Searching for Water on an Exoplanet
Tuesday, April 3, 2007
Science Update - A look at CfA discoveries from recent journals

An "exoplanet" is an extra-solar planet -- a planet orbiting a star other than our own Sun. Over 120 exoplanets have been discovered in the past decade in a scientific revolution that has been led to a very significant degree by SAO. Astronomers trying to identify the key processes involved in the formation of the Earth look to these extra-solar examples to test their models and refine their understanding of our home.

CfA astronomer (and past CfA student) David Charbonneau, together with seven of his colleagues, has used the Spitzer Space Telescope to obtain an infrared spectrum of the the planet orbiting the nearby star (63 light-years distant) HD 189733. This star has a Jupiter-sized planet orbiting very close by, at only 3.1% of the distance of the Earth from the Sun, and so much closer than even Mercury from our Sun. As as result of its proximity to the star, this exoplanet's atmosphere is thought to be quite hot, more than about 1000 kelvin, making it easier to see in the infrared than most other known exoplanets. Easier maybe...but certainly not easy. The infrared light from this exoplanet is still only about 0.5% of the flux from its parent star. Using meticulous data acquisition and analysis techniques, however, the team was able to extract the light of the planet from starlight.

To their surprise the astronomers did not see any evidence for the atmospheric molecules that virtually all the models had predicted: water, and methane. In fact, the spectrum of the atmosphere showed no molecular features at all. The reason for the absence of water is a mystery. Indeed, the scientists point out that simply reducing the quantity of water predicted to be in the atmosphere will not solve the problem because then the models predict other characteristics that are not observed. The intriguing result needs confirmation, and the team plans to repeat their measurements, but it also suggests that there is a fundamental gap in our understanding of the formation of planetary atmospheres around exoplanets.