The mass of dwarf planet Eris.
ABSTRACT The discovery of dwarf planet Eris was followed shortly by the discovery of its satellite, Dysnomia, but the satellite orbit, and thus the system mass, was not known. New observations with the Keck Observatory and the Hubble Space Telescopes show that Dysnomia has a circular orbit with a radius of 37,350 +/- 140 (1-sigma) kilometers and a 15.774 +/- 0.002 day orbital period around Eris. These orbital parameters agree with expectations for a satellite formed out of the orbiting debris left from a giant impact. The mass of Eris from these orbital parameters is 1.67 x 10(22) +/- 0.02 x 10(22) kilograms, or 1.27 +/- 0.02 that of Pluto.
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ABSTRACT: Comet C/1853 E1 (Secchi) has a hyperbolic orbit with eccentricity 1.01060 and perihelion outside of the Earth's orbit. Integrating the orbit with barycentric coordinates backwards to 50000 AU, the approximate edge of the Oort cloud, shows that the orbit remains hyperbolic. This is still true even if plutoids additional to Pluto are included in the integration. Nor does including Galactic tidal and disc effects and possible nongravitational forces change the orbit to a high eccentricity ellipse. Although certain factors, such as unknown massive plutoids, gravitational effects by interstellar gas clouds, or unmodelled nongravitational forces operating on the comet, could change this situation, the tentative conclusion that the origin of this comet is extrasolar remains the one most consistent with the observations (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)Astronomische Nachrichten 02/2012; 333(2). · 1.12 Impact Factor
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ABSTRACT: Aims. The goal of this work is to characterize the ensemble thermal properties of the Centaurs / trans-Neptunian population. Methods. Thermal flux measurements obtained with Herschel/PACS and Spitzer/MIPS provide size, albedo, and beaming factors for 85 objects (13 of which are presented here for the first time) by means of standard radiometric techniques. The measured beaming factors are influenced by the combination of surface roughness and thermal inertia effects. They are interpreted within a thermophysical model to constrain, in a statistical sense, the thermal inertia in the population and to study its dependence on several parameters. We use in particular a Monte-Carlo modeling approach to the data whereby synthetic datasets of beaming factors are created using random distributions of spin orientation and surface roughness. Results. Beaming factors η range from values <1 to ~2.5, but high η values (>2) are lacking at low heliocentric distances (rh < 30 AU). Beaming factors lower than 1 occur frequently (39% of the objects), indicating that surface roughness effects are important. We determine a mean thermal inertia for Centaurs/ TNO of Γ = (2.5 ± 0.5) J m-2 s−1/2 K-1, with evidence of a trend toward decreasing Γ with increasing heliocentric (by a factor ~2.5 from 8–25 AU to 41–53 AU). These thermal inertias are 2–3 orders of magnitude lower than expected for compact ices, and generally lower than on Saturn’s satellites or in the Pluto/Charon system. Most high-albedo objects are found to have unusually low thermal inertias. Our results suggest highly porous surfaces, in which the heat transfer is affected by radiative conductivity within pores and increases with depth in the subsurface.Astronomy and Astrophysics 01/2013; 557(2013-A60):1-19. · 4.48 Impact Factor
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ABSTRACT: The first refereed science papers based on data with the Keck II natural guide star (NGS) and laser guide star (LGS) adaptive optics (AO) system were published in 2000 January and 2005 May, respectively. As of the end of 2012 a total of 260 refereed science papers have been published based on Keck NGS AO data and 152 with the Keck LGS AO system. This paper provides an overview of the first dozen years of scientific productivity with Keck AO and the lessons that can be drawn from this experience. The performance and limitations of the existing Keck AO systems are also discussed along with the technical developments currently underway to improve the scientific reach of these systems. The Keck AO capabilities are in high demand by a broad science community, a community that has expanded as new capabilities, especially LGS AO, have been added.Publications of the Astronomical Society of the Pacific 07/2013; 125(929):798-808. · 3.23 Impact Factor
The Mass of Dwarf Planet Eris
Michael E. Brown* and Emily L. Schaller
category called “dwarf planets”: objects in orbit
around the Sun that are large enough to be in
hydrostatic equilibrium but have insufficient
mass to gravitationally dominate their region of
the solar system. Eris is larger than Pluto (2, 3)
and thus the largest currently known member of
The subsequent discovery of Dysnomia (4),
a satellite of Eris, presented the opportunity to
directly measure the mass of Eris by determining
the Keplerian orbit of the satellite. Observations
of Eris and Dysnomia were obtained on 20, 21,
30, and 31 August 2006 (UT) with the Keck
Observatory laser guide star adaptive optics
(LGS AO) system (5, 6). Observations from the
Hubble Space Telescope (HST) were taken on 3
surements of the relative positions of Dysnomia
on these six nights (Fig. 1 and table S1) plus the
position from the discovery on 10 September
2005, we determined the orbit of Dysnomia by
using a Powell c2minimization scheme to find
to fit a purely circular orbit in which the five free
parameters are semimajor axis, orbital period,
inclination, longitude of the ascending node, and
6.5 or a reduced c for nine degrees of freedom
(14 x, y coordinates minus 5 orbital parameters)
of 0.7, indicating an excellent fit to the model.
Expanding the model to allow an eccentric orbit
gives a best-fit eccentricity of ~0.007 and only a
marginally lower reduced c2of 0.6, suggesting
evidence for a noncircular orbit. Derived orbital
elements along with uncertainties from Monte
Carlo analysis appear in table S2.
