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Boss A.P.,Carnegie Institution of Washington
Astrophysical Journal Letters | Year: 2010

Forming giant planets by disk instability requires a gaseous disk that is massive enough to become gravitationally unstable and able to cool fast enough for self-gravitating clumps to form and survive. Models with simplified disk cooling have shown the critical importance of the ratio of the cooling to the orbital timescales. Uncertainties about the proper value of this ratio can be sidestepped by including radiative transfer. Three-dimensional radiative hydrodynamicsmodels of a disk with amass of 0.043M⊙ from 4 to 20AUin orbit around a 1M⊙ protostar showthat disk instabilities are considerably less successful in producing self-gravitating clumps than in a disk with twice this mass. The results are sensitive to the assumed initial outer disk (To) temperatures.Models with To =20 K are able to form a single self-gravitating clump, whereas models with To =25 K form clumps that are not quite self-gravitating. These models imply that disk instability requires a disk with a mass of at least ∼0.043M⊙ inside 20 AU in order to form giant planets around solar-mass protostars with realistic disk cooling rates and outer-disk temperatures. Lower mass disks around solar-mass protostars must rely upon core accretion to form inner giant planets. © 2010 The American Astronomical Society. All rights reserved.

Boss A.P.,Carnegie Institution of Washington
Annual Review of Earth and Planetary Sciences | Year: 2012

Isotopic abundances of short-lived radioisotopes such as 26Al appear to provide precise chronometers of events in the early Solar System, assuming that they were initially homogeneously distributed. However, both 60Fe and 26Al were likely formed in a supernova and then injected into the solar nebula in a highly heterogeneous manner. Conversely, the abundances in primitive meteorites of the three stable oxygen isotopes exhibit mass-independent fractionations that somehow survived homogenization in the solar nebula. Both the presence of refractory particles in Comet 81PWild 2 and the anomalously high crystallinity observed in protoplanetary disks may require large-scale outward radial transport from the hotter inner disk regions, even as disk gas accretes onto the central protostar. We examine here theoretical efforts to solve these seemingly disparate cosmochemical puzzles and conclude that the mixing and transport produced by a phase of marginal gravitational instability appears to meet all of these constraints. © 2012 by Annual Reviews. All rights reserved.

Elkins-Tanton L.T.,Carnegie Institution of Washington
Annual Review of Earth and Planetary Sciences | Year: 2012

Theory and observations point to the occurrence of magma ponds or oceans in the early evolution of terrestrial planets and in many early-accreting planetesimals. The apparent ubiquity of melting during giant accretionary impacts suggests that silicate and metallic material may be processed through multiple magma oceans before reaching solidity in a planet. The processes of magma ocean formation and solidification, therefore, strongly influence the earliest compositional differentiation and volatile content of the terrestrial planets, and they form the starting point for cooling to clement, habitable conditions and for the onset of thermally driven mantle convection and plate tectonics. This review focuses on evidence for magma oceans on planetesimals and planets and on research concerning the processes of compositional differentiation in the silicate magma ocean, distribution and degassing of volatiles, and cooling. © 2012 by Annual Reviews. All rights reserved.

Sheppard S.S.,Carnegie Institution of Washington
Astronomical Journal | Year: 2010

Extreme outer solar system objects have possible origins beyond the Kuiper Belt edge, high inclinations, very large semimajor axes, or large perihelion distances. Thirty-three such objects were observed in this work to determine their optical colors. All three objects that have been dynamically linked to the inner Oort Cloud by various authors ((90377) Sedna, 2006 SQ372, and (87269) 2000 OO67) were found to have ultra-red surface material (spectral gradient, S 25). Ultra-red material is generally associated with rich organics and the low inclination "cold" classical Kuiper Belt objects (KBOs). The observations detailed here show that very red material may be a more general feature for objects kept far from the Sun. The recently discovered retrograde outer solar system objects (2008 KV42 and 2008 YB 3) and the high inclination object (127546) 2002 XU93 show only moderately red surfaces (S 9) very similar to known comets, suspected dead comets, Jupiter and Neptune Trojans, irregular satellites, D-type asteroids, and damocloids. The extended or detached disk objects, which have large perihelion distances and are thus considered to be detached from the influence of the giant planets but yet have large eccentricities, are found to have mostly moderately red colors (10 ≲ S ≲ 18). The colors of the detached disk objects, including the dynamically unusual 2004 XR190 and (148209) 2000 CR105, are similar to the scattered disk and Plutino populations. Thus the detached disk, scattered disk, Plutino, and high inclination "hot" classical objects likely have a similar mix of objects from the same source regions. Outer classical KBOs, including (48639) 1995 TL8, were found to have very red surfaces (18 ≲ S ≲ 30). The low inclination "cold" classical KBOs, outer classical KBOs and possibly the inner Oort Cloud appear to be dominated by ultra-red objects (S ≳ 25) and thus do not likely have a similar mix of objects as the other outer solar system reservoirs such as the scattered disk, detached disk, and Trojan populations. A possible trend was found for the detached disk and outer classical Kuiper Belt in that objects with smaller eccentricities have redder surfaces irrespective of inclinations or perihelion distances. There is also a clear trend that objects more distant appear redder. © 2010 The American Astronomical Society.

Boss A.P.,Carnegie Institution of Washington
Astrophysical Journal | Year: 2011

Doppler surveys have shown that more massive stars have significantly higher frequencies of giant planets inside ∼3AU than lower mass stars, consistent with giant planet formation by core accretion. Direct imaging searches have begun to discover significant numbers of giant planet candidates around stars with masses of ∼1 M ⊙ to ∼2 M ⊙ at orbital distances of ∼20AU to ∼120AU. Given the inability of core accretion to form giant planets at such large distances, gravitational instabilities of the gas disk leading to clump formation have been suggested as the more likely formation mechanism. Here, we present five new models of the evolution of disks with inner radii of 20AU and outer radii of 60AU, for central protostars with masses of 0.1, 0.5, 1.0, 1.5, and 2.0 M ⊙, in order to assess the likelihood of planet formation on wide orbits around stars with varied masses. The disk masses range from 0.028 M ⊙ to 0.21 M ⊙, with initial Toomre Q stability values ranging from 1.1 in the inner disks to ∼1.6 in the outer disks. These five models show that disk instability is capable of forming clumps on timescales of ∼103yr that, if they survive for longer times, could form giant planets initially on orbits with semimajor axes of 30AU to ∼70AU and eccentricities of ∼0 to ∼0.35, with initial masses of ∼1 M Jup to ∼5 M Jup, around solar-type stars, with more protoplanets forming as the mass of the protostar (and protoplanetary disk) is increased. In particular, disk instability appears to be a likely formation mechanism for the HR 8799 gas giant planetary system. © 2011. The American Astronomical Society. All rights reserved.

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