Kepler 78-b is our closest twin yet -- at least in terms of size and mass.
Kepler-78b: Too close for comfort. (Art by Karen Teramura at UHIfA)
Scientists following up on data from the Kepler planet-hunting telescope have identified Earth’s closest twin yet—at least in terms of size and mass. Measuring only 1.2 times the radius of Earth, Kepler -78b is now the smallest planet for which we also have a mass: about 1.7 times Earth’s. The two planets have roughly the same density, which means Kepler-78b is probably made of rock and iron too.
That’s pretty much where the similarity ends, though. Kepler-78b orbits perilously close to its host star—so close, in fact, that its year lasts only 8.5 days, and surface temperatures are some 2,000 degrees hotter than they are here. No water, no life, and no good explanation—at least not yet—for how such a small planet could end up so close to its star. According to current theory, the star should have been two to three times bigger than it is today when the planet formed. But if -78b started off in its current location, “the planet’s orbit would be inside the star itself,” which is clearly ...