On December 5th, 2011, the Kepler Science team announced that they had confirmed a planet roughly the size of ours, in orbit around a star very similar to the Sun.
What's more, the distance of this planet from its star was perfect. It wasn't too close, and it wasn't too far away.
If this planet were the Earth, life could live there.
It's called Kepler-22b.
The parent star, Kepler-22 is very similar to our Sun. Classified as a sun-like G-type star, it is very dim as seen from Earth and measurements of dips in light as the planet passes in front are very difficult to make.
Nevertheless, the Kepler Space Telescope first noticed this star as having a planet that is possibly like Earth almost immediately. A mere three days after being declared operationally ready, Kepler took notice that something was interesting in orbit around this small, dim, Sun-like star located 600 light years away.
Followup observations on the ground and from the Spitzer Space Telescope confirmed the planets existence.
Kepler-22b, the first Earth-sized planet found by Kepler in a habitable zone around another star, has been confirmed. Of the 54 habitable zone planet candidates currently under scrutiny by the Kepler Space Telescope, this is the first to be confirmed.
Kepler-22b has a radius 2.4 times that of the Earth, putting this planet in a category commonly called a super-Earth. It takes 290 days to travel once around around its sun, which is slightly smaller and cooler than ours.
The promise of Kepler-22b, however, is tempered by what we do not know about it. There is much hidden from us. The most glaring gap in our knowledge is that we don't know what it's made of. It could be a rocky planet like Earth, or it could be gaseous and more like Neptune.
It may even be covered entirely of water, an ocean planet with an altogether new set of possibilities for life.
What we do know is that while Kepler-22b is Earth-like, it is hardly our twin. At 2.4 times the radius of the Earth, if it is rocky and very dense, then life could be very hard here. We also do not know if it even contains an atmosphere.
And if it does have an atmosphere, how much heat does the atmosphere trap? If it traps too much, then Kepler-22b could be more like Venus, a planet known to be very inhospitable to life.
If it's a Neptune-like planet, made entirely of gas, then the chances for life, at least as we know it to be, are impossible. Living here, any life would take on a completely unexpected form.
The chances for finding out the answers to many of these questions about Kepler-22b, while possible, are somewhat remote with our current levels of technology. For example, using radial velocity measurements, a spectral technique that allows us to find out chemical compositions of stars, planets and their atmospheres, in addition to the planet itself by measuring the wobble of the star as the planet circles it, is not feasible for a star so far away with such a small planet in orbit around it.
Perhaps the biggest promise offered by the confirmation of Kepler-22b isn't the planet itself, but lies in the fact that it was found so soon after we started looking for planets like the Earth. With Kepler, the number of Earth-sized candidate planets has increased 200 percent, and the number of super-Earth candidates has increased 140 percent since February 2011. This bodes very well indeed for our future search for life in the galaxy.
The discovery of Kepler-22b as an Earth-like planet in a habitable zone is encouraging, but that by itself offers limited hope in our search for life elsewhere in our galaxy. So far, all we know is that Kepler-22b is a smallish planet in orbit around a star like our Sun at a distance that is very promising.
It made be rocky, like the Earth, or it may be gaseous, like Neptune, but for us to know the true promise of Kepler-22b, we need to keep looking. Keep collecting information with as many instruments as we can bring to bear.
Because it's what we DON'T know about Kepler-22b that offers us the most promise.
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