Two new planets orbiting other stars have recently been discovered using NASA's orbiting Kepler telescope. And while every new planet discovery is pretty amazing, normally two more add to the hundreds already confirmed wouldn't really be newsworthy. However, these two weren't discovered by professional astronomers! They were found by members of the Planet Hunters "citizen scientists" team; regular folks who have volunteered to sift through data returned by the observatory in hopes of finding far-flung worlds. One of the planets found orbits its star with a period of just under 10 days, and the other orbits a second star in just under 50 days. Both are much more massive than Earth; the first is 2.65 times and the second over 8 times our diameter. The relatively lower mass means the first one might be rocky (as opposed to a gas giant) but the short period means it's hot, far hotter than Earth. Both planets transit their stars as seen from Earth. In other words, they pass directly in front of their stars from our point of view, blocking the light a wee bit. This drop can be measured, and the planet detected. By knowing how big the star is (a dwarf, a giant, whatever) the period of the planet can be found, and the size of the planet can be determined by how much light is blocked, too. The Kepler observatory is staring at about 100,000 stars all the time to look for these mini-eclipses, and astronomers use a fleet of software to automatically tag suspicious changes in starlight. But it's pretty hard to look through all the potential planet data, and that's why Planet Hunters was set up: let people go through the data themselves, using their keen eyes and powerful brains to look for anything that might be a planet. And it worked! The two planets discovered were just announced in a paper led by the Kepler team (PDF). Here's a plot showing one of the transits:

That's the data for KIC 10905746, the 2.65 Earth-mass planet with the 10 day orbital period. The top half shows the brightness as measured by Kepler; the star is a variable star which means it changes its own intrinsic brightness with a period of every few days. That makes this a difficult target! The red lines mark the transits spotted by the planet hunters; you can see where the brightness dips more than usual. Hard to see, aren't they? But once you get the period of the planet, you can then "fold" the data, cutting it up into time intervals based on that period. Observations that were taken at different times -- but show the planet in the same position relative to the star (for example, every time the planet is right in the middle of the transit) -- can then be added together, cleaning up the noise. That's in the bottom half, which shows all the transits stacked up (and with the star's own changes mathematically removed). The dimming of the star's light is much more clear -- the red line is the best fit to the event. But note the scale: the planet only blocks 0.2% of the star's light! It's pretty amazing that we can see it at all. Remarkably, both stars in this case were flagged as potential planet-bearers by the software, but also removed from the list to make followup observations by the same code! In the case of the star shown, it's because the star was thought to be a giant near the end of its life, making followup observations and analysis too difficult. However, the human brain is pretty good at his sort of thing, and several people on the Planet Hunters site picked out the planetary transit. I'll note that the planet orbiting the binary star announced last week was also spotted by a Planet Hunter! The Kepler team had already spotted it as well, so they get the credit, but still. It doesn't take a professional to find even the really weird planets out there. Pretty cool, and good indicator that this citizen scientist project has a bright (or very slightly dimmed, I suppose) future ahead of it. And you can still participate! All you have to do is go to the Planet Hunters website, sign up, and get cracking. Who knows? Maybe you'll find another world... ... and I have to wonder. An Earth-like planet orbiting a Sun-like star will have a very weak transit. A computer would have a very hard time picking that out of the data, but we humans are pattern-finding machines. Will the first exoEarth be found by a professional astronomer, or instead by some science enthusiast who decided one day to check out this Planet Hunters thing...? Image credit: transit art: ESO/L. Calçada

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