From the 30 August 2006 HST image at a
wavelength of 0.6 mm, we measured a relative
brightness ratio between the two objects of only
of Dysnomia is inconsistent with the dynamical-
majority of KBO satellites (7), but detailed
simulations show that such small satellites can be
formed from the debris after a giant impact (8). A
collisionally produced satellite of the size of
Dysnomia that tidally evolved outward from an
initial location near the Roche limit would be
predicted to have a roughly 15-day circular orbit
[Supporting Online Material (SOM) text], con-
to the low mass of Dysnomia,this outward orbital
he discovery of Kuiper belt object (KBO)
2003 UB313 (1), now officially named
Eris, prompted the recent reevaluation of
expansion would have slowed the spin period of
Eris by only a part in ~10−5.
Whereas the other two KBO systems that
appear to be products of giant impacts, Pluto and
2003 EL61, contain multiple satellites, satellites
almost an order of magnitude fainter than
Dysnomia can be ruled out beyond the orbit of
Dysnomia from deep HST observations (SOM
text). For a purely tidally evolved system, any
satellite beyond the orbit of Dysnomia must be
larger than Dysnomia, and thus such a system
can be ruled out. Interior to ~0.4 arc sec, how-
ever, the expected fractional brightness of a
tidally evolved satellite is ~0.0007, which is
beyond our detection ability (SOM text). Any
additional purely tidally evolved satellites of Eris
would be expected to be closer and fainter than
these limits. Although such additional small faint
satellites cannot be ruled out, the current limits
and the apparently circular orbit of Dysnomia
suggest that Eris might indeed be a single-
From the period and semimajor axis of the
orbit of Dysnomia, we can use Kepler’s laws to
calculate a mass for the Eris-Dysnomia system
of 1.66 ×1022± 0.02 × 1022kg or 1.27 ±0.02 of
the mass of Pluto. With any plausible assump-
tions of albedo and density, Dysnomia’s mass in
the system is negligible. In addition to being the
largest, Eris is also the most massive known
From this mass measurement and the previ-
ous size measurements, we can calculate the
density of Eris. The initial indirect IRAM radio-
metric measurement suggested a diameter of
3000 ± 400 km (2), whereas the later HST
direct measurement found a smaller diameter of
2400 ± 100 km (3). By using the more direct
measurement with the smaller uncertainty, we
is consistent with the moderately high 2.03 ±
0.06, 2.06 ± 0.01, and ~ 2.6 g cm−3densities
EL61, respectively (9–11). Using the earlier in-
direct IRAM diameter measurement would give
a density of only 1.2 ± 0.6 g cm−3, which is
significantly lower than other objects of compa-
rable size in the outer solar system, giving con-
fidence, although not confirmation, in the more
direct HST diameter measurement with the
Recent direct and indirect measurements of
lower-than-expected densities for objects in the
outer solar system and thus a deficit of rocky
high densities of Eris, Pluto, Triton, and 2003
EL61, in contrast, all require rock fractions of
~70% or higher (14), as anticipated from ex-
pected cosmochemical abundances in the proto-
References and Notes
1. M. E. Brown, C. A. Trujillo, D. L. Rabinowitz, Astrophys. J.
635, L97 (2005).
2. F. Bertoldi, W. Altenhoff, A. Weiss, K. M. Menten,
C. Thum, Nature 439, 563 (2006).
3. M. E. Brown, E. L. Schaller, H. G. Roe, D. L. Rabinowitz,
C. A. Trujillo, Astrophys. J. 643, L61 (2006).
4. M. E. Brown et al., Astrophys. J. 639, L43 (2006).
5. P. L. Wizinowich et al., Pub. Astron. Soc. Pacific 118, 297
6. Materials and methods are available on Science Online.
7. P. Goldreich, Y. Lithwick, R. Sari, Nature 420, 643 (2002).
8. R. M. Canup, Science 307, 546 (2005).
9. M. W. Buie, W. M. Grundy, E. F. Young, L. A. Young, S. A.
Stern, Astron. J. 132, 290 (2006).
10. B. A. Smith et al., Science 246, 1422 (1989).
11. D. L. Rabinowitz et al., Astrophys. J. 639, 1238 (2006).
12. D. C. Jewitt, S. S. Sheppard, Astron. J. 123, 2110 (2002).
13. J. A. Stansberry et al., Astrophys. J. 643, 556 (2006).
14. W. B. McKinnon, J. I. Lunine, D. Banfield, in Neptune and
Triton, D. P. Cruikshank, Ed. (Univ. Arizona Press, Tucson,
1995), pp. 807–878.
15. This research is supported by a Presidential Early Career
Award to M.E.B. In addition, E.L.S. is supported by a
NASA graduate student research fellowship. We thank
J. Aycock, R. Campbell, A. Conrad, K. Grace, J. Lyke,
C. Melcher, C. Sorenson, M. van Dam, and C. Wilburn at
Keck Observatory, without whom these complicated LGS
AO observations would not have been possible.
Supporting Online Material
Materials and Methods
Tables S1 and S1
29 December 2006; accepted 14 March 2007
Division of Geological and Planetary Sciences, California
Institute of Technology, Pasadena, CA 91125, USA.
*To whom correspondence should be addressed. E-mail:
Fig. 1. The projected orbit of Dysnomia around
Eris. Observations are show as crosses of the size
of the 1-s uncertainty. The predicted positions
at the time of observations are shown by open
circles. The solid circle in the center is 10 times
the actual angular size of Eris.
VOL 31615 JUNE 2007