In this section, I discuss how the presence of Jupiter and Saturn, but especially the former, has profoundly shaped the volatile inventory of Earth. The conclusion of this section is that the spatial location and mass distribution of giant planets will predetermine the existence and habitability of terrestrial planets. I define terrestrial planets as those bodies made primarily of rock (including metals, such as iron-nickel), with solid surfaces capable of holding volatiles in liquid and gaseous (as well as solid) form. The possible variations on the theme of terrestrial planets have been thoroughly discussed elsewhere (38), as has the complexity of physical processes leading to continuous habitability of Earth through time (39).
Although dynamical simulations of terrestrial planet formation have appeared previously in the literature (40), little effort has been made to look at the stability of terrestrial planets in systems with giant planet configurations other than our own and specifically at orbital configurations like those in observed systems. Very recently, J. Chambers§ simulated the orbital evolution of planetesimals and planetary embryos (larger aggregations approaching the size of the terrestrial planets) for a broad spectrum of giant planet orbits. He concludes that terrestrial planets could form and exist stably at 1 AU in the presence of Jovian-mass planets in circular orbits as tight as 3 AU or with masses several times that of Jupiter (but residing again at 5 AU).
Giant planets on eccentric orbits, on the other hand, make terrestrial planet formation more difficult by increasing the eccentricities of the orbits of the planetesimals themselves. However, Chambers points to systems with detected giant planets on moderate-period eccentric orbits, 14 Herculis and Epsilon Eridani, as being potentially habitable. Both the stars in this case are K-2 dwarfs with luminosities nearly three times less than the Sun's. For those systems, the habitable zone (where liquid water is stable) resides closer to the star than for our Sun, and it turns out that the inner edge of these compact habitable zones is dynamically stable against perturbations by the giant planets in both systems. The possible existence of these stable habitable zones illustrates the complexity involved in drawing conclusions about the potential for habitability of terrestrial planets in systems with diverse architectures.
It is possible that the first Earth-sized planet we detect and study around another star, perhaps with a facility like Darwin or Terrestrial Planet Finder (41), will have no atmosphere or no atmospheric water vapor (and, by inference, no surface water) at all. There are strong (but not fully conclusive) arguments that Earth's oceanic and crustal water budget was not derived locally, i.e., from planetesimals formed at 1 AU. The principal such argument is compositional: the water content of asteroids in the main belt appears to decline with decreasing semimajor axis, from the carbonaceous chondrites (10% by mass) to the enstatite chondrites (as low as 0.05% in mass). Because Earth formed inward of the asteroid belt, and temperatures in planet-forming disks rather generally must increase with decreasing semimajor axis, the planetesimals at 1 AU could have been dry. Other arguments pro and con have been offered (42), but there is some consensus that our volatile budget reflects significant contributions from distal orbits.
The strongest constraint on the source material of Earth's crustal water comes from the oceanic deuterium-to-hydrogen ratio (D/H). Here one must distinguish between crustal water and deep mantle water, because there is disagreement whether the deep reservoir has a different D/H value or even exists (42). The oceanic [Standard Mean Ocean Water (SMOW)] D/H ratio, 150 parts per million, is 5 to 6 times the solar system's primordial value measured in Jupiter (43). The SMOW value is also a factor of 2 to 3 times lower than that obtained for D/H in water in three long-period comets, all of which come from the Oort Cloud (44), thus ruling out Oort Cloud comets as the sole or even principal source of Earth's ocean water (44). Chondritic meteorites, while exhibiting large variations in D/H in hydrated minerals, on average have D/H close to SMOW. Because carbonaceous parent bodies are generally thought to reside in the asteroid belt, it is compelling to consider whether most of Earth's water came from the primordial asteroid belt. Models addressing this hypothesis must account for both the total mass of crustal water, which is perhaps several times the mass of the ocean itself, and the SMOW value of D/H.
