Useful comments by the reviewer and editor greatly improved the presentation of the material. Preparation of the paper and some of the work described herein were supported by the National Aeronautics and Space Administration Origins and Planetary Atmospheres Programs.
AU, astronomical unit; D/H, deuterium-to-hydrogen ratio; SMOW, Standard Mean Ocean Water.
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This paper was submitted directly (Track II) to the PNAS office.
"Solar-type" stars are considered here to be F-, G-, and K-type stars, on which the Doppler spectroscopic search for planetary companions is focused. The Sun itself is a G-type star, so designated in a well-characterized sequence based on spectral classification, so my use of the term "solar-type" is a loose one.
There is no standardized nomenclature in this field. In this paper, the term "Jovian mass" does not denote an object of exactly one Jupiter mass, but rather one that ranges from 0.1-13 Jupiter masses. The upper mass is the minimum mass for deuterium fusion in solar-composition objects (7), a convenient and perhaps not entirely arbitrary (8) cutoff for planets. The term "giant planets" used earlier in the text is defined here to refer specifically to bodies in the mass range given above that are primarily hydrogen-helium, with a greater or lesser admixture of heavier elements. Such objects implicitly are like our own giant planets in rough composition. There is only one extrasolar planet that we can assign with confidence to be a "giant planet," because we know its radius, hence bulk density and composition. In our own system, Uranus and Neptune barely qualify, because they are so rich in heavy elements; some planetologists call them "ice giants" to distinguish them from Jupiter and Saturn.
§ American Astronomical Society Division for Planetary Sciences Meeting 2000, Pasadena, Talk no. 31.02.