Hubble smelt who dealt it

Contributed by
Mar 19, 2008
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Scientists have used Hubble to detect methane in the atmosphere of a planet orbiting another star for the first time and to confirm the existence of water vapor there as well.

This is not an easy thing to do! The planet is so close to its star that they can't be separated in an image, and the star totally overwhelms the light from the planet. However, when the planet is between the star and us (think of it as a tiny and very distant eclipse) the starlight passes through the planet's atmosphere, which absorbs some of the light. Methane is very good at sucking down infrared light in fact, so when you look in the infrared you see the star looking a bit dimmer.

If you take a spectrum -- break the light up into individual colors -- you can get pretty detailed information about the starlight. In this case, the absorption by methane in the planet's atmosphere is pretty clear.

A spectrum is like a fingerprint: it identifies the element or molecule, how much of it there is, and even in some cases its temperature. The spectrum obtained using Hubble indicates the presence of methane and water vapor.

This is an important first step in understanding exoplanetary atmospheres, especially since we don't know all that much about these giant Jupiters that orbit close in to their parent stars. In this case, the planet, called HD 189733b, is about 63 light years away, and orbits its star in just about two days. We don't expect there to be life on it -- the tops of the clouds are at about 1700 degrees Fahrenheit -- but just finding these two molecules means they must be abundant in other planetary systems.

Now here's a dash of some slightly chillier water on these findings. First, they aren't surprising: I'd be far more surprised if we didn't find these molecules in the planet's atmosphere. We've seen plenty of both water and methane in space; in fact I worked on spectra of a brown dwarf that showed water vapor (literally steam in this case) in its atmosphere -- plus, the water vapor was previously detected using Spitzer (the telescope, not the governor). Methane is a very simple molecule, and should form anywhere there is carbon, hydrogen, and low enough temperatures so that the molecule doesn't break apart.

To me, the import of these findings is that with equipment we already have, we can probe the atmosphere of a planet over 600 trillion kilometers away. That's cool. That implies very strongly that with the next generation of space and ground-based telescopes being built now, we'll soon have an even better glimpse into the air out there.

And yes, I know that methane is not what makes, um, human emissions smell bad, but I couldn't resist the title.