Monthly Archives: December 2011

Earth Sized Planets!

Kepler20.jpgKepler just won’t quit!  Just after discovering a slightly-larger-than-Earth planet in the Habitable Zone, Kepler now announces a pair of actually-Earth-sized planets.  These planets are in very, very short period orbits (6 and 20 days), and so sit very, very close to their parent star (Mercury orbits the Sun every 88 days).  They’re super hot!

This bodes very well for Kepler‘s ultimate goal — combining the features of the Kepler-20 and Kepler-22 systems:  Earth sized planets in the Habitable Zone, maybe even with giant planets further out (or closer in?).  It will take a while – these planets were discovered despite their very small size because they orbit so frequently that Kepler could see them transit their parent star many, many times, and so the signal built up over time.  A planet in the Habitable Zone will only transit 3-7 times over the course of the mission, and so the analysis is much, much harder, especially for such small planets.
Unfortunately, we’re in the middle of a big upgrade to exoplanets.org and our undergraduate maintainers Katherina and Eunkyu are away for the holidays, so Kepler-20 may have to wait a little while before it appears in the Exoplanet Orbit Database.  Soon though!

Kepler-22b: A 2.4 Earth-radius Planet in the Habitable Zone of a Sun-like Star

Pop the champagne!  

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The Kepler mission always promised to find rocky planets in the Habitable Zones of Sun-like stars with the transit method.
This means that it watches around 100,000 stars very carefully from space, waiting to see if any of them get briefly dimmer due to the passage of a planet in front of it.  If the star has a close-in giant planet, the dimming is more likely (the alignment has to be just right for this to work), frequent (the planet will go around quickly), and more easily detected (the planet blocks a lot of light from the star).  If the planet is small and far away from its central star then the dimming is less likely, infrequent, and very hard to detect.  Kepler can barely detect a planet like Earth going around a star like the Sun — but it can do it!
Kepler-22 b is bigger than Earth — it has around 2.3 times the Earth’s radius.  We don’t know what it is made of or what its composition is, but at that size there is nothing like it in the Solar System.  It could be a big ball of rock, a huge “water world” with oceans hundreds or thousands of miles deep, or maybe even a smaller rocky planet with an enormous atmosphere of hydrogen, water and other gasses.  We don’t know because it is too small for us to detect with radial velocities, so we cannot measure its mass.  The Kepler team has done an enormous amount of work to “validate” this planet, which is their jargon for proving that the signal is really coming from a planet, and not from any of hundreds of possible other sources (the most common such “false positive” is a grazing eclipsing binary pair of stars that happen to appear right next to a brighter star, which “dilutes” the eclipse signature into what looks like a planetary transit).
Kepler-22 b sits in the “Habitable Zone” of its host star, which is a G star like the Sun.  This means that it probably has a surface temperature compatible with liquid water, if it has a solid surface at all.  This doesn’t mean we know we could live there, but it is the first planet we’ve found that we might be able to live on.  This concept of the Habitable Zone was first rigorously developed by Penn State University professor James Kasting.

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Bill Boruki, the Kepler Principle Investigator, pointed out how lucky we are to have this system now.  The planet takes almost a year to orbit its star (it’s a lot like Earth that way, and it takes so long because it is in the Habitable Zone — if it were closer to the star it would orbit more quickly).  In general Kepler needs to see a planet transit twice to measure the orbital period (the period between transits) and then a third time to confirm that the signal is real (it has to happen again at precisely the first period or it’s not actually a planet).
  
But the Kepler mission has only been up for around 2 or 3 years, which means that the first transit happened almost right away —  in fact just 3 days into the mission!  This made the 3rd transit occur with as soon as possible, given the period, which is why this is the first discovery announced.  There will be more.
A few months back co-I Dimitar Sasselov gave a TED talk that caused quite a stir.  He implied that Kepler had already discovered 140 “Earth like” planets and that they were in the process of finding which ones were Habitable.  Dr. Sasselov was careful to point out that this was a statistical result — he couldn’t point to any particular planet as being “Earth like”, but the fact that there were so many detections and Kepler was so good was a sign that eventually there would be a lot of discoveries.  He was saying that the writing was on the wall, and it was good news.
The reaction was confusing and mixed.  One website declared “Kepler Co-Investigator Spills The Beans: Lots of Earth-like Planets.” Since the Kepler team had not validated any of these planets, they had not made a big announcement about them, but some venues took this TED talk in the UK to be NASA’s big “announcement” about Kepler.   NASAwatch.com asked “Is the Kepler team hiding something? Why is Sasselov talking about data that the Kepler team said that they did not want to discuss yet?”  Dr. Sasselov and Kepler issued a clarifying statements, which only seemed to make some venues strangely irate.
With that backdrop, think about Kepler-22 b.  Kepler saw this planet only 3 days after it began its mission.  It would have been immediately flagged as a planet candidate by the team.  It may even have been among the 140 candidates in Dr. Sasselov’s talk.  In some sense, this planet was “discovered” almost 3 years ago, but it has taken that long to find this needle in a haystack among many, many other, similar signals.  Of course, NASA hasn’t been hiding anything — nature has!  Data analysis is hard.
The discovery and rapid announcement of Kepler-22 is a tribute to the hard work of the Kepler team.  With a few more years of data, the Kepler team should be able to validate a planet that is no larger than earth in its Habitable Zone — a virtual Earth twin (that will be around 6 times harder than this one!).  Astronomers worldwide will have to do a huge amount of work to learn what that planet is made of, whether it has water on its surface, and what it might be like there, but at least we’ll know where to look!
Disclaimer:  I have no affiliation with Kepler or any pertinent nonpublic knowledge about its misison.