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- The occurrence of Jovian planets and the habitability of planetary systems

The past 5 years have seen the field of extrasolar giant planets mature from one in which the principal question to be addressed was, "Do planets exist around other stars?" to one in which comparative planetology and cosmogony could be conducted on multiple systems. More than 2 years passed after the discovery of the first extrasolar planet, 51 Peg B (1), before the debate was settled over whether a planet or stellar pulsations were responsible for the oscillating Doppler shift that is the telltale signature of the radial velocity technique (2). Today the situation is vastly different. Over 50 nearby stars, all roughly similar in spectral type to the Sun, have companions detected by radial velocity (3), at least one system has multiple planets (4), and one planet (HD209458b) can be directly detected as it transits its parent star (5). These data have enabled meaningful statistics to be accumulated on the frequency of planets around solar-type starsdagger (4), as well as allowed modeling to reveal the bulk density and early history of one planet (6).

The most striking, and oft-quoted, characteristic of the extrasolar planet menagerie is the preponderance of Jovian-massDagger planets at small orbital distances from their parent stars. Although the apparent statistical overrepresentation of such tight orbits in the observed cohort of planets is biased by the fact that Doppler spectroscopy is most sensitive to smaller orbital semimajor axes (9), the mere existence of such objects forces a paradigm shift in our expectations regarding planetary system architectures. Leaving aside just for the moment the issue of whether giant planets could form in place at small orbital distances or must migrate inward, the presence of giant planets scattered uniformly from 0.04 astronomical units (AU) through 3 AU has enormous implications for the frequency of habitable Earth-like planets in the galaxy. What fraction of solar-type stars might be precluded from having Earth-like planets through occupation of the habitable zone by giant planets? Do the processes of giant planet formation and dynamical evolution generally suppress or encourage the production of habitable planets, in terms of planetary growth, supply of volatiles and organic material to the habitable zone, and long-term collision rates of planetesimal debris with habitable planets?

In this paper, I review and extend existing models and their foundational observations that constrain the frequency of formation of giant planets around solar-type stars, as well as the rough distribution of their orbits and their effect on the incidence of terrestrial planets. I also show how modeling of the origin of Earth's oceans through dynamical scattering of planetesimals allows some constraints on equivalent scenarios of delivery of water and other volatiles to extrasolar terrestrial planets.

My intention here is to focus on giant planets themselves and to provide a guide to and extension of the literature on their nature, abundance, and quantitative effects on other components of planetary systems. Other recent work of related interest concerns planetary system habitability in terms of a key indicator such as the carbon-to-oxygen ratio (10), the specific orbital positions of terrestrial planets around stars (11), or moons around giant planets (12). General discussions about the formation and habitability of terrestrial planets have appeared recently in the scientific (13) and popular literature (14).

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