The Half-Astrophysicist Blog

Kepler-22b: Not Quite Earthlike Yet!

Yesterday, the Kepler team announced the discovery of a new planet: Kepler-22b. It has been widely reported in the media as

This artist's conception illustrates Kepler-22b, a planet known to comfortably circle in the habitable zone of a sun-like star. Image credit: NASA/Ames/JPL-Caltech

Earth-like, capable of supporting life and even one story speculating if it could be our future home (I do not vouch for this being a reliable site).

So, what was really found. First, a review of how Kepler works. Kepler is basically staring at one area of the sky and monitoring the brightnesses of about 150,000 stars. If a planet is orbiting the star, there is a chance it will line up and pass directly in front of the star. Kepler will see the star get a bit dimmer for a little while (usually on the order of hours) as the planet transits (passes in front of) the star. When we say a bit dimmer, it means only one part in 10,000 or so, not very easy to detect which is one of the reasons this is a space based mission. From this information, you can tell if there is a planet there. Based on how long the transit takes, you can get an estimate of the radius of the planet (but not its mass…that requires follow up spectroscopy where you measure the stars wobble as the planet orbits it).

In order to qualify as a planet candidate, you need to see the star dim three times. First time, you know something happened but it might not have been a planet. Second time, pretty much same thing. Third time, you can say you have a planet candidate if the time between the first and second transit is the same as the time between the second and third transit. This pattern indicates the star is dimming with a regular period just like you would expect if a planet was orbiting it.

Kepler got lucky with this planet. Its first transit was detected only three days after Kepler started taking science data. The next one, 290 days later and the third, another 290 days later. The planet, dubbed Kepler-22b was found about as quick as it could be since its first transit was detected almost immediately (imagine if you missed it by one day…then you would have had to wait another 290 days! It’s almost certain there are planets that were just missed).

So Kepler-22b orbits the star every 290 days. It’s a little closer to its star than Earth is to the Sun, but that’s okay since the star is a little smaller and not quite as bright. It sits in what is commonly called the habitable zone: a place where a planet similar to Earth could have liquid water on its surface.

There’s the catch: Similar to Earth. Kepler-22b is about 2.4 times the diameter of Earth, so quite a bit bigger (that’s almost 14 times the volume!) It’s not clear if the planet is predominantly rocky like Earth. The planet could have a very thick or non-existent atmosphere. It could rotate way too fast or way too slow (resulting in a hot side and a frozen side).  It could have its axis of rotation tipped on its side like Uranus. There are lots of ways it could be far from an Earth-like planet!

Of course this just gets astronomers excited to study it further. We can determine its mass by measuring the Doppler shift it imparts on the star as the planet orbits and hence determine if it is a rocky planet like Earth. Advanced spectroscopic techniques can be used to examine the planets atmosphere. These further studies (which you can bet are already being planned) will start to unravel some of the mysteries of Kepler-22b. I think Kepler Program Scientist Douglas Hudgins says it well, “This is a major milestone on the road to finding Earth’s twin.”

Lost in this hoopla is that they also announced over 1,000 new planet candidates (and that it looks like about 99% of their candidates are turning out to be planets, so they have a pretty good batting average).

The good news is that even if Kepler-22b turns out to be not quite so habitable, there are a lot more in the pipeline awaiting confirmation (looks like around 140 more, give or take a few based on where you draw the line for the habitable zone). These all need to be followed up and maybe in the next few years, we will nail down that true Earth-like planet.

December 6, 2011 - Posted by | Exoplanets, Kepler

1 Comment »

  1. Nice summary. I hope to live long enough to see a future space telescope actually resolve what one of these new worlds looks like visually.

    Comment by Bill | December 22, 2011 | Reply


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