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Architectures of Planetary Systems and Implications for Their Formation

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Architectures of Planetary Systems and Implications for Their Formation Architectures of planetary systems and implications for SPECIAL FEATURE their formation Eric B. Ford1 Center for Exoplanets and Habitable Worlds, and Department of Astronomy and Astrophysics, The Pennsylvania State University, State College, PA 16803 Edited by Adam S. Burrows, Princeton University, Princeton, NJ, and accepted by the Editorial Board March 26, 2014 (received for review December 3, 2013) Doppler planet searches revealed that many giant planets orbit migrate through either a gaseous protoplanetary disk or close to their host star or in highly eccentric orbits. These and a planetesimal disk. If giant planet cores form beyond the ice subsequent observations inspired new theories of planet forma- line, then planetesimal disks would rarely be massive enough to tion that invoke gravitation interactions in multiple planet systems drive the large-scale migration needed to form a hot Jupiter. On to explain the excitation of orbital eccentricities and even short- the other hand, a gaseous protoplanetary disk could power period giant planets. Recently, NASA’s Kepler mission has identi- a rapid migration, perhaps too rapid (10). It is unclear how the fied over 300 systems with multiple transiting planet candidates, planets would avoid migrating all of the way into the host star or including many potentially rocky planets. Most of these systems why the migration would be halted to leave planets with orbital include multiple planets with closely spaced orbits and sizes be- periods of ∼2–5 days (11). The apparent pileup of hot Jupiters tween that of Earth and Neptune. These systems represent yet with orbital periods of a few days could be the result of cen- another new and unexpected class of planetary systems and pro- soring, i.e., those planet that continued to migrate closer to their vide an opportunity to test the theories developed to explain the host star were either accreted onto the star, destroyed, or re- properties of giant exoplanets. Presently, we have limited knowl- duced in mass due to stellar irradiation and mass loss. Even in edge about such planetary systems, mostly about their sizes and this case, a stopping mechanism must be invoked to produce the orbital periods. With the advent of long-term, nearly continuous observed giant planets with orbital periods beyond ∼7 days, monitoring by Kepler, the method of transit timing variations because tides rapidly become inefficient with increasing (TTVs) has blossomed as a new technique for characterizing the orbital separations. ASTRONOMY gravitational effects of mutual planetary perturbations for hun- dreds of planets. TTVs can provide precise, but complex, con- Eccentricity Excitation plus Tidal Circularization. Unlike disk mi- straints on planetary masses, densities, and orbits, even for gration, eccentricity excitation followed by tidal circularization planetary systems with faint host stars. In the coming years, naturally explains the pileup of hot Jupiters at orbital periods of astronomers will translate TTV observations into increasingly 2–7 days due to the rapid onset of tidal effects. The large ec- powerful constraints on the formation and orbital evolution of centricities required to initiate circularization could be generated planetary systems with low-mass planets. Between TTVs, im- in a variety of ways. The simplest scenario is planet–planet proved Doppler surveys, high-contrast imaging campaigns, and scattering, as it requires only one additional massive planet (7). microlensing surveys, astronomers can look forward to a much In the case of two planets and no additional perturbers, the better understanding of planet formation in the coming decade. initial ratio of semimajor axes must be small enough to permit close encounters. Such scenarios may arise naturally for giant hot-Jupiters | super-Earths planets due to rapid mass growth. Alternatively, a system with more than two planets (8, 12–15) will naturally approach in- Hot Jupiters stability on a much longer timescale. Even for a simple three- Radial velocity (RV) surveys have discovered over 400 planets, planet system, the timescale until close encounters can easily most with masses larger than that of Jupiter (www.exoplanets. exceed 10 million years (14), by which time the protoplanetary org; refs. 1, 2). Many of the early RV discoveries were “hot disk will have dissipated. Jupiters,” planets with orbital periods of up to several days and Alternatively, a system of two (or more) planets may become masses comparable to that of Jupiter or Saturn (e.g., ref. 3). As unstable due to an external perturbation, such as secular inter- the timespan of observations has increased, the median orbital actions with a binary companion (16) or a stellar flyby (17). This can lead to strong planet scattering, often following a prolonged period of RV-discovered planets has steadily increased to more – than a year. Now, we know that hot Jupiters are a relatively rare phase of weaker interactions (18 20). Each close encounter outcome of planet