3 February 2011

Planets, Planets Everywhere!

Posted by Ryan Anderson

For there is a single general space, a single vast immensity which we may freely call Void; in it are innumerable globes like this one on which we live and grow.

– Giordano Bruno, 1584

It’s looking more and more like Bruno was right. Yesterday the Kepler Space Telescope released its second batch of data, revealing an astounding 1235 new exoplanet candidates!

Artist's rendition of the view from a habitable exomoon.

For the uninitiated, Kepler is a space telescope whose whole purpose is to stare at the same patch of sky for several years, non-stop. Unlike other space telescopes like Hubble, the goal with Kepler is not to take pretty pictures. The goal is to carefully monitor the light of 156,000 stars. Stars vary in brightness all the time with the comings and goings of starspots and flares, but Kepler is watching for dips in brightness that repeat at a regular interval, hinting that something may be orbiting the star.

Planetary systems are typically flat like a CD, and the only way that we can see a planet blocking the light from its star is if its orbit is perfectly edge-on to our line of sight. As you can imagine, this is pretty unlikely. That’s why Kepler is monitoring so many stars, and it seems to have payed off.

The data released yesterday are from the first 4.5 months of observations, and already Kepler has detected 1235 possible new exoplanets! In most of these cases, follow-up observations have to be made by Kepler and other telescopes to be sure that the dip in brightness seen is due to a planet and not, say, a starspot or a dim stellar companion. Still, with this many candidates, it looks like Kepler has opened the exoplanet floodgates.

Even better, we can get a lot more information about transiting exoplanets than we can about those detected just by watching the wobble of their parent star. Since transiting planets actually pass in front of the star, we can get an estimate of their radius and their mass, which leads to an estimate of their density and therefore composition. 68 of the new candidate planets appear to be Earth sized, 288 are super-Earth sized, 622 are more like Neptune and 165 are Jupiter-sized. Even better, 54 of the possible planets orbit in their star’s habitable zone, and some of the stars observed had whole systems of planets. One star, dubbed Kepler-11, has at least six planets orbiting very close to it. If they were somehow transplanted to our solar system they would be closer to the sun than Venus.

A comparison of the sizes of planets discovered by Kepler. Credit: NASA/Tim Pyle

And remember, these results are just from the first 4.5 months of observations! For all the potential planets found so far, there are sure to be hundreds or thousands more with more distant orbits that have not yet brought them around to block their star’s light. The avalanche of new exoplanets is just beginning. Kepler is only looking at a small patch of the sky – about the size of your fist held at arm’s length – and it has already identified 1235 potential exoplanets. Since Kepler can only see planets in systems that just happen to be edge-on, those 1235 planets are probably only 1% of all the solar systems out there. Just doing a back of the envelope calculation, if Kepler is seeing only 1% of planetary systems out there, then there are around 8 planets for every ten stars. In a galaxy like the milky way with 200 billion or so stars, that works out to at least 160 billion planets. Only one of the 1235 candidate Kepler planets so far is both Earth-sized and in the habitable zone, but if you scale that up, it gives 128 million habitable Earth-like planets.

And that’s just extrapolating from the 1235 candidates found so far.

Kepler’s mission will continue for another 3+ years, and Kepler’s results are going to lead to a surge in other exoplanet-related science. Suddenly theorists will have dozens of solar systems to try to simulate, not just ours. Observers will be busy trying to confirm Kepler’s detections, searching for the faint signatures of even-smaller planets, and making even more difficult measurements, like looking for evidence of moons around the exoplanets or detecting the composition of the exoplanet atmospheres based on how they absorb starlight. All of that work is going to be fascinating and mind-blowing, but for now it just makes me happy to look up at the sky and know that the galaxy is full of worlds – both like our own and totally alien – and that we are finally finding them.

A map of the exoplanet candidates in Kepler's field of view. Credit: NASA/Tim Pyle