Starlight used to determine distant planet’s mass

Nature : To date, the method used to weigh a distant planet involves measuring its radial velocity—the gravitational pull it exerts on its star; the pull appears as a tiny wobble in the star’s orbit. But that method generally works only on large planets orbiting bright stars. To find smaller exoplanets near fainter stars, Julien de Wit and Sara Seager of MIT have developed a technique that uses a planet’s transmission spectrum, or the tiny amount of starlight detected around the planet’s perimeter as it transits in front of its star. As the light passes through the planet’s atmosphere, its spectrum is altered and can yield clues to the planet’s atmospheric properties and mass, from which other characteristics can be determined, such as whether the planet is a gas giant or rocky and suitable for life. Because the technique requires the fine measurement of minuscule amounts of light, it could work especially well with the newer generation of instruments being planned, such as the James Webb Space Telescope, set to launch in 2018.