Alpha Centauri is the closest star system to our own — a bit more than four light-years away. And a few years ago, it became even more intriguing when astronomers in Switzerland announced the discovery of a planet orbiting one of its stars. Since then, other astronomers have found that if the planet is real, it’s no more than a few times as massive as Earth. Most of the planets discovered in other star system are more massive than Earth, because big planets are easier to find. But it’s the smaller ones — like Earth — that are most likely to host life. Alpha Centauri consists of three stars, and the astronomers discovered the possible planet around Alpha Centauri B, the second-brightest member of the system. The astronomers detected a tiny shift in the star’s motion toward and away from us. They attributed that shift to the gravitational pull of a small planet. But measuring the planet’s exact mass depends not just on the star’s back-and-forth shift. Astronomers also must know the tilt
