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Project Progress

The existence of close-in extrasolar planets provides an opportunity to evaluate the delivery of water and pre-biotic molecules to planets, via collisions in the early stages of planet formation. Over 150 planets are known to orbit stars other than the sun, and many of these extrasolar planets occur quite close to their stars and have orbital periods of only a few days. This close-in orbital geometry has two important implications. First, it is believed that these planets migrated inward by interactions with a protoplanetary disk. This migration process is likely to have provided ample opportunity for the outer envelopes and atmospheres of close-in planets to become enriched by collisions and accretion, similar to the collision processes that enriched the early earth. Therefore studying atmospheric composition of close-in extrasolar planets, even ones not in the habitable zone, is of great interest for astrobiology.

A second implication that follows from the existence of close-in extrasolar planets is that such planets have higher probabilities of transiting their stars, and the transit geometry provides methods to detect and characterize their atmospheres. D. Deming collaborated with T. Brown (HAO), D. Charbonneau (Harvard CfA), J. Harrington (Cornell Univ.), and L. J. Richardson (GSFC), to search for carbon monoxide absorption in the atmosphere of the transiting extrasolar planet HD 209458b. High spectral resolution observations were obtained in the 2-micron atmospheric window using the Keck telescope on Mauna Kea. Although the analysis did not detect the presence of carbon monoxide, the upper limit from these Keck data requires that the atmosphere must have a high opacity, to a degree which may require new models for the planetary atmosphere.

Working with S. Seager (Carnegie DTM), L. J. Richardson (GSFC), and J. Harrington (Cornell Univ.), D. Deming used the Spitzer Space Telescope to detect the secondary eclipse of HD 209458b at 24-microns wavelength. The secondary eclipse of a transiting extrasolar planet is the time when the planet disappears behind the star, and reappears. The amplitude of the secondary eclipse in the combined light of the system is a direct measurement of the infrared brightness of the planet. Simultaneous with the Deming et al. measurement of HD 209458b, a group at Harvard CfA headed by D. Charbonneau detected the secondary eclipse of another transiting planet at 4.5- and 8-microns, also using Spitzer. These Spitzer detections are the first direct measurements of planets orbiting other stars, and they open a new era in astronomy and astrobiology, where we can now directly study and compare alien worlds to those in our own solar system.

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