Researchers have weighed a planet orbiting a distant star by measuring the starlight passing through its atmosphere. The technique could accelerate the hunt for Earth-like worlds. Knowing the mass of an extrasolar planet, or exoplanet, is an important step in determining whether it is rocky, and thus potentially home to life. To find that mass, astronomers usually rely on radial-velocity measurements, in which tiny wobbles in a star’s orbit reveal the gravitational tug (and hence mass) of an orbiting planet. Radial velocity works best for big planets that orbit bright, steadily shining stars, so exoplanet scientists would like to have methods that they can apply to other planets around other types of stars, including fainter ones. In a paper appearing today in Science1, Julien de Wit, a planetary scientist at the Massachusetts Institute of Technology in Cambridge, and his supervisor, Sara Seager, report that they can calculate a planet’s mass by analysing the spectrum of its star’s light.