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May 6th, 2012

steuard: (physics)
Sunday, May 6th, 2012 10:30 pm
One of the things that still blows my mind about our studies of planets around other stars (beyond the simple fact that we can see them at all) is that astronomers are actually able to figure out the composition of their atmospheres. That's possible when a planet passes between us and its star: it creates a tiny shadow that dims the star's light ever so slightly, and an even tinier fraction of that light passes through the planet's atmosphere. So if the spectral pattern of the star's light changes when the planet is in the way, we can analyze the changes to tell us what's its air is made of.

We think we understand the technique pretty well, but it would be great if we could test it out on a planet whose atmosphere we already understand. If only there were some known planet expected to pass between us and its star, we could point the Hubble at it and check this calculation against the known result. Well, hey! The planet Venus is going to transit across the sun on June 5th (the next time will be in 2117: watch it (carefully), and take your kids!). Only one problem: pointing the Hubble straight at the sun would destroy its sensitive optics (much like staring at it with unprotected eyes: be careful!).

So what are they going to do instead? Point the Hubble at the moon. The idea is that studying the much weaker reflected light of the sun (and briefly, of a tiny bit of the atmosphere of Venus) will be a decent test of those models. As long as they take a careful sample of the reflected spectrum for many hours before the transit, they can get an accurate baseline reading. Then by taking equally careful (and lengthy) measurements during the transit, they can measure the difference when Venus is present. If all goes well, the measurements will yield the same "no life on this planet!" signal that we've already established by looking straight at it.

Studying Venus by staring at the moon: crazy, but awesome. (Here's the original article: Hubble to Use Moon as Mirror to See Venus Transit.)