UNIVERSITY PARK, Pa. -- A team of astronomers has measured the mass and size of a Mars-sized planet orbiting a red dwarf star about 200 light years from our solar system. The planet, named Kepler-138b, is the first exoplanet smaller than the Earth to have both its mass and its size measured. A paper by the team, which includes astronomers at Penn State, NASA Ames Research Center, the SETI Institute and the University of Chicago, will be published in the journal Nature on June 18.
Planet Kepler-138b is one of three planets that orbit the star Kepler-138 and that pass in front of it on every orbit as viewed from Earth -- a maneuver that astronomers call a transit. "Each time a planet transits the star, it blocks a small fraction of the star's light, allowing us to measure the size of the planet," said Daniel Jontof-Hutter, a research associate in astronomy at Penn State who led the study."We also measured the gravity of all three planets, using data from NASA's Kepler mission, by precisely observing the times of each transit," Jontof-Hutter said. The astronomers also were able to measure the masses of these planets. "Each planet periodically slows down and accelerates ever so slightly from the gravity of its neighboring planets. This slight change in time between transits allowed us to measure the masses of the planets," Jontof-Hutter explained. After measuring both the mass and size of an exoplanet, astronomers then can calculate its density and its bulk composition.