A very recent model for the origin of Earth's oceans provides a mechanism for deriving Earth's water budget from the asteroid belt (19). The model, unlike previous ones, quantifies in a chronological fashion the supply of terrestrial water from multiple sources that wax and wane in importance at different times keyed to the gravitational scattering of planetesimals by Jupiter and Saturn. Because it tracks the accretion of the terrestrial planets as well as the scattering of planetesimals, the model predicts when, during Earth accretion, different sources of water become available. Before the time that the Earth reached half its present mass, icy bodies from the Jupiter-Saturn region as well as small bodies from the primordial asteroid belt supplied water to the Earth. This water would have been trapped deep in the planet as well as lost through subsequent very large impacts.
Late in the accretional history of Earth, the dynamical environment of the primordial asteroid belt as shaped by Jupiter evolved to the point where large planetary embryos existed there. These embryos, by virtue of being built from objects in the 2-4 AU region, were rich in water with D/H of the carbonaceous chondrites. The presence of Jupiter pumped up the eccentricities of the embryo orbits so that they crossed the orbit of the growing Earth. The model predicts high collision probabilities between Earth and embryos originally on distal orbits, so that as much as 10 times the current oceanic inventory of water could have been delivered to Earth with appropriate D/H (19). Finally, after accretion of the Earth was complete, a late infall of icy material, essentially comets scattered from the Uranus-Neptune region and the Kuiper Belt, impacted Earth to contribute no more than 10% of Earth's water. This "late veneer," previously proposed (45) to be the source of most of Earth's water (46), cannot be a primary source according to the dynamical calculations. Although we do not know the D/H for Kuiper Belt comets, it plausibly is no less than that of Oort Cloud comets, having been derived from outer solar system material little processed from the nascent molecular cloud. The very small contribution of comets to Earth's oceans derived from the dynamical calculations is fully consistent with the relative D/H values for comets versus SMOW.
From the point of view of the present survey, the details of the story for Earth in particular are not important. Indeed in the study described above, assumptions were made about the length of time over which Jupiter formed; different assumptions might lead to different outcomes in terms of water abundance and timing of delivery to Earth (19). The key point is a general one: the timing of delivery and amounts of volatiles delivered depend on the masses, orbital configurations, and timing of formation of the giant planets in a given system. The complexity of the story implies that it is difficult to extrapolate from one case study to another without running a full numerical simulation. However, some general inferences can be made (sketched in Fig. 3). Define the "snowline" of the protoplanetary disk as the orbital distance beyond which water ice is stable and modestly inward of which water can exist in hydrated minerals. The presence of Jupiter near the snowline of our protoplanetary disk was crucial to pumping up the orbital eccentricities of relatively proximal hydrated ("wet") embryos in the primordial asteroid belt, making them potentially available to deliver large amounts of water to Earth.
In systems that lack a large giant planet at or near the snowline, pumping of orbital eccentricities among wet embryos was limited to mutual gravitational interactions among them. Numerical simulations suggest this is not sufficient to create embryo orbits that would cross the orbits of terrestrial planets in the habitable (liquid water) zone (47). Conversely, in a system where a giant planet formed inward of the snowline, terrestrial planets should be relatively bereft of water, because wet embryos would be scattered outward very efficiently by the forming giant planet. On terrestrial planets in such systems, some amount of water will be available from distant comets but not much, given the difficulty of scattering the latter inward. The extrapolations given above depend, of course, on the assumption that giant planets form before terrestrial planets. Indeed, there is no reason why a terrestrial planet cannot form quickly during the time the gaseous disk is still present, but migration of such planets inward to the parent star could be quite rapid (28).
The main point, that the water budget delivered to the habitable zone depends sensitively on the existence and properties of giant planets, can be extended to other volatiles as well, including organics. However, there is a potentially interesting twist: the organic and volatile content of solid bodies is a sensitive function of temperature and other conditions of formation. Although most of Earth's water may have come from asteroids, it is possible in principle that most of the organic molecules came instead from comets (48). Therefore the composite-volatile picture of a habitable terrestrial planetwater plus life-forming elements and monomerswill be an even more complex function of the distribution and properties of neighboring giant planets.