Astronomers measure first Mars-sized exoplanet

Scientists have measured the mass and size named of the first exoplanet smaller than the Earth named Kepler-138b.

The Mars-sized planet has been orbiting a red dwarf star about 200 light years from our solar system, and 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.

Daniel Jontof-Hutter, a research associate in astronomy at Penn State who led the study which also involved astronomers at NASA Ames Research Center, the SETI Institute, and the University of Chicago, said that each time a planet transited the star, it blocked a small fraction of the star's light, which allowed them to measure the size of the planet.

He added that they 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 Kepler-138b is less than 1/10th an Earth mass and half the size of the Earth. In kilograms, Kepler-138b is 4 followed by 23 zeros (4 times 10 to the power of 23 kilograms). In kilometers, its diameter is about 6,600.

According to coauthor Jason Rowe the measurements were consistent with a variety of compositions, and favor compositions that were mostly rocky.

Eric Ford, coauthor of the study added that the results demonstrate the rapid progress of exoplanet science and the enduring value of data collected by the Kepler mission.

The paper is published in the journal Nature.