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Implications of Extrasolar Planets for Understanding Planet Formation

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Implications of Extrasolar Planets for Understanding Planet Formation 27 Mar 2002 8:47 AR AR154-05.tex AR154-05.SGM LaTeX2e(2001/05/10) P1: GKD 10.1146/annurev.earth.30.091201.140357 Annu. Rev. Earth Planet. Sci. 2002. 30:113–48 DOI: 10.1146/annurev.earth.30.091201.140357 Copyright c 2002 by Annual Reviews. All rights reserved IMPLICATIONS OF EXTRASOLAR PLANETS FOR UNDERSTANDING PLANET FORMATION Peter Bodenheimer and D.N.C. Lin UCO/Lick Observatory, University of California, Santa Cruz, California 95064; e-mail: [email protected], [email protected] Key Words extrasolar planets, planetary systems, Solar System formation ■ Abstract The observed properties of extrasolar planets and planetary systems are reviewed, including discussion of the mass, period, and eccentricity distributions; the presence of multiple systems; and the properties of the host stars. In all cases, the data refer to systems with ages in the Ga range. Some of the properties primarily reflect the formation mechanism, while others are determined by postformation dynamical evolutionary processes. The problem addressed here is the extraction of information relevant to the identification of the formation mechanism. The presumed formation sites, namely disks around young stars, therefore, must provide clues at times much closer to the actual formation time. The properties of such disks are briefly reviewed. The amount of material and its distribution in the disks provide a framework for the development of a model for planet formation. The strengths of, as well as the problems with, the two major planet formation mechanisms—gravitational instability and core accretion–gas capture—are then described. It is concluded that most of the known planetary systems are best explained by the accretion process. The timescales for the persistence of disks and for the formation time by this process are similar, and the mass range of the observed planets, up to approximately 10 Jupiter masses, is naturally explained. The mass range of 5–15 Jupiter masses probably represents an overlapping transition region, with planetary formation processes dominating below that range and star formation processes dominating above it. INTRODUCTION 2 Planets have low masses, <10 times the masses of their central stars, and lu- 3 minosities that can be as high as 10 times the solar luminosity (L ) during brief periods of the formation phase, but luminosities are generally in the range 6 10 of 10 –10 L . Even nearby planets are generally spatially separated by less than 1 arc second from their host stars. The combination of these effects makes detection of planets difficult. The available detection methods (Marcy & Butler 1998, Perryman 2000) include (a) periodic Doppler shift variations in the line-of- sight velocity of the central star, as determined from displacements in frequency of spectral lines or pulsar timing; (b) periodic small positional shifts of the central 0084-6597/02/0519-0113$14.00 113 20 Mar 2002 7:46 AR AR154-05.tex AR154-05.SGM LaTeX2e(2001/05/10) P1: GKD 114 BODENHEIMER LIN star as it orbits around the center of mass of a planetary system; (c) direct detec- tion of the reflected or emitted light of the planet; (d ) observation of the periodic dimming of the light of the star caused by the transit of a planet; and (e) departures from symmetry in the light curve of a gravitationally lensed star (Sackett 2001; Peale 1997, 2001). Practically all of the detections made so far have resulted from the Doppler method, although there are a few candidates for direct imaging and one detected transit of a planet whose existence was previously known through the Doppler technique. The first detection of an extrasolar planetary system was made by Wolszczan & Frail (1992) from precise measurements of the arrival times of pulses from the pulsar PSR 1257 12. The two planets in that system whose properties are well determined have masses+ (3.4/sin i)M and (2.8/sin i)M and orbital distances of 0.36 and 0.47 AU, respectively, where M is the mass of the Earth and i is the angle (generally unknown) between the line of sight and the normal to the orbital plane. The mutual gravitational perturbations of the planets lead to fluctuations in the observed arrival times of the pulses (Wolszczan 1994), constraining i to be most likely >60, so that the masses are indeed only a few M . Two other objects may be present in this system. The presence of planets is rare among the pulsars, with only one other pulsar, PSR B1620–26 in the globular cluster M4, having a confirmed planet whose mass is most likely to be 10 Jovian masses (MJ) or less (Thorsett et al. 1999). The orbital separation is approximately 60 AU (Ford et al. 2000), and the planet orbits an inner binary system consisting of the pulsar and a white dwarf in a one-half year orbit. The first extrasolar planet (ESP) around a solar-type star was discovered by Mayor & Queloz (1995), who measured the periodic Doppler shift in the spectral lines of the star 51 Pegasi. This star displayed an amplitude in the line-of-sight 1 1 velocity of 59 m s , compared with a measurement accuracy of 13 m s , leading to a mass of (0.47/sin i )MJand an orbital period of 4.23 days. This discovery was confirmed by Marcy & Butler (1995), and further discoveries were soon announced of planets around the stars 47 Ursae Majoris (Butler & Marcy 1996), 70 Virginis (Marcy & Butler 1996), 61 Cygni B (Cochran et al. 1997), Coronae Borealis (Noyes et al. 1997), Bo¨otis (Butler et al. 1997), Andromedae (Butler et al. 1997), and 55 Cancri (Butler et al. 1997). The minimum masses of these objects are in the range of 0.47 to 6.6 MJ. Previous to these discoveries, a companion to the star HD 114762 with a minimum mass of 11 MJ had been reported by Latham et al. (1989); this object apparently lies near the borderline between planets and brown dwarfs. At present, the list of planetary companions around main-sequence stars with masses similar to that of the Sun includes more than 60 members. Some of these systems contain multiple planets, including Andromedae with three planets and at least five systems with two planets each. Detailed updated information on all ESPs is available from the Extrasolar Planets Encyclopedia (http://www.obspm.fr/planets). The detection rate in the range of parameters in which current Doppler searches can find planets—namely, masses down to 0.1–0.4 MJ and separations less than 20 Mar 2002 7:46 AR AR154-05.tex AR154-05.SGM LaTeX2e(2001/05/10) P1: GKD PLANET FORMATION 115 1 3 AU, corresponding to a velocity semiamplitude of >20ms —is approximately 6–8 systems per 100 solar-type stars studied (D. Fischer, private communication). The velocity limit is expected to go lower in the future. The transit method is most likely to detect close-in planets (<1 AU) but could, in principle, reach down to the range of 1 M . In contrast, searches for direct detections are likely to yield relatively high-mass planets far (>100 AU) from the star, and gravitational lensing is sensitive to planets in the Neptune-Jupiter mass range at separations of 1–10 AU. No firm lensing detections have been reported so far, implying that less than one third of stars in the 0.3 M mass range have Jovian mass companions in the separation range of 1.5 to 4 AU (Sackett 2001). In the following sections, we review the general properties of the ESPs around main-sequence stars, discuss the observed and theoretical properties of disks that provide the environment for planet formation, describe the two main formation models for planetary systems, and conclude with an analysis of the clues that the ESPs provide for an identification of the actual mode of planet formation. OBSERVATIONAL PROPERTIES OF EXTRASOLAR PLANETS The general properties of ESPs around main-sequence stars are summarized in reviews by Marcy & Butler (1998, 1999) and Marcy et al. (2000). Figure 1 shows the discovery space surveyed to date. The semimajor axis of the planet’s orbit is plotted as a function of Mp sin i, where Mp is the actual mass of the planet. Orbits fall within 3 AU, although with a longer time baseline, Jupiter-mass planets will be detectable at greater separations. A considerable number fall inside 0.1 AU (note that the orbit of Mercury about the Sun is at 0.39 AU), with the smallest separation of only 0.038 AU (8.8 solar radii). The apparent cutoff at this separation still remains to be explained. The detection of so many Jupiter-mass planets with small separations was a major surprise, in view of the fact that the giant planets in our Solar System reside outside 5 AU. The anomaly can be explained by the hypothesis that these giant planets actually formed at 5 to 10 AU away from the star and then migrated inward, losing orbital angular momentum as a result of the torque exerted on the planet by the disk (Goldreich & Tremaine 1980, Ward 1997, Lin et al. 1996). Timescales for this process can be quite short; for example, a planetary core of 10 M at 5 AU has a characteristic migration timescale of only a few times 104 years. This rapid migration is one of the major unsolved problems of planet formation. The migration may be stopped at small distances from the star as a result of (a) dispersal of the disk, (b) tidal interactions between planet and star, or (c) truncation of the inner part of the disk by the stellar magnetic field.
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