For the first time ever, astronomers have measured the size and mass of an exoplanet smaller than Earth.

A team of astronomers from Penn State, NASA Ames, the University of Chicago, and the SETI Institute have measured the mass of a Mars-sized exoplanet located 200 light-years away. That’s astonishing, particularly considering that this planet, called Kepler-138b, has a mass that’s just 6.7% that of the Earth. The new study now appears at the science journal Nature.


Kepler-138 is a red dwarf encircled by at least three planets. The outer two planets, Kepler-138c and Kepler-138d, are are slightly larger than Earth (~1.6 Earth radii) and too close to their host star to be habitable. Previous studies measured the masses of the two outer planets, but the new work incorporated additional Kepler data, enabling measurement of the mass of the Mars-sized inner planet while improving the mass measurements of the outer planets.

Penn State astronomer Daniel Jontof-Hutter explained how they did it in a statement:

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. We also measured the gravity of all three planets, using data from NASA’s Kepler mission, by precisely observing the times of each transit. 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.

Once astronomers know the mass and size of an exoplanet, they can calculate its density and bulk composition. In this case, Kepler-138b appears to be rocky, but more work will be required to confirm this.

“Kepler-138 b is roughly 3,000 times less massive than the first exoplanet whose density was measured 15 years ago,” added co-author Eric Ford, also from Penn State. “We now are working to discover and characterize rocky planets in the habitable zones of nearby stars.”

Related: This is how you weigh objects in space

This plot shows the masses and sizes of the smallest exoplanets for which both quantities have been measured. The solar system planets (shown in red) are for comparison. (Caption and image credit: NASA Ames/W. Stenzel)

“These results demonstrate the rapid progress of exoplanet science and the enduring value of data collected by the Kepler mission,” said F0rd.


Read the entire study at Nature: “The mass of the Mars-sized exoplanet Kepler-138 b from transit timing”.

Contact the author at and @dvorsky. Top image: Artistic impression of the Kepler-138 system. The sizes of the planets have been exaggerated. Credit: SETI Institute/Danielle Futselaar