The Earth, as well as all smaller bodies within the Solar System, consists almost entirely of compounds that are condensable under reasonable conditions. In contrast, more massive planets contain a considerable fraction of light gases. About 90% of Jupiter's mass is H and He, and these two light elements make up ^75% of Saturn. The large amounts of H and He contained in Jupiter and Saturn imply that these planets must have formed within ~107 years of the collapse of the Solar System's natal cloud, before the gas in the protoplanetary disk was swept away. The two largest planets in our Solar System are generally referred to as gas giants, even though these elements aren't gases at the high pressures that most of the material in Jupiter and Saturn is subjected to. Analogously, Uranus and Neptune are frequently referred to as ice giants, even though the astrophysical ices (such as H2O, CH4, H2S, and NH3) that models suggest make up the majority of their mass (Hubbard, Podolak, & Stevenson 1995) are in fluid, rather than solid, form. Note that whereas H and He must make up the bulk of Jupiter and Saturn because no other elements can have such low densities at plausible temperatures, it is possible that Uranus and Neptune are primarily composed of a mixture of 'rock' and H/He.
Lithium and heavier elements constitute <2% of the mass of a solar composition mixture. The atmospheric abundances of volatile gases heavier than helium (excluding neon, which was predicted prior to Galileo Probe measurements to be substantially depleted through gravitationally induced settling; Roulston & Stevenson 1995) are ~ 3x solar in Jupiter (Young 2003), a bit more enriched in Saturn, and substantially more for Uranus and Neptune. The bulk enhancements in heavy elements relative to the solar value are roughly 5, 15, and 300 times for Jupiter, Saturn, and Uranus/Neptune, respectively. Thus, all four giant planets accreted solid material substantially more effectively than gas from the surrounding nebula. Moreover, the total mass in heavy elements varies by only a factor of a few among the four planets, while the mass of H and He varies by about two orders of magnitude between Jupiter and Uranus/Neptune.
The extrasolar planet discoveries of the past decade have vastly expanded our database by increasing the number of planets known by more than an order of magnitude. The distribution of known extrasolar planets is highly biased towards those planets that are most easily detectable using the Doppler radial velocity technique, which has been by far the most effective method of discovering exoplanets. These extrasolar planetary systems are quite different from our Solar System; however, it is not yet known whether our planetary system is the norm, quite atypical, or somewhere in between.
Nonetheless, some unbiased statistical information can be distilled from available exo-planet data (Marcy et al. 2004, 2005): Roughly 1% of Sun-like stars (chromospherically quiet, late F, G and early K spectral class main sequence stars) have planets more massive than Saturn within 0.1 AU. Approximately 7% of Sun-like stars have planets more massive than Jupiter within 3 AU. Planets orbiting interior to ~0.1 AU, a region where tidal circularization timescales are less than stellar ages, have small orbital eccentricities. The median eccentricity observed for planets on more distant orbits is 0.25, and some of these planets travel on very eccentric orbits. Within 5 AU of Sun-like stars, Jupitermass planets are more common than planets of several Jupiter masses, and substellar companions that are more than ten times as massive as Jupiter are rare. Stars with higher metallicity are much more likely to host detectable planets than are metal-poor stars (Gonzalez 2003; Santos et al. 2003; Fischer & Valenti 2005), with the probability of hosting an observable planet varying as the square of stellar metallicity (Marcy et al. 2005). Low-mass main-sequence stars (M dwarfs) are significantly less likely to host one or more giant planets with orbital period(s) of less than a decade than are Sun-like stars. Multiple-planet systems are more common than if detectable planets were randomly assigned to stars. Two of the three extrasolar giant planets with well-measured masses and radii, HD 209458b and TrES-1, are predominantly hydrogen (Charbonneau et al. 2000; Burrows, Sudarsky, & Hubbard 2003; Alonso et al. 2004), as are Jupiter and Saturn. However, the third such planet to be observed, HD 149026b, which is slightly more massive than Saturn, appears to have comparable amounts of hydrogen and helium and heavy elements (Sato et al. 2005), making it intermediate between Saturn and Uranus in terms of bulk composition, but more richly endowed in terms of total amount of 'metals' than is any planet in our Solar System.
Transit observations have also yielded an important negative result: Hubble Space Telescope photometry of a large number of stars in the globular cluster 47 Tucanae failed to detect any transiting inner giant planets, even though ~17 such transiting objects would be expected if the frequency of such planets in this low metallicity cluster were the same as that for Sun-like stars in the solar neighborhood (Gilliland et al. 2000). In contrast, it appears likely that a ^3 MJ planet is orbiting ^20 AU from the pulsar PSR B1620-26 white dwarf binary system, which is located in the globular cluster Messier 4. This system has been taken to be evidence for ancient planet formation in a low metallicity (5% solar) protoplanetary disk by Sigurdsson (1993) and Sigurdsson et al. (2003). Sig-urdsson's formation scenario requires a fairly complex stellar exchange to account for the planet in its current orbit. There is a much more likely explanation for the planet orbiting PSR B1620-26, which requires neither planetary formation in a low metallicity disk, nor stellar exchange. This system has two post-main sequence stars sufficiently close to have undergone disk-producing mass transfer during the white dwarf's distended red giant phase, which occurred within the past 109 years (Sigurdsson et al. 2003). Such a metals-enriched disk could have been an excellent location for the giant planet to form, and growth within such a disk would fit well with both planet formation theories and the observed strong correlation of planetary detections with stellar (and presumably proto-stellar disk) metallicity. Sigurdsson (1993) noted the possibility that the planet formed in a post-main sequence disk, but he discounted this scenario because he relied on the planetary growth timescales given by Nakano (1987), whose model requires an implausibly long 4 x 109 years to form the most distant known major planet within our Solar System, Neptune.
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