HARPS spectrograph therefore has the precision required to
detect a new category of planets, namely habitable super-Earths.
This sensitivity was expected from simulations of intrinsic stellar
signals15, and actual observations of planetary systems14.
is capable of reaching the precision needed to search for habitable
super-Earths around solar-type stars using the radial-velocity tech-
nique. However, it requires an important investment in observation
time, and thus only few targets can be observed over several years.
Recent statistical analyses and theoretical models of planetary forma-
tion suggest that low-mass rocky planets and especially Earth twins
should be common12,38–40. We are therefore confident that we are on
the right path to the discovery of Earth analogues.
Received 3 August; accepted 13 September 2012.
Published online 17 October 2012.
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Table 1 | Orbital parameters of the planet orbiting a Centauri B
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Velocity semi-amplitude (ms21)
Minimum mass (Earth masses)
Number of data points
O2C residuals (ms21)
BJD, barycentric Julian date; O2C, observed minus calculated.
RV (m s–1)
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Supplementary Information is available in the online version of the paper.
Acknowledgements The data presented here were obtained with the ESO 3.6-m
telescope at La Silla Paranal Observatory, Chile. We thank the Swiss National Science
N.S. and X.D. acknowledge support by the European Research Council/European
Community underFP7throughStartingGrantagreement number239953, aswell as
fromFundacaoparaaCı ˆenciaeaTecnologia(FCT)throughprogrammeCı ˆencia2007
CTE-AST/098528/2008 and PTDC/CTE-AST/098604/2008.
the ESO programme ‘Searching for Earth-analogs around nearby stars with HARPS’.
results and contributed to the manuscript.
Author Information Reprints and permissions information is available at
www.nature.com/reprints. The authors declare no competing financial interests.
and requests for materials should be addressed to X.D. (email@example.com)
and F.P. (firstname.lastname@example.org).
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