REVIEW Extrasolar Planets: Constraints for Planet Formation Models Nuno C
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REVIEW Extrasolar Planets: Constraints for Planet Formation Models Nuno C. Santos,1,3* Willy Benz,2 Michel Mayor3 they require less observational time. However, Since 1995, more than 150 extrasolar planets have been discovered, most of them in these short-period planets were unexpected, orbits quite different from those of the giant planets in our own solar system. The because giant planets were thought to have number of discovered extrasolar planets demonstrates that planetary systems are formed on orbits beyond the ice line, the point common but also that they may possess a large variety of properties. As the number of where the nebula becomes cold enough for detections grows, statistical studies of the properties of exoplanets and their host stars ices to condensate, thereby maximizing the can be conducted to unravel some of the key physical and chemical processes leading to density of solids available for accretion (7). the formation of planetary systems. Continuous planet searches have revealed giant planets with orbital periods as short as 1.2 days n 1992, Wolszczan and Frail made the About 10% of the discovered exoplanets (8) or as long as È10 years (9), although this discovery that planets far from our own have an orbital period of less than 5 days. upper limit is probably due to observational I solar system were in orbit around the These planets are easier to detect because limitations. Some of the planets are on ec- pulsar PSR 1257þ12 (1). After years of false starts and retracted announcements, it was thrilling that definitive evidence had finally been found for smaller orbiting bodies in dis- 10,000 tant stellar environments. At the same time, pulsars are not anything like our own Sun, and any planets orbiting them would not be ex- HD 114762 b pected to harbor life as in our solar system. For 1000 this reason, researchers had long been eager to observe planets in orbit around Sun-like stars, Jupiter and in 1995, a first planet was detected around the star 51 Pegasi (2, 3). Since then, more than 100 Saturn 150 other planets have been found, most of sin i (M ) 2 these discovered, or at least confirmed, with m 51 Peg b radial-velocity techniques (4), in which the HD 16141 b stellar wobble of the star moving about the center Neptune HD 49674 b of mass of the star-planet system is measured 10 with spectral Doppler information. These dis- 55 Cnc e coveries have advanced our understanding of µ Ara c planet dynamics and planet formation. The increase in precision and continuity of 1 Earth the current radial-velocity surveys has given astronomers the possibility of unveiling a large variety of planets. In less than 10 years, the 1990 1995 2000 2005 lowest known detectable planetary mass has Epoch (years) decreased by more than one order of magnitude Fig. 1. Minimum mass of the known extrasolar planets orbiting solar-type stars, in Earth masses as a (Fig. 1), reflecting the jump in measurement function of the year of the discovery. The masses of Jupiter, Saturn, Neptune, and the Earth are marked precision from È10 m sj1 to È3msj1 at the for comparison (blue lines). A few benchmark cases (red dots) are labeled: the probable brown-dwarf end of the century (5), and more recently companion to HD 114762 (65), 51 Peg b (2), HD 49674 b (66), and the record-holders, recently crossing the barrier of 1 m sj1 precision with discovered very low mass companions to 55 Cnc and m Ara (11, 12). Data are as of June 2005. state-of-the-art spectrometers such as the High Accuracy Radial Velocity Planet Searcher (HARPS) (6). Table 1. Extrasolar planets transiting their host stars, as confirmed in June 2005. Other candidates exist, although their characteristics are still not well constrained (e.g., OGLE-TR-10). System Planet mass (M ) Planet radius (R ) Orbital period (days) Reference 1 Jup Jup Centro de Astronomia e Astrofı´sica da Universidade T T de Lisboa, Observato´rio Astrono´mico de Lisboa, Tapada HD 209458 0.69 0.05 1.35 0.06 3.52 (46, 67) T T da Ajuda, 1349-018 Lisboa, Portugal. 2Physikalisches OGLE-TR-56 1.45 0.23 1.23 0.16 1.21 (8, 68) T T 0.13 Institut, Universita¨t Bern, Sidlerstrasse 5, CH-3012 OGLE-TR-111 0.53 0.11 1.00 0.06 4.02 (48) 3 T T 0.07 Bern, Switzerland. Observatoire de Gene`ve, 51 chemin OGLE-TR-113 1.35 0.22 1.08 0.05 1.43 (40) des Maillettes, 1290 Sauverny, Switzerland. OGLE-TR-132 1.19 T 0.13 1.13 T 0.08 1.69 (40, 69) TrEs-1 0.75 T 0.07 1.08 T 0.18 3.03 (49) *To whom correspondence should be addressed. 