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CALTECH GE 133 - FORMATION OF THE OUTER PLANETS

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FORMATION OF THE OUTER PLANETSJACK J. LISSAUERNASA Ames Research Center, Space Science Division, MS 245-3, Moffett Field, CA 94035, USAReceived: 15 April 2004; Accepted in final form: 19 August 2004Abstract. Models of the origins of gas giant planets and ‘ice’ giant planets are discussed and relatedto formation theories of both smaller objects (terrestrial planets) and larger bodies (stars). The mostdetailed models of planetary formation are based upon observations of our own Solar System, ofyoung stars and their environments, and of extrasolar planets. Stars form from the collapse, andsometimes fragmentation, of molecular cloud cores. Terrestrial planets are formed within disksaround young stars via the accumulation of small dust grains into larger and larger bodies untilthe planetary orbits become well enough separated that the configuration is stable for the lifetimeof the system. Uranus and Neptune almost certainly formed via a bottom-up (terrestrial planet-like)mechanism; such a mechanism is also the most likely origin scenario for Saturn and Jupiter.Keywords: planet formation, giant planets, solar nebula1. IntroductionThere is convincing observational evidence that stars form by gravitationally-in-duced compression of relatively dense regions within molecular clouds (Lada etal., 1993; André et al., 2000). The nearly planar and almost circular orbits of theplanets in our Solar System argue strongly for planetary formation within flattenedcircumstellar disks. Observations by Goodman et al. (1993) indicate that typi-cal star-forming dense cores inside dark molecular clouds have specific angularmomentum > 1021cm2s−1. When these clouds undergo gravitational collapse,this angular momentum leads to the formation of pressure-supported protostarssurrounded by rotationally-supported disks. Such disks are analogous to the pri-mordial solar nebula that was initially conceived by Kant and Laplace to explainthe observed properties of our Solar System (e.g., Cassen et al., 1985). Obser-vational evidence for the presence of disks of Solar System dimensions aroundpre-main sequence stars has increased substantially in recent years (McCaughreanet al., 2000). The existence of disks on scales of a few tens of astronomical unitsis inferred from the power-law spectral energy distribution in the infrared overmore than two orders of magnitude in wavelength (Chiang and Goldreich, 2000).Observations of infrared excesses in the spectra of young stars suggest that thelifetimes of protoplanetary disks span the range of 106– 107years (Strom et al.,1993; Alencar and Batalha, 2002).Dust within a protoplanetary disk initially agglomerates via sticking/local elec-tromagnetic forces. The later phases of solid body growth are dominated by pair-CSpringer 2005Space Science Reviews 116: 11–24, 2005.DOI: 10.1007/s11214-005-1945-312 J. J. LISSAUERwise collisions of bodies that also influence one another’s trajectories gravitation-ally. Terrestrial planets continue to grow by pairwise accretion of solid bodiesuntil the spacing of planetary orbits becomes large enough that the configuration isstable to gravitational interactions among the planets for the lifetime of the system(Safronov, 1969; Wetherill, 1990; Lissauer, 1993; 1995; Chambers, 2001; Laskar,2000). The largest uncertainty in our understanding of solid planet formation is theagglomeration from cm-sized pebbles to km-sized bodies that are referred to asplanetesimals. Collective gravitational instabilities (Safronov, 1969; Goldreich andWard, 1973) might be important, although turbulence could prevent protoplanetarydust layers from becoming thin enough to be gravitationally unstable (Weiden-schilling and Cuzzi, 1993). Recent calculations suggest that high metallicity disksmay form planetesimals via gravitational instabilities, but that dust in disks withlower solids contents may not be able to overcome turbulence and settle into asubdisk that is dense enough to undergo gravitational instability (Youdin and Shu,2002). Planetesimal formation is a very active research area (Goodman and Pindor,2000; Ward, 2000), and results may have implications for our estimates of theabundance of both terrestrial and giant planets within our galaxy.Our understanding, such as it is, of planet formation comes from a widely di-verse range of observations, laboratory studies and theoretical models. Detailedobservations obtained from the ground and from space are now available for theplanets and many smaller bodies (moons, asteroids and comets) within our SolarSystem. Studies of the composition, minerals and physical structure have beenused to deduce conditions within the protoplanetary disk (Hewins, Jones and Scott,1996). Data on the now more numerous known extrasolar planets are less detailedand more biased, yet still very important. Observations of young stars and theirsurrounding disks provide clues to planet formation now taking place within ourgalaxy. Laboratory experiments on the behavior of hydrogen and helium at highpressures have been combined with gravitational measures of the mass distributionwithin giant planets deduced from the trajectories of passing spacecraft and moonsto constrain the internal structure and composition of the largest planets in ourSolar System.Theorists have attempted to assemble all of these pieces of information togetherinto a coherent model of planetary growth. But note that planets and planetarysystems are an extremely heterogeneous lot, the ‘initial conditions’ for star andplanet formation vary greatly within our galaxy (Mac Low and Klessen, 2004),and at least some aspects of the process of planet formation are extremely sensitiveto small changes in initial conditions (Chambers et al., 2002).The remainder of this chapter concentrates on the formation of bodies muchlarger than Earth yet substantially smaller than the Sun. Observations of giantplanets in our Solar System and beyond are summarized in Section 2. Formationmodels are reviewed in Section 3, and conclusions are given in Section 4.FORMATION OF THE OUTER PLANETS 132. ObservationsAbout 90% of Jupiter’s mass is H and He, and these two light elements make up∼75% of Saturn. The two largest planets in our Solar System are generally referredto as gas giants even though these elements aren’t gases at the high pressures thatmost of the material in Jupiter and Saturn is subjected to. Analogously, Uranus andNeptune are frequently referred to as ice giants even though


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CALTECH GE 133 - FORMATION OF THE OUTER PLANETS

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