24 Progress of Theoretical Physics Supplement No. 158, 2005Observed Properties of Exoplanets:Masses, Orbits, and MetallicitiesGeoffrey Marcy,1, ∗)R. Paul Butler,2Debra Fischer,3Steven Vogt,4Jason T. Wright,1Chris G. Tinney5andHughR.A.Jones61Astronomy Department, University of California, Berkeley, CA 94720, USA2Department of Terrestrial Magnetism, Carnegie Institution of Washington,5241 Broad Branch Rd NW, Washington DC 20015-1305, USA3Department of Physics and Astronomy, San Francisco State University,San Francisco, CA 94132, USA4UCO/Lick Observatory, University of California at Santa Cruz,Santa Cruz, CA 95064, USA5Anglo-Australian Observatory, PO Box 296, Epping 1710, Australia6Centre for Astrophysics Research, University of Hertfordshire,Hatfield, AL10 9AB, UKWe review the observed properties of exoplanets found by the Doppler technique that hasrevealed 152 planets to date. We focus on the ongoing 18-year survey of 1330 FGKM typestars at Lick, Keck, and the Anglo-Australian Telescopes that offers both uniform Dopplerprecision (3 m s−1) and long duration. The 104 planets detected in this survey have minimummasses (M sin i)aslowas6MEarth, orbiting between 0.02 and 6 AU. The core-accretionmodel of planet formation is supported by four observations: 1) The mass distribution risestoward the lowest detectable masses, dN/dM ∝ M−1.0. 2) Stellar metallicity correlatesstrongly 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 MEarthhas been discovered. The distribution ofsemi-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 eccentricityis e =0.25, and even planets beyond 3 AU reside in eccentric orbits, suggesting that thecircular orbits in our Solar System are unusual. The occurrence “hot Jupiters” within 0.1 AUof FGK stars is 1.2±0.2%. Among stars with one planet, 14% have at least one additionalplanet, occasionally locked in resonances. Kepler and COROT will measure the occurrenceof earth-sized planets. The Space Interferometry Mission (SIM) will detect planets withmasses as low as 3 MEarthorbiting within 2 AU of stars within 10 pc, and it will measuremasses, orbits, and multiplicity. The candidate rocky planets will be amenable to follow-upspectroscopy by the “Terrestrial Planet Finder” and Darwin.§1. IntroductionIn the past 10 years, 152 exoplanets have been discovered orbiting 131 normalstars by using the Doppler technique to monitor the gravitational wobble inducedby a planet, as previously summarized.56), 59)Multiple planet systems have beendetected around 17 of the 131 planet-bearing stars, found by superimposed multipleDoppler periodicities.59), 83)Remarkable statistical properties have emerged from the152 planets:• Planet mass distribution: dN /dM ∝ M−1.0(Fig. 1)∗)E-mail: [email protected] Properties of Exoplanets 25• >7% of stars have giant planets within 5 AU, most beyond 1 AU (Fig. 2)• Hot Jupiters (a<0.1 AU) exist around 1.2% of FGK stars• Eccentric Orbits are common, with a median of e = 0.25 (Fig. 3)• Planet occurrence rises rapidly with stellar metallicity (Fig. 6)• Multiple planets are common, often in resonant orbits (Fig. 7)Four planets of extraordinarily low mass have been found. Three have Neptune-like masses of Msin i of 21, 15, and 18 MEarthorbiting host stars, GJ 436, 55 Cancri,and HD 190360, respectively.12), 60), 83)The fourth planet is likely the first “super-Earth” with Msin i =6.0MEarthand P =1.94 d, orbiting the star, GJ 876.68)Apparently, planet formation can populate the mass range between that of Uranusand Earth.The first direct image of an exoplanet has finally occurred with the VLT/NACOand HST/NICMOS images of 2M1207 and its companion separated by 773 mas (54AU projected separation).17), 71)At the likely age (8 Myr) of this system in theTW Hydrae association, the IR photometry implies a mass of 2–5 MJupbased onatmospheric models of such young, warm planets.6), 11)The second epoch HST ob-servations (Schneider, private communication) confirm that the companion is boundto the primary, rendering it the first planetary mass companion ever imaged.To date, 7 planets are known that cross the disk of their star, 5 found pho-tometrically by the dimming of the star.3), 9), 79)However, the two closest starswith transiting planets (HD 209458, HD 149026) were found first by the Dopplermethod.13), 34), 70)The fractional dimming of the star’s flux gives a direct measureof the radius of the planet relative to the stellar radius that is determined fromstellar modelling. The edge-on orbit and Doppler measurements give the planetmass. The resulting densities of these planets are in the range of 0.2–1.4 g/cm3verifying the expectation that the planets are gaseous (albeit with liquid metal-lic hydrogen interiors).13), 34)During transit, starlight passing through the planet’satmosphere has allowed detection of its constituents, notably sodium and hydro-gen.14), 82)During eclipse of the planet by the star, measurements of the diminishedinfrared flux in narrow bands permit assessment of the atmospheric temperatureand other atmospheric constituents such as water vapor and methane.15), 19)Fromprecise Doppler measurements, including the Rossiter effect (in which the planetblocks a portion of the rotating star’s hemisphere, causing a net Doppler shift), thetidal heating has been shown to be negligible, leaving the large radius of HD 209458unexplained.49), 87)Most remarkable is HD 149026 that has only 1.21 MSatbut ahigh density of 1.4 g/cm3(twice Saturn’s), implying the existence of a massive rockycore of 70 MEarth.70)The standard theory for the formation of gas giant planets is the “core-accretion”model that begins with dust particles colliding and growing within a protoplanetarydisk to form rock-ice planetary cores.1), 30), 33), 39), 42)–44), 51), 53), 86)If the core becomesmassive enough while gas remains in the disk, it gravitationally accretes nearby gas,acquiring an extended gaseous envelope.7), 8), 61), 65)Support for the core-accretionmodel has come from HD 149026b with its massive rocky core and implied high abun-dance of heavy elements, suggesting that core formation dominated any acquisition26 Marcy, Butler, Fischer, Vogt, Wright, Tinney and Jonesof gas, as discussed by Sato et al.
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