The 212 currently identified extrasolar planets (that is, planets around stars other than our Sun) have known masses and orbits, but they are otherwise rather mysterious objects. For example, although all of them are massive enough to have their own atmospheres, almost nothing is known about these atmospheres. The reason is simple: only a few of these planets have been directly detected at all due to the very bright glare of their nearby stars. Nearly all of them were discovered indirectly from the wobbling motions of their parent stars. Studying a planetary atmosphere is therefore a formidable task, but one which is of tremendous interest to people trying to determine how the Earth's atmosphere formed and evolved.
CfA astronomer David Charbonneau and seven of his colleagues have now succeeded in studying the atmosphere of the planet around the nearby star known as HD189733. The planet, a bit more massive than Jupiter, orbits very close to its star - only 3% of the distance of the Earth from Sun, and so much closer to its star than Mercury is from the Sun. Every 2.2 days the planet completes an orbit that takes it across the face of the star as seen from the Earth (a "transit"). Knowing the orbital parameters very precisely, the astronomers used the Spitzer Space Telescope spectrometer to stare at the star just when the planet was at the center of its transit, and they repeated the observations a month later.
In the infrared this planet shines with about one-half of one percent as much light as does its star -- not much, but enough for modern instruments. Using sophisticated data analysis techniques to insure they were really looking at the planet's light and not at extraneous scattered signal, the scientists obtained an infrared spectrum of the planet at the wavelength where they had expected to see the telltale signature of atmospheric water vapor. But, to their surprise, none was seen, nor was there any indication of methane gas, another atmospheric constituent they had anticipated from theoretical models. It is still too soon to determine the implications of these unexpected results, and whether they imply, for example, that the models of extrasolar planetary atmospheres are more generally in error. The new result, however, emphasizes the dramatic, even astonishing ability of scientists on Earth to study the chemistry of the atmospheres of exotic planets around distant stars.