The Kepler planet hunting space telescope has done pretty well finding planets that exist in the "habitable zone" around alien stars, but so far, all of these planets have been significantly larger than Earth. Today, NASA announced that Kepler has discovered the first Earth-sized planets orbiting another star.
We've got a couple different ways of spotting planets orbiting around other stars. One is through the little gravitational "wobble" that an orbiting planet will cause in its parent star. But the smaller the planet is, the less noticeable is the wobble, and for a planet that's about the same mass as Earth, the wobble is too small for us to detect.
The other method that we use (specifically, what Kepler uses) is to watch for changes in the brightness of a star as a planet orbits around it. If a planet gets between a star and Kepler's sensors, the spacecraft can detect that tiny little drop in brightness as the planet blocks out some of the star's light, and the exact amount that the star dims can clue us in to the size of the planet.
NASA announced today that Kepler has managed to spot telltale dimming around a star about 1,000 light years away called Kepler-20, suggesting that there are not one, but two planets out there close to the same size as Earth. The star boasts a total of five planets, and two of them, Kepler-20e and Kepler 20-f, are 0.87 times the radius of Earth and 1.03 times the radius of Earth, respectively. It's just like home! Except for the bad news, that is.
The bad news is that these planets are both way, way too close to their parent star, and if you set foot on either one you'd find yourself turned into a charred cinder faster than you could say "wow it's hot." So that's no good. The reason that this is such an exciting discovery, though, is that for the first time we've got both pieces of the puzzle: exoplanets in the habitable zone, and exoplanets the size of Earth. What this means is that it's just a matter of time before we find an alien planet with the magical combination of the right size and the right place: Earth 2.0.