formation. Nevertheless, their existence and between giant planets leads to a small perturbation to their their orbital properties provide important clues to the planet- formation process. Significance Disk Migration. Before the discovery of hot Jupiters, planet-for- Prior to the discovery of exoplanets, astronomers fine tuned mation theories had been focused on explaining properties of the theories of planet formation to explain detailed properties of solar system (4). The large masses of hot Jupiters imply a sub- the solar system. The discovery of exoplanets has significantly stantial gaseous component and therefore rapid formation, be- increased our appreciation for the diversity of planetary sys- fore the protoplanetary disk is dispersed. In situ formation of the tems in nature. In this article we review the current un- rocky cores of hot Jupiters is problematic due to the high tem- derstanding of the late stages of evolution of planetary perature and low surface density of the disk so close to their host systems, focusing on observational constraints from radial ve- star. Therefore, theorists explain hot Jupiters starting with the locity and transit observations. formation of a rocky core at larger separations from the host star, followed by accretion of a gaseous envelope and migration Author contributions: E.B.F. wrote the paper. to their current location. The mechanism for migration is less The author declares no conflict of interest. clear. There are two broad classes of models: a gradual migration This article is a PNAS Direct Submission. A.S.B. is a guest editor invited by the Editorial through a disk (5, 6) or the excitation of a large eccentricity Board. followed by tidal circularization (7–9). In principle, planets could 1E-mail: [email protected]. www.pnas.org/cgi/doi/10.1073/pnas.1304219111 PNAS Early Edition | 1of6 Downloaded by guest on October 1, 2021 orbits (21). Thus, it is typically a series of close encounters (AU) before scattering (39) than if scattering commenced at that excites the two planets’ orbital eccentricities until one planet’s several AU. pericenter is small enough to initiate tidal circularization. In Another key observational result is the realization that hot practice, planetary systems that form two giant planets may well Jupiters are seldom accompanied by additional planets close to form additional massive planets, leading to a series of planet– their host star. This was foreshadowed by radial velocity planet planet scattering events and substantially increasing the proba- searches (2) and dramatically confirmed by Kepler observations bility for one to achieve a pericenter of just a few stellar radii. of transiting hot Jupiters (40), as these provide precise con- Another possible mechanism for eccentricity excitation straints on both small planets with orbital periods of weeks to involves secular (i.e., long term) perturbations by one or more months (via photometry) and low-mass planets in or near mean- distant bodies (e.g., more planets, a brown dwarf, or binary motion resonances (via transit timing variations). This result is stellar companion). If there is a large mutual inclination between consistent with the broad predictions of hot-Jupiter formation the inner planet and the outer companion, then large eccen- via eccentricity excitation plus tidal circularization, but is in stark tricities are possible, even for systems with large orbital period contrast to the predictions of disk-migration models (41, 42). ratios (16, 22, 23, 24). This could be particularly relevant for Thus, the isolation of hot Jupiters suggests that there is a strong planets orbiting one member of a binary (or higher multiple) star upper limit to the fraction of hot Jupiters formed via disk mi- system, even though the orbital period of the two stars is often gration. Recent radial velocity follow up of systems with hot much larger than the orbital period of the planet. Jupiters has found long-term radial velocity accelerations in For small mutual inclinations, exciting an eccentricity large roughly half of the surveyed hot Jupiters (43). The location of enough to trigger tidal circularization requires a substantial an- the second-closest planet in systems with hot Jupiters bolsters the gular momentum deficit (AMD) and a series of planets that hypothesis that hot Jupiters may frequently commence scattering ∼ serve to couple the inner giant planet to outer planets, which are while at an orbital distance 1 AU. more likely to have a significant initial AMD (25). If the planets are widely spaced, then it is possible to construct initial con- Summary of Hot-Jupiter Formation. In summary, planet formation ditions that lead to the inner planet’s pericenter dropping to only from a gaseous disk likely leads to forming many planets on low- eccentricity orbits. Initially, close encounters lead to collisions a few stellar radii (26). However, for more typical initial con- and increasing planet masses. Once the planets become massive ditions, the secular interactions lead to close encounters between enough to eject bodies from the gravitational potential well of planets that result in collisions and/or ejections via planet–planet the star, ejections become more common.
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