Carbon dioxide detected in extrasolar gas giant

NASA has announced another milestone in the study of extrasolar planets: the detection of carbon dioxide in the atmosphere of an extrasolar gas giant. Using the Hubble Space Telescope’s near infrared camera and multi-object spectrometer, astronomers at the Jet Propulsion Laboratory were able to resolve the infrared spectral signature of CO2 in the atmosphere of HD 189733b, a gas giant orbiting a star 63 light-years away. This is a hot Jupiter-style planet, meaning a gas giant that orbits its primary very closely – in this case with a 2.2 day orbit. Hot Jupiters are fairly common, and probably result from the gas giant spiraling inwards from an unstable orbit… which would likely destroy any closer terrestrial worlds that were there to begin with. That’s bad news for life in this one particular star system, but the discovery of CO2 proves that we now have a sound method for detecting the gas around other worlds.

Here’s an example of how useful this information could be, even for a gas giant that itself is unlikely to support life. Earlier observations of HD 189733b also detected methane (CH4) and water vapor in its atmosphere. In our own solar system Jupiter has an atmosphere of mostly hydrogen and helium, with trace amounts of water vapor and methane but virtually no carbon dioxide. Jupiter’s atmosphere has very little oxygen bound up inside its atmospheric molecules, but HD 189733b appears to have significantly more oxygen atoms in its bulk composition. This is good, because an oxidizing planet can only form in an oxidizing solar nebula, and such an oxygen-rich setting for planet formation will tend to form more molecular water. Whatever terrestrial planets circle the star HD 189733a probably are swimming in water, literally, as long as they survived the transit of HD 189733b from an outer system giant to an inner system bully. The odds aren’t good. But elsewhere in systems where water-bearing gas giants circle stably and far from their star, water-bearing and potentially life-bearing inner worlds are very likely to exist.

Now we’ve seen CO2, H2O, CH4 and sodium (a still earlier study) in the atmospheres of alien gas giants. The list of found gases is filling up. I wonder how long before we can add O2 or O3 to that list? If we can, and we don’t detect those molecules in the atmosphere of some hot Jupiter – which could evolve molecular O2 or ozone via photolysis of CO2 under intense stellar irradiation – we’ll have likely found that big, flashing “LIFE” sign everyone is looking for.


~ by Planetologist on December 20, 2008.

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