The past few years have seen the discovery of dozens of planets outside the solar system. Most of them have been gas giants, like Jupiter or Saturn. But a few have been rocky planets like the Earth or Mars. However, even those planets have been much larger than Earth. For example, Kepler-22b, which was recently found in the “habitable zone” around its star, is about twice the size of the Earth.
However, NASA announced today that its Kepler mission has discovered two (probably) rocky planets that are about the same size as the Earth. Both planets are in the Kepler-20 solar system, which has five total planets and is about 1,000 light-years away. Kepler-20e is slightly smaller than Venus, while Kepler-20f is slightly larger than Earth, measuring 1.03 times its radius.
Neither planet, alas, would be a good place to build a summer home. The surface temperatures of Kepler-20e average out at about 800 degrees, and on Kepler-20f things are kept at a toasty 1400 degrees.
That doesn’t take away from the significance of the finding, though. As the lead author of the study, Francois Fressin, stated in the NASA press release, “This discovery demonstrates for the first time that Earth-size planets exist around other stars, and that we are able to detect them.”
The goal of the Kepler mission is to discover Earth-sized planets that are in orbit in the habitable zone of their stars. At this point, the mission has now found a planet in the habitable zone, and two planets the size of Earth. I’m willing to bet that it’s only a matter of time before we find a planet that meets both criteria.