Direct Detection of ExoplanetsJ.-L. BeuzitLaboratoire d’Astrophysique de GrenobleD. MouilletObservatoire Midi-Pyr´en´eesB. R. OppenheimerAmerican Museum of Natural HistoryJ. D. MonnierUniversity of Michigan, Ann ArborDirect detection of exoplanets from the ground is now within reach of existing astronomicalinstruments. Indeed, a few planet candidates have already been imaged and analyzed and thecapability to detect (through imaging or interferometry) young, hot, Jupiter-mass planets exists.We present here an overview of what such detection methods can be expected to do in thenear and far term. These methods will provide qualitatively new information about exoplanets,including spectroscopic data that will mature the study of exoplanets into a new field ofcomparative exoplanetary science. Spectroscopic study of exoplanet atmospheres promises toreveal aspects of atmospheric physics and chemistry as well as internal structure. Astrometricmeasurements will complete orbital element determinations partially known from the radialvelocity surveys. We discuss the impact of these techniques, on three different timescales,corresponding to the currently available instruments, the new “Planet Finder” systems underdevelopment for 8 to 10-m telescopes, foreseen to be in operation in 5 to 10 years, and the moreambitious but more distant projects at the horizon of 2020.1. INTRODUCTIONSince the discovery of a planet around a solar-type starten years ago by Mayor and Queloz (1995), the study ofexoplanets has developed into one of the primary researchareas in astronomy today. More than 170 exoplanets havebeen found orbiting stars of spectral types F to M, with asignificant fraction in multi-planet systems (see the chapterby Udry et al. for a review). These exoplanets have beendiscovered using indirect detection methods, in which onlythe planet’s influence on the host star is observed.Indirect detection techniques include radial velocitymeasurements, which detect the movement of a star due toa planet’s gravitational influence (see the chapter by Udryet al.; Marcy et al., 2003), photometric transit observa-tions, which detect the variation in the integrated stellarflux due to a planetary companion passing through the lineof sight to its host star (see the chapter by Charbonneau etal.), as well as astrometry, which also detects stellar mo-tion (Sozzetti, 2005), and gravitational microlensing, whichinvolves unrepeatable observations biased toward planetswith short orbital periods (Mao and Paczynski, 1991; Gouldand Loeb, 1992).Almost all of the currently known exoplanets have beendetected by radial velocity measurements. Due to the un-known inclination angle of the orbit, only a lower limit ofthe mass can actually be derived for each individual planetcandidate. The statistical distribution of exoplanets can stillbe obtained thanks to the large number of detections. Theseexoplanets typically have masses similar to those of the gi-ant, gaseous planets in our own solar system. They aretherefore generally referred to as Extrasolar Giant Planets,or EGPs. Since 2004, a few planets with minimum massesranging between 6 and 25 Earth masses have been detectedby radial velocity measurements (Butler et al., 2004; Riveraet al., 2005; Bonfils et al., 2005) and very recently a 5.5Earth mass planet was discovered by gravitational lensing(Beaulieu et al., 2006).These indirect methods have proven to be very success-ful in detecting exoplanets, but they only provide limited in-formation about the planets themselves. For example, radialvelocity detections allow derivation of a planet’s orbital pe-riod and eccentricity as well as a lower limit to its mass dueto the unknown inclination angle of the orbit. Photomet-ric transit observations provide information about a planet’sradius, and, with great effort, limited measurements of thecomposition of its upper atmosphere (Jha et al., 2000). Inaddition, large radial velocity surveys with sufficient preci-sion only started about a decade ago. Thus, they are sen-sitive only to exoplanets with relatively small orbital peri-ods, typically corresponding to objects at distance smallerthan a few AUs from their parent stars. Currently, these1surveys are completely insensitive to planets at separationscomparable to those of Jupiter and Saturn in our solar sys-tem. Finally, the accuracy of radial velocity measurementsstrongly biases the detections based on the type of the hoststars toward old, quite and solar mass (G-K) stars. Futureextensive search for transiting planets will suffer similar bi-ases regarding orbital periods and stellar types.Direct detection and spectroscopy of the radiation fromthese exoplanets is necessary to determine their physical pa-rameters, such as temperature, pressure, chemical composi-tion, and atmospheric structure. These parameters are crit-ically needed to constrain theories of planet formation andevolution. Furthermore direct detection enables the studyof planets in systems like our own. In these respects, directdetection is complementary to the indirect methods, espe-cially to the radial velocity technique.However, direct observation of exoplanets is still at theedge of the current capabilities, able to reveal only the mostfavorable cases of very young and massive planets at largedistances from the central star. (See Section 2.1). The majorchallenge for direct study of the vast and seemingly diversepopulation of exoplanets resides in the fact that most of theplanets are believed to be 106to 1012times fainter than theirhost stars, at separations in the subarcsecond regime. Thisrequires both high contrast and spatial resolution.Various instrumental approaches have been proposed forthe direct detection of exoplanets, either from the ground orfrom space. In particular, several concepts of high-orderadaptive optics systems dedicated to ground-based large(and small) telescopes have been published during the pastten years (Angel and Burrows, 1995; Dekany et al., 2000;Mouillet et al., 2002; Macintosh et al., 2002; Oppenheimeret al., 2003). Interferometric systems, using either a nullingor a differential phase approach have also been described(see Section 4). Space missions have also been proposed,relying either on coronagraphic imaging (TPF-C observa-tory, see the chapter by Stapelfeldt et al.) or on interfer-ometry (TPF-I and Darwin projects, e.g. Mennesson etal., 2005; Kaltenegger and Fridlund, 2005, and referencestherein).In this chapter, we
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