DETECTION OF THERMAL EMISSION FROM AN EXTRASOLAR PLANETDavid Charbonneau,1Lori E. Allen,1S. Thomas Megeath,1Guillermo Torres,1Roi Alonso,2Timothy M. Brown,3Ronald L. Gilliland,4David W. Latham,1Georgi Mandushev,5Francis T. O’Donovan,6and Alessandro Sozzetti1,7Received 2005 February 3; accepted 2005 March 1ABSTRACTWe present Spitzer Space Telescope infrared photometric time series of the transiting extrasolar planet systemTrES-1. The data span a predicted time of secondary eclips e, correspondin g to the passage of the p lanet behind t hestar. In both bands of our observations, we detec t a flux decrement with a timing, amplitude, and duration as pre-dicted by published parameters of the system. This signal represents the first direct detection of (i.e., the observationof photons emitted by) a planet orbiting another star. The observed eclipse depths (in units of relative flux) are0:00066 0:00013 at 4.5 mand0:00225 0:00036 at 8.0 m. These estimates provide the first observationalconstraints on models of th e therma l emission of hot Jupiters. Assuming that the planet emits as a blackbody, weestimate an effective temperature of Tp¼ 106 0 50 K. Under the additional assumptions that the planet is in ther-mal equilibrium with the radiation from the star and emits isotropically, we find a B ond albedo of A ¼ 0:31 0:14.This would imply that the planet absorbs the majority of stellar radiation incident upon it, a conclusion of significantimpact to atmospheric models of these objects. We a lso compare our data to a previously published model of theplanetary th ermal emission, which predicts prominent spect ral features i n our observa tional bands due to water andcarbon monoxide. This mode l adequ ately rep roduces the observed planet-to-star flux ra tio at 8 .0 m; howev er, itsignificantly overpredicts the ratio at 4.5 m. We also present a n estimate of the timing of the s econdary eclipse,which we use to place a strong constraint on the expression e cos !,wheree is the o rbital eccentricity and ! is thelongitude of periastron. The resulting upper limit on e is sufficiently small that we conclude that tidal dissipation isunlikely to provide a significant source of energy interior to the planet.Subject h ead inggs: binaries: eclipsing — infrared: stars — planetary systems — stars: individual (TrES-1) —techniques: photometric1. INTRODUCTIONExtrasolar planets that transit their parent stars are particularlyvaluable, since they afford direct estimates of key physical pa-rameters of the object. Moreover, it is only for bright stars thatdirect follow-up studies of the planet are likely to succeed. Indeed,the numerous follow-up o bservations of the brightest knowntransiting system HD 209458 (Charbonneau et al. 2000; Henryet al. 2000) include the detection (Charbonneau et al. 2002) andupper limits (Richardson et al. 2003a, 2003b; Deming et al. 2005)of absorption features in the planetary atmosphere, the discoveryof a cloud of escaping hydrogen atoms (Vidal-Madjar et al. 2003),and a search for circumplanetary rings and Earth-sized satellites(Brown et al. 2001). The Optical Gravitational Lensing Experi-ment (OGLE) survey (Udalski et al. 2002a, 2002b, 2003, 2004)has yielded five transiting planets (Bouchy et al. 2004; Konackiet al. 2003, 2005; Moutou et al. 2004; Pont et al. 2004) withreasonably precise estimates of the planetary masses and radii;however, the direct follow-up studies described above are im-peded for these obje cts because of their great d istance and re-sulting faintness. The exclusive opportunities afforded by brightsystems have provided strong motivation for numerous wide-field, small-aperture surveys for these objects (Alonso et al. 2004;Bakos et al. 2004; Christian et al. 2004; Dunham et al. 2004;O’Donovan et al. 2004; Pepper et al. 2004). The first success forthe wide-field approach occurred only recently with the detec-tion of TrES-1 (Alonso et al. 2004). This hot Jupiter has a massMp¼ 0:76 0:05ðÞMJupand radius Rp¼ð1:04þ0:080:05ÞRJupand islocated 0.0394 AU from the central K0 V star (Sozzetti et al.2004).Soon after the discovery of TrES-1, we proposed for Direc-tor ’s Discretionary time on the Spitzer Space Telescope to moni-tor photometrically the TrES-1 system with the goal of detectingthe thermal emission of the planet through observation of thesecondary eclipse (i.e., the passage of the planet behind thestar). Detection of this signal is of interest for several reasons.First, it would constitute the first direct detection of an extra-solar planet, i.e., the detection of photons emitted by the planet.Second, the amplitude of the secondary eclipse can be compareddirectly to theoretical models of the planetary atmosphere and,under the assumption of blackbody emission, yields an estimateof the effective temperature of the planet. Third, the timing andduration of the secondary eclipse place constraints on the orbitaleccentricity that are much more restrictive than those from theradial velocity orbit alone. A nonzero eccentricity requires asource of excitation (such a s a second planet), and the dampingof this orbital eccentr icity provides an internal energy sourcethat could slow the contraction of the radius of the transitingplanet (Bodenhe ime r et al. 20 01, 2003). Conver sely, a zero ec-centricity would rule out this scenario.In this paper, we p resent the detection of the thermal emissionfrom the extrasolar planet TrES-1 and discuss the implicationsfor our understanding of the plane t.1Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge,MA 02138; [email protected] de Astrof ı´sica de Canarias, 38200 La Laguna, Tenerife, Spain.3High Altitude Observatory, Na tional Center for Atmospheric Research,3450 Mitchell Lane, Bou lder, CO 80307.4Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD21218.5Lowell Observatory, 1400 West Mars Hill Road, Flagstaff, AZ 86001.6California Institute of Technology, 1200 East California Boulevard, Pasadena,CA 91125.7Home institution: Department of Physics and Astronomy, University ofPittsburgh, 100 Allen Hall, 3941 O’Hara Street, Pittsburgh, PA 15260.523The Astrophysical Journal, 626:523–529, 2005 June 10# 2005. The American Astronomical Society. All rights reserved. Printed in U.S.A.2. OBSERVATIONS AND TIME SERIES PRODUCTIONThe Infrared Array Camera (IRAC; Fazio et al. 2004) onSpitzer obtains simultaneous images in
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