Only a small number of worlds around other stars look anything like Earth: roughly the same size and at the right distance from their star for liquid water to be present. But are these Earth-like exoplanets really made from the same sort of stuff—a rocky surface, an iron core, and just a dash of water? A study presented here today at a meeting of the American Astronomical Society suggests that exoplanets, at least up to 1.6 times the mass of Earth, follow pretty much the same recipe as our home. So if we’re looking for life out there, we can probably ignore anything bigger than that.
NASA’s Kepler satellite has detected the greatest number of exoplanets. It detects them by the dip in brightness they cause when they pass in front of their parent star. This decrease allows researchers to deduce the diameter of the planet but not its mass. Measuring mass requires an entirely different technique. As an exoplanet orbits its star, its own gravity makes the star wobble back and forth very slightly—the heavier the planet, the greater the wobble. Astronomers can measure the wobble by measuring the frequency of a star’s light very accurately with an instrument called a spectrograph. When the star is wobbling away from Earth its light is stretched out and reduced in frequency; when it moves toward us, the light is bunched up and its frequency increases.