Monday, December 5, 2011

Kepler Finds A Possibly Live One

The Kepler mission has been looking for planets that might have liquid water (and therefore life as we know it) and it looks like they have their first confirmed candidate - the oh-so-memorably-named Kepler 22b.  Kepler 22b, or just 22b for short, has a radius 2.4 times as big as the Earth's and orbits a star that has roughly 2% less mass than the Sun every 289 days. That places it right at the inner edge of that star's habitable zone (the range of distances from their star where planets might have liquid water), as illustrated by this lovely NASA graphic.

If you assume an Earth-like greenhouse effect for 22b (which is a big assumption considering Venus and Mars have drastically different atmospheres than Earth), the mean surface temperature (assuming it has a surface) would be a balmy but not unreasonable 22 degrees Celsius compared to Earth's mean surface temperature of about 14 degrees Celsius.

NASA likes to call this type of planet a "Super-Earth", however that's something of a misnomer as planets with more than twice the radius of Earth probably aren't primarily rocky planets like Earth, Venus, and Mars but rather more like smaller, defrosted versions of our solar system's ice giants, Neptune and Uranus.  Using planetary structure models, one can map out the range of possible compositions for a planet of a given radius (remember with Kepler's transit data they know the planet's size but not it's mass).

As you can see, 22b likely has a composition with significant amounts of hydrogen and helium, which means it may have a very thick, deep atmosphere. Alternatively it could have very large amounts of water, but at this point there's just no way to tell what it's made of as the planet is too far from it's star to be detected using the radial velocity technique, which can determine a planet's mass.  It's possible that space telescopes like Hubble and Spitzer might be able to get some information on the composition of the planet's atmosphere, but most likely this one is going to have to wait for new telescopes and instruments to be characterized more fully.

The best part is that 22b is not alone.  The Kepler team only officially announces a planet as discovered when they can confirm it using another telescope (here they used Spitzer to verify a transit), but the list of "planet candidates" in habitable zones is growing.  As of the now, there are about a half-dozen planet candidates in habitable zones that are smaller than 22b. 

As Kepler approaches it's third anniversary, expect those numbers to increase dramatically.  You can already see the trend by comparing the numbers of planets from the June 2010 (in blue), February 2011 (in red), and December 2011 (in yellow) data releases.

You can find the official NASA press release here and the slides from that press release (which are the source of these lovely images) here.


  1. This is really cool stuff.  I hope to see a day where we discover life on another planet. :)

  2. Yes, in all likelihood, Kepler-22b is a "warm Neptune" with a thick gaseous atmosphere and no solid surface. It's important to note that the moniker "super-Earth" is just a code name for the rough size and does not imply that we have any information on the planet's composition. I actually kinda liked how the NASA graphics sort of suggested this by making the picture look somewhat enigmatic (can't tell if its all atmosphere or if there's a continent down there). 

    The popularity of this story just goes to show you what the words "habitable zone" will do... we've discovered gas giants in the habitable zone before (which probably got their fanfare at the time) and given that Kepler-22b is certainly smaller, though probably not qualitatively different, means that it's actually not that new of a discovery in some sense. As Nick pointed out, though, Kepler will surely deliver more and more interesting planets along these lines. The "candidates" are generally 80-90% true planets, so its practically certain that we've got plenty of nice small temperate worlds out there. 

    You mentioned measuring the mass with the radial velocity (Doppler wobble) technique, which would indeed be great. However, it's not plausible for the vast majority of Kepler targets: Kepler's goal was to get a good estimate of the frequency by detecting large numbers of worlds in a small patch of sky, which requires observing around stars that are too faint for radial velocity (and Hubble and Spitzer can't say anything about the atmospheres either). The opposite extreme, covering the whole sky to finding the brightest, most interesting targets is the goal of the proposed TESS mission. 

    It turns out, the best way to measure the mass of faint Kepler planets is through a different technique where you detect the gravitational influence between planets in a multiple planet system (as most exoplanetary systems are). These is known as transit timing variations. These are often weak and sometimes only an upper limit mass is derivable, but the nice thing is that all you need is Kepler data and patience (as it can take years for the signals to evolve to the point where a unique mass measurement is possible). We also need NASA to decide to fund Kepler through an extended mission, which is actually going to be a harder sell than you might imagine since the budget is so tight. 

    Anyway, I'm on the Kepler group working on these and we're excited about the possibilities, though frankly a true Earth twin (habitable zone 1 Earth mass planet around a Sun-like star) is most likely out of reach. However, we'll measure masses for several (maybe dozens) of planets in the small Earth-like size regime and perhaps from that and from what we learn about planet formation we'll be able to reasonably extrapolate to some of these habitable-zone Earth candidates. 

    Anyway, feel free to contact me (e-mail in my CV) if you have any Kepler questions. Also keep your eyes open for more Kepler announcements at the Texas AAS meeting in a few weeks...

    Darin Ragozzine

    PS I'm a satisfied lurker on theeternaluniverse and hope you all keep up the good work!

  3. Darin,

      Thanks for the comment.  By the way, you really need to "leak" some great rumor from Kepler as a test to follow how internet rumors spread.  Let me suggest some of the following tactics:

    1.  Write a fake letter in the Nature style with a title and abstract stating an earthsize planet in a habitable zone with a rocky surface a similar atmpophere has been found and print it at a printer accessible to many people at the CFA and just leave it there a couple days and see what happens.  

    2.  Then make some anonymous comments on key places on the internet swearing the same thing.

    3. Okay I give up, but it would be great to see how to start a great rumor and track how it builds once started.


To add a link to text:
<a href="URL">Text</a>