Detection of Thermal Emission from an Extrasolar Planet

TLDR: In this paper, the authors present Spitzer Space Telescope infrared photometric time series of the transiting extrasolar planet system TrES-1, which represents the first direct detection of photons emitted by a planet orbiting another star.

Abstract: We present Spitzer Space Telescope infrared photometric time series of the transiting extrasolar planet system TrES-1. The data span a predicted time of secondary eclipse, corresponding to the passage of the planet behind the star. In both bands of our observations, we detect a flux decrement with a timing, amplitude, and duration as predicted by published parameters of the system. This signal represents the first direct detection of (i.e. the observation of photons emitted by) a planet orbiting another star. The observed eclipse depths (in units of relative flux) are 0.00066 ± 0.00013 at 4.5 µm and 0.00225±0.00036 at 8.0 µm. These estimates provide the first observational constraints on models of the thermal emission of hot Jupiters. Assuming that the planet emits as a blackbody, we estimate an effective temperature of Tp = 1060 ±50 K. Under the additional assumptions that the planet is in thermal equilibrium with the radiation from the star and emits isotropically, we find a Bond albedo of A = 0.31 ± 0.14. This would imply that the planet absorbs the majority of stellar radiation incident upon it, a conclusion of significant impact to atmospheric models of these objects. We also compare our data to a previously-published model of the planetary thermal emission, which predicts prominent spectral features in our observational bands due to water and carbon monoxide. This model adequately reproduces the observed planet-to-star flux ratio at 8.0 µm, however it significantly over-predicts the ratio at 4.5 µm. We also present an estimate of the timing of the secondary eclipse, which we use to place a

Figures

Fig. 1.—Top: Binned 8.0 m time series. The best-fit model eclipse curve has a depth of FII ¼ 0:00225 and a timing offset of tII ¼ þ8:3 minutes and is plotted as the solid line. A model of the same depth but tII ¼ 0 is shown as the dashed line. Bottom: The 1, 2, and 3 confidence ellipses on the eclipse depth and timing offset.

Fig. 3.—Solid black line shows the Sudarsky et al. (2003) model hot Jupiter spectrum divided by the stellar model spectrum (see text for details). The open diamonds show the predicted flux ratios for this model integrated over the four IRAC bandpasses (which are shown in gray and renormalized for clarity). The observed eclipse depths at 4.5 and 8.0 m are overplotted as black diamonds. No parameters have been adjusted to the model to improve the fit. The dotted line shows the best-fit blackbody spectrum (corresponding to a temperature of 1060 K), divided by the model stellar spectrum. Although the Sudarsky et al. (2003) model prediction is roughly consistent with the observations at 8.0 m, the model overpredicts the planetary flux at 4.5 m. The prediction of a relatively large flux ratio at 3.6 m should be readily testable with additional IRAC observations.

Fig. 2.—Top: Binned 4.5 m time series. The best-fit model eclipse curve [excluding the data at times 0:05 < t (days) < 0:03; see text] has a depth of FII ¼ 0:00066 and a timing offset of tII ¼ þ8:1 minutes and is plotted as the solid black line. A model of the same depth but tII ¼ 0 is shown as the dashed line. The best-fit model when these additional 9.8% of data are included is shown as the gray line; the estimated depth is similar, but a significant timing offset is found. Bottom: The black ellipses are the 1, 2, and 3 confidence ellipses on the eclipse depth and timing offset for the restricted data set; the corresponding ellipses for the complete data set are shown in gray.

TABLE 1 Times of Primary and Secondary Eclipse

Fig. 4.—The 1 , 2 , and 3 confidence spaces of the orbital eccentricity e and the longitude of periastron !. Since it is the combined expression e cos ! that is constrained by the time of secondary eclipse, there exists a small range of ! (near cos ! ¼ 0) for which a significant eccentricity is permitted. For this situation to occur, however, the orbital ellipse would need to be very nearly aligned with our line of sight. Additional radial velocity observations will further restrict the allowed parameter space.