Hubble spies carbon dioxide 600 trillion kilometers away!

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Dec 9, 2008
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Astronomers using the Hubble Space Telescope have, for the first time, detected carbon dioxide in the atmosphere of a planet orbiting another star.

Artist drawing of the planet and its star
Artist's conception of the gassy planet. Click to embiggen.

This is pretty astonishing! Here's the scoop:

CO2 absorbs and emits light at very specific wavelengths in the infrared. Theses wavelengths are like fingerprints, indicating the existence of the gas. Using a near-infrared camera on Hubble, a team led by astronomer Mark Swain (I went to grad school with him! Nice work, Mark!) took data of the planet HD 189733b, which orbits its parent star every 2.2 days. They got a spectrum of the star and the planet at the same time, and then waited until the planet was behind the star and got a spectrum of just the star by its lonesome. By subtracting the star's spectrum from the star+planet spectrum, they got the spectrum of just the planet itself.

That's how they saw the light emitted from the CO2. Now, that sounds easy enough, but in practice this is incredibly difficult and detailed work. I've done work like it, and I'm here tell you this is quite an achievement. It's the first time it's ever been done. As a bonus, they also detected carbon monoxide.

Mind you, this planet is 63 light years away -- that's over 600 trillion kilometers (about 380 trillion miles)!

Almost certainly, these gases are not biotic; that is, weren't created by life. Even though the star is a bit cooler than the Sun, the planet is less than 5 million km (3 million miles) from the star, so it's hot. Most likely, the gases are abiological in origin, but this new result has interesting implications for life. If we do spot a planet that is more suitably situated for life, we might be able to detect the presence of gases indicating that biology is going on. We'll need better telescopes and all that, but think of this current work as a proof-of-concept.

As it happens, methane and water vapor have been detected in this planet's atmosphere as well. Again, they are probably not biological, but this shows that we are getting pretty good at picking through the compositions of alien worlds... and that in this case the planet must have a whopping greenhouse effect going on. And it's not like it needs it, that close to the star.

This is a pretty cool discovery. Along with the other gases detected, and the direct images we've made of planets around other stars, this is a fantastic time to be studying planets. Even ones a long, long way away.

Image credit: ESA, NASA, M. Kornmesser (ESA/Hubble) and STScI