I know I said my next post would be on quantum mechanics, but I figured I'd cover what happened today first.

As you've probably heard, today marked the n$^{th}$ transit of Venus across the sun. (In fact, it's still going on right now if you're in a part of the world where the sun is still up.) Some folks in the MIT Planetary Science department put together an event to check out the transit from the top of building 37. Unfortunately, being in Massachusetts means Massachusetts weather, and we didn't have any clear sky to actually see the transit. Looks like I'll just have to wait another 105 years to see it in person.

However, not all was lost. Before the big event, there was a great talk on exoplanets given by one of Sara Seager's post-docs. Unfortunately, I don't remember his name and can't find it anywhere. Maybe a commenter could let me know?

Now, first things first: an exoplanet is just a planet that orbits some star other than our own. It turns out that we now know of over 700 exoplanets and have several thousand potential candidates for objects that we think are probably exoplanets, many of which are probably earth-sized. Turns out we're not so alone after all.

So, how does one go about finding these exoplanets? It's difficult to look for them directly- planets tend to be small and generally don't emit their own light. One way to do it indirectly is by watching for transits! You look at a given star, and you see if the flux (intensity) of light coming from the star drops for some time and then goes back to normal. This indicates that something has temporarily passed in front of it, blocking the light, and that thing is likely a planet. This gives you information about the ratio of the size of the planet vs. the star, and using other techniques (such as absorption spectroscopy, described below) to figure out the size of the star gives you their absolute sizes.

There's also another way to look at this. Imagine that we're looking at a planet that orbits behind a star. Before the planet goes out of sight behind the star, it looks bright since it's reflecting some of the starlight back towards us. This extra flux from the reflection off of the planet "disappears" (as far as we're concerned) when the planet passes behind the star, and then "reappears" as it comes back into our line of sight. We therefore get a similar effect- a temporary drop in the flux coming from the star.

You can get a long way with this, but let's say that we're even more ambitious. Say we wanted to figure out whether those planets could support life. An important criterion to consider is the composition of the atmosphere. So how do you figure out what an atmosphere is made of? It turns out that different gasses absorb light of different frequencies. For instance, N$_2$ (molecular nitrogen) has absorption line at ~2300 cm$^1$. This means that if we look at light that has passed through a bunch of N$_2$ gas, we will see barely any with wavelength 2300 cm. Since chemists have boldly mapped out the absorption spectrum of every gas that you would expect to find in an atmosphere, this allows us to determine the atmospheric content of planets and stars that are hundreds of thousands of light-years away. For exoplanets, all we need to do is take measurements of the way the flux changes when the exoplanet goes through a transit at a bunch of different wavelengths, and we have accurate information about what those planets are like!

I say "all we need to do" as if this were a trivial task. Of course, actually performing these measurements and sifting out the data from hundreds of thousands of stars is a monumental effort. But that's why we've got many so smart astronomers working on the job. Science prevails!

P.S. It turns out Sara Seager is an advisor for this new company called Planetary Resources. They want to mine asteroids. Do I see some unobtanium in our future? http://astrobites.com/2012/05/19/discussing-planetary-resources-with-sara-seager/

$^1$. Molecular Absorption spectra: http://www.coe.ou.edu/sserg/web/Results/results.htm