0.04 E-mail: [email protected] www.sciencemag.org SCIENCE VOL 310 14 OCTOBER 2005 251 R EVIEW centric orbits (10) more typical of some within the disk_s lifetime. Recent extensions of accretion model is sufficiently advanced to comets in the solar system, whereas others the core accretion model, including disk evo- begin to allow quantitative calculations to be are in multiple planet systems (11). Finally, lution and planet migration (18), have shown made and thus permits a direct comparison although the most recently discovered plan- that, provided the planet survives, migration with giant planets in (22)andoutside(21)our ets have masses only one order of magnitude speeds up core growth, resulting in giant planet solar system. The direct collapse model is in a larger than Earth (11–13), some behemoths formation well within the inferred disk lifetimes. state where only qualitative statements can be have more than 15 times the mass of Jupiter In the direct collapse scenario (19), giant made without the possibility to compare quan- (14). It is not clear whether the more massive planets form directly from the gravitational titatively with observations. of these companions should be classified as fragmentation and collapse of the protoplan- Having at first painfully exposed our still planets at all. According to the pre-1995 plan- etary disk within a few dynamical time scales. sizable lack of understanding, the growing et formation theories, none of these objects Although this is an appealing feature that number of exoplanets discovered is now al- were supposed to exist. greatly simplifies a number of complicated lowing a statistical analysis of their proper- To understand the meaning of these ob- processes, this model has its own difficulties ties (10, 23–26) as well as those of their host servations, models of planetary system for- as well. For example, high-resolution simu- stars (27, 28), thereby providing invaluable mation and evolution are required. One giant lations of this process show that planets tend constraints on the physical and chemical planet formation scenario is the processes involved in the forma- core accretion model. In this tion of these systems. model, a solid core is first formed by the accretion of planetesimals. Statistical Properties As the core grows, it eventually 1.6 of Exoplanets becomes massive enough to grav- Before 1995, all our understand- itationally bind some of the neb- ing of planet formation was based ular gas, thus surrounding itself 1.4 on studies of one system, the solar with an envelope. The subsequent system. The failure of our theories evolution of this core-envelope ) to explain the diversity of the structure has been studied in detail Jup 1.2 more than 150 exoplanets has (7), and it has been shown that the markedly shown the necessity for solid core and the gaseous envelope further observational guidance. In grow in mass, the envelope remain- 1 the case of the solar system, this ing in quasistatic and thermal equi- Radius (R guidance is provided by in situ librium. During this phase, the measurements that allow a detailed energy radiated by the gas is sup- 0.8 study of structure, composition, pliedbyenergyreleasedfromthe isotopic abundances, and often accretion of planetesimals. As the time scales. In the case of the core mass reaches a critical value Eof exoplanets, this guidance is pro- 0.6 the order of 15 Earth masses (M])at vided by a careful statistical anal- 5 astronomical units (AU), but ysis of the distribution of masses, depending on different physical periods, and orbital eccentricity, as parameters, such as the solid accre- 0.4 0.8 1.2 1.6 well as of the chemical properties tion rate onto the core^, radiative of the host star. Both approaches Mass (MJup) losses can no longer be offset by are needed. planetesimal accretion and the en- Fig. 2. Mass-radius diagram for the giant planet companions for which a The existence of giant planets velope starts to contract. This transit event has been detected. Red symbols denote planets discovered with orbital periods of less than increases the gas accretion rate, by the OGLE survey (8, 40, 48), the green symbol denotes the planet È10 days, the so-called ‘‘hot Ju- orbiting the star TrEs-1 (49), and the blue symbol denotes HD 209458 b which in turn raises the radiative (46). The positions of Jupiter and Saturn are also marked (black open piters,’’ poses important difficul- energy losses, causing the process to squares). Two isodensity curves, with densities r of 0.4 and 1.0 g cmj3, ties to conventional as well as accelerate, leading to the very rapid are also shown for comparison. The planet orbiting HD 209458 presents unconventional giant planet forma- buildup of a massive envelope.