When Extrasolar Planets Transit Their Parent StarsDavid CharbonneauHarvard-Smithsonian Center for AstrophysicsTimothy M. BrownHigh Altitude ObservatoryAdam BurrowsUniversity of ArizonaGreg LaughlinUniversity of California, Santa CruzWhen extrasolar planets are observed to transit their parent stars, we are granted unprece-dented access to their physical properties. It is only for transiting planets that we are permitteddirect estimates of the planetary masses and radii, which provide the fundamental constraints onmodels of their physical structure. In particular, precise determination of the radius may indicatethe presence (or absence) of a core of solid material, which in turn would speak to the canonicalformation model of gas accretion onto a core of ice and rock embedded in a protoplanetary disk.Furthermore, the radii of planets in close proximity to their stars are affected by tidal effectsand the intense stellar radiation; as a result, some of these “hot Jupiters” are significantly largerthan Jupiter in radius. Precision follow-up studies of such objects (notably with the space-basedplatforms of the Hubble and Spitzer Space Telescopes) have enabled direct observation of theirtransmission spectra and emitted radiation. These data provide the first observational constraintson atmospheric models of these extrasolar gas giants, and permit a direct comparison withthe gas giants of the Solar system. Despite significant observational challenges, numeroustransit surveys and quick-look radial velocity surveys are active, and promise to deliver anever-increasing number of these precious objects. The detection of transits of short-periodNeptune-sized objects, whose existence was recently uncovered by the radial-velocity surveys,is eagerly anticipated. Ultra-precise photometry enabled by upcoming space missions offersthe prospect of the first detection of an extrasolar Earth-like planet in the habitable zone of itsparent star, just in time for Protostars and Planets VI.1. OVERVIEWThe month of October 2005, in which the fifth Protostarsand Planets meeting was held, marked two important eventsin the brief history of the observational study of planets or-biting nearby, Sun-like stars. First, it was the ten-year an-niversary of the discovery of 51 Pegb (Mayor and Queloz,1995), whose small orbital separation implied that similarhot Jupiters could be found in orbits nearly co-planar to ourline of sight, resulting in mutual eclipses of the planet andstar. Second, October 2005 heralded the discovery of theninth such transiting planet (Bouchy et al., 2005a). This se-lect group of extrasolar planets has enormous influence onour overall understanding of these objects: The 9 transitingplanets are the only ones for which we have accurate esti-mates of key physical parameters such as mass, radius, and,by inference, composition. Furthermore, precise monitor-ing of these systems during primary and secondary eclipsehas permitted the direct study of their atmospheres. As aresult, transiting planets are the only ones whose physicalstructure and atmospheres may be compared in detail to theplanets of the Solar system, and indeed October 2005 wasnotable for being the month in which the number of objectsin the former category surpassed the latter.Our review of this rapidly-evolving field of study pro-ceeds as follows. In Section 2, we consider the physicalstructure of these objects, beginning with a summary of theobservations (Section 2.1) before turning to their impact onour theoretical understanding (Section 2.2). In Section 3,we consider the atmospheres of these planets, by first sum-marizing the challenges to modeling such systems (Sec-tion 3.1), and subsequently reviewing the detections andupper limits, and the inferences they permit (Section 3.2).We end by considering the future prospects (Section 4)for learning about rocky planets beyond the Solar systemthrough the detection and characterization of such objectsin transiting configurations.12. PHYSICAL STRUCTURE2.1. Observations2.1.1. Introduction. When a planet transits, we can ac-curately measure the orbital inclination, i, allowing us toevaluate the planetary mass Mpldirectly from the minimummass value Mplsin i determined from radial-velocity obser-vations and an estimate of the stellar mass, M?. The plane-tary radius, Rpl, can be obtained by measuring the fractionof the parent star’s light that is occulted, provided a reason-able estimate of the stellar radius, R?, is available. Withthe mass and radius in hand, we can estimate such criti-cally interesting quantities as the average density and sur-face gravity. Hence, the information gleaned from the tran-siting planets allows us to attempt to unravel the structureand composition of the larger class of extrasolar planets,to understand formation and evolution processes (includingorbital evolution), and to elucidate physical processes thatmay be important in planetary systems generically. Fig. 1shows the mass-radius relation for the 9 known transitingplanets, with Jupiter and Saturn added for comparison. Itis fortunate that the present small sample of objects spans amoderate range in mass and radius, and appears to containboth a preponderance of planets whose structure is fairlywell described by theory, as well as a few oddities that chal-lenge our present knowledge.We begin by describing how the objects shown in Fig. 1were identified and characterized, and, along the way, weilluminate the limitations that these methods imply for ourefforts to understand extrasolar planets as a class. By def-inition, transiting planets have their orbits oriented so thatthe Earth lies nearly in their orbital plane. This is an uncom-mon occurrence; assuming random orientation of planetaryorbits, the probability that a planet with orbital eccentricity,e, and longitude of periastron, $, produces transits visiblefrom the Earth is given byPtr= 0.00451AUaR?+ RplR1 + e cos(π2− $)1 − e2which is inversely proportion to a, the orbital semi-majoraxis. All known transiting planets have orbital eccentricitiesconsistent with zero, for which the last factor in the aboveequation reduces to unity.The radii of Jovian planets are typically only about 10%of the stellar radii. The transits known to date result in a0.3 − 3% diminution of the stellar flux reaching the Earth.These transits last for 1.5 − 3.5 hours, and accurate ground-based characterizations of these events are challenging. Thepaucity and
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