The Kepler mission, a NASA project to find and profile planets outside of our solar system, has announced the discovery of two potentially habitable exosolar planets.
The planets are part of a system, Kepler-62, which is thought to contain five roughly earth-sized planets. The biggest is almost twice earth's size; the smallest slightly more than half.
That alone is notable, because the normal way the Kepler mission identifies planets is by measuring the gravitational they exert on their star. A big enough planet will pull the star slightly closer to the earth when it's on one side, and slightly further when it's on the other. That causes a minute fluctuation in the brightness of the star, measured from our planet, which the Kepler orbital observatory can sense.
But only the largest gas giants have such an effect, and while they are noteworthy finds in themselves, they aren't habitable. To find smaller planets, the mission looks at stars which have other fluctuations in light – due to planets passing in front of them. They then have to model every possible reason why those fluctuations could occur, and hope that they find that the most likely cause is exoplanets traversing the star.
The planets which they've found this way aren't just earth sized, though. Two of them, each measuring around 1.5 times the size of earth, are roughly the same distance away from their star as we are. Their orbits take a third and two thirds of an earth year each, but, because their star is less bright than our sun, they receive 1.2 and 0.4 times the light, respectively, that we do.
That will equal one hot planet and one cold one – but either of them might be in the so-called "habitable zone", where liquid water can exist. And liquid water is the only universal prerequisite we know for life.
The authors note that their method cannot tell if the planets are even rocky, as opposed to gas giants, let alone whether they actually have an atmosphere or water. But they are some of the best candidates we've found to date.
And crucially, we've found them fast. The Kepler telescope has been in orbit for around half its expected life, but it's already produced a vast amount of data to crunch. It's discovered almost 3,000 possible exoplanets, and has already found one that could have water. If the hit rate stays high, there could be many more.