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In 2007, astronomers discovered that a scorching-hot gas planet beyond 63 light years from our solar system is steaming with water vapour, the findings emerged from observations made with NASA's Spitzer Space Telescope. Now, it seems the planet HD 189733b, one of several "hot Jupiters", suffers from a high concentration of atmospheric carbon dioxide as revealed by near-infrared spectroscopy. The discovery represents a small, but significant, step to finding life's building materials on planets beyond the solar system. The balmy planet has a temperature of 1000 Kelvin and zips closely around its star in an orbit lasting just two Earth-days and earlier predictions suggested that water vapour might be abundant in its atmosphere although until last year there was scant evidence for this. Giovanna Tinetti, formerly a European Space Agency fellow at the Institute d'Astrophysique de Paris in France and now at University College London, and colleagues published data in Nature in 2007 that showed this hot Jupiter to be wet. "We're thrilled to have identified clear signs of water on a planet that is trillions of miles away," said Tinetti at the time. Subsequently, data revealed the planet to have methane in its atmosphere too. Now, NASA and ESA scientists working with Hubble Space Telescope data have shown that carbon dioxide is abundant in the atmosphere of the hot Jupiter. The researchers suggest that while HD 189733b, which lies in the constellation Vulpecula, is far too hot to sustain life as we know it, this discovery is nevertheless a significant chemical finding for exobiologists. The results appear in the 9th December 2008 issue of Astrophysical Journal Letters. "This [discovery] is exciting because Hubble is allowing us to see molecules that probe the conditions, chemistry, and composition of atmospheres on other planets," explains Mark Swain of The Jet Propulsion Laboratory in Pasadena, USA. "Thanks to Hubble we're entering an era where we are rapidly going to expand the number of molecules we know about on other planets." These latest Hubble observations are essentially a proof-of-principle, demonstrating that the basic components of terrestrial life, carbon, hydrogen, oxygen in the form of small molecules, carbon dioxide, methane, and water, can be observed on distant worlds. The results are, of course, just a single, small step on the road to discovering extraterrestrial life, should it exist. Swain and team used Hubble's Near Infrared Camera and Multi-Object Spectrometer (NICMOS) to study infrared light emitted from the planet. They explain that gases in the planet's atmosphere absorb certain wavelengths of light from the planet's hot glowing interior. They identified not only carbon dioxide, but also carbon monoxide. This is, they add, the first time a near-infrared emission spectrum has been obtained for an extrasolar planet. The observations were made possible because the orbit of HD 189733b is tilted edge-on to the Earth. This means that it is eclipsed by its parent star every 2.2 Earth-days. This allows the astronomers to effectively subtract the light of the star from the light from the planet and the star by measuring spectra before eclipse and during totality when the planet is hidden. "The carbon dioxide is the main focus of the excitement, because that is a molecule that under the right circumstances could have a connection to biological activity as it does on Earth," Swain says. "The very fact that we're able to detect it, and estimate its abundance, is significant for the long-term effort of characterizing planets both to find out what they're made of and to find out if they could be a possible host for life." Tinetti adds that, "In the terrestrial planets of our solar system, carbon dioxide plays a crucial role in the stability of climate. On Earth, carbon dioxide is also one of the ingredients of photosynthesis and a key element in the carbon cycle. Our observations represent a great opportunity to understand the role of this gas in the atmospheres of hot-gaseous and highly irradiated planets". The NASA/ESA/CSA James Webb Space Telescope launches in 2013, astronomers hope to use the same approach to analyse spectroscopically biomarkers on exoplanets of similar size to the Earth. Related links:
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