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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.)
Monday, May 7th, 2012 04:20 am (UTC)
That was awesome, thanks for sharing.
Monday, May 7th, 2012 04:33 am (UTC)
That is awesome! At One Hundred Fun Hundered this weekend, the kids spent quite a bit of time at a model showing this effect. They had a nice booth set up where the kids could click "capture data" on a computer, then turn a crank to move some planets around a light bulb and between the bulb and a photon detector, showing instant results on the computer.

--Beth
Edit: fix link quotes
Edited 2012-05-07 04:33 am (UTC)
Monday, May 7th, 2012 07:49 pm (UTC)
I take it the spectrographic instruments on SOHO (http://en.wikipedia.org/wiki/Solar_and_Heliospheric_Observatory) aren't up to the task?
Monday, May 7th, 2012 08:07 pm (UTC)
That's an interesting question. My first guess is that using a telescope like Hubble (and the instruments it has) would be a closer match to the sort of data that they'd get from actual extrasolar planet observations. If anything, I'd guess that SOHO's instruments might be too good: it's designed to study a star right up close, in ways that just aren't available when you're looking many light-years away.
Monday, May 7th, 2012 08:32 pm (UTC)
This is, as everyone else has said, totally awesome.

Did you get the almost-but-not-quite Peril Sensitive Sunglasses from Mudd for the event? I'm wondering how well they'll work. Hope the weather is decent enough to find out. Given the (PST) times listed for the event in the Mudd flier, it looks like the transit will start with the sun already getting low in the sky out here on the East coast...

(Also, woo, excellent use for my science/astronomy user icon!)
Tuesday, May 8th, 2012 01:51 am (UTC)
I do in fact have the Sunglasses of Omnipresent Lethal Peril from Mudd. (Kim and I each have a pair, in fact.) They block light awfully well: I've gotten a good look at the very dim looking filaments of a few (bright!) light bulbs, for instance.

We won't be able to see the whole transit from here (I hear that even the folks in Alaska are recommending that people go all the way to Hawaii instead), but several professors here are getting together to host a public viewing of the first hour or two. We've got a solar filter for our 8" telescope, and we're going to see if we can project it on a screen somehow (or if everyone will need to take turns at the eyepiece). This assumes that it's not overcast, of course.