DETECTION OF EXTRASOLAR GIANT PLANETS

Eight extrasolar planet candidates have now been identified, all revealed by Keplerian Doppler shifts in their host stars. The masses (m sin i) lie between 0.5 and 7 MJUP, and the semimajor axes are less than 2.1 astronomical units (AU). Doppler detectability favors high masses and small orbits, and improvements will render Saturn masses detectable within a few years. The substellar mass function (dN/dM) for companions is roughly flat from 70 down to 10 MJUP, but it exhibits a sharp increase for masses below 5 MJUP. For three of these companions (47 UMa, ρ Crb, and 55 Cnc), their circular orbits must be primordial (not tidally induced), indicating formation in a disk, as presumed for Solar System planets. Eccentric orbits may be explained by gravitational perturbations, either by companion stars, other planets, or disk resonances. The detections imply that ∼6% of solar-type stars have giant planets within 2 AU. The small orbits (a < 2 AU) imply that the planets formed either in situ, without the benefit of ice grains, or suffered inward migration. Orbital decay within 1 Myr in disks appears inevitable and may shape the planet mass distribution. The observed stability of spectral line shapes suggests that nonradial stellar oscillations do not affect the planet detections.