Observed Properties of Exoplanets: Masses, Orbits, and Metallicities

Abstract

We review the observed properties of exoplanets found by the Doppler technique that has revealed 152 planets to date. We focus on the ongoing 18-year survey of 1330 FGKM type stars at Lick, Keck, and the Anglo-Australian Telescopes that offers both uniform Doppler precision (3 m s−1) and long duration. The 104 planets detected in this survey have minimum masses (M sin i) as low as 6 MEarth, orbiting between 0.02 and 6 AU. The core-accretion model of planet formation is supported by four observations: 1) The mass distribution rises toward the lowest detectable masses, dN/dM ∝ M−1.0. 2) Stellar metallicity correlates strongly with the presence of planets. 3) One planet (1.3 MSat) has a massive rocky core, MCore ≈ 70 MEarth. 4) A super-Earth of ∼ 7 MEarth has been discovered. The distribution of semi-major axes rises from 0.3 – 3.0 AU (dN/d log a) and extrapolation suggests that ∼12% of the FGK stars harbor gas-giant exoplanets within 20 AU. The median orbital eccentricity is e = 0.25, and even planets beyond 3 AU reside in eccentric orbits, suggesting that the circular orbits in our Solar System are unusual. The occurrence “hot Jupiters” within 0.1 AU of FGK stars is 1.2±0.2%. Among stars with one planet, 14% have at least one additional planet, occasionally locked in resonances. Kepler and COROT will measure the occurrence of earth-sized planets. The Space Interferometry Mission (SIM) will detect planets with masses as low as 3 MEarth orbiting within 2 AU of stars within 10 pc, and it will measure masses, orbits, and multiplicity. The candidate rocky planets will be amenable to follow-up spectroscopy by the “Terrestrial Planet Finder” and Darwin.