Because we live on a small world, it's natural to think intelligent life elsewhere does, too, but small planets orbiting other stars are tough to study. Now, for the first time, astronomers have measured the approximate mass and density of an extrasolar planet smaller than Earth. Earlier work indicated the planet blocks so little of its sun's light—a red dwarf named Kepler-138—that it must be only the size of Mars. In this artist's conception, the small world is in the foreground at left, its position shifted by gravitational tugs from two other planets, one of which is shown at lower right. As astronomers report online today in Nature, this gravitational interaction reveals that the little world is roughly 6.6% as massive as Earth—intermediate between Mercury (5.5%) and Mars (10.7%)—and roughly 2.6 times denser than water. That's two-thirds as dense as Mars, which might suggest it has more rock and less iron than the Red Planet. In fact, the numbers are so uncertain that no firm conclusion about its composition is yet possible, but future observations should yield tighter constraints. In any event, the planet lies so close to its sun—revolving every 10.3 days—that its climate probably resembles hostile Mercury's, making it an unlikely abode for alien life.