Size and mass of small exoplanet measured

For the first time, scientists have managed to measure the size and mass of an exoplanet smaller than Earth, paving the way for new understanding of the diversity of planets in the Universe.


Plot showing the masses and sizes of the smallest exoplanets for which both quantities have been measured. Credit: NASA/Ames W Stenzel


Researchers have measