Mars-sized exoplanet is smallest to have its mass measured

The smallest exoplanet to be weighed is now a Mars-sized world 200 light-years away in the constellation Lyra. Kepler 138b is nearly half the diameter of Earth with about 7 percent of its mass, researchers report in the June 18 Nature. Unlike Earth and chilly Mars, however, 138b is roasted to just over 300° Celsius by its dim, red sun: The exoplanet swings around its star in a little over 10 days.

Astronomers calculated 138b’s mass by measuring how strongly the planet and two others in the Kepler 138 system tug on one another gravitationally. Kepler 138b’s mass and size also reveal that the planet’s density is about 2.6 grams per cubic centimeter — similar to that of pure rock. 

Editor’s note: The distance to Kepler 138 and temperature of Kepler 138b were updated on June 17, 2015 to reflect more recent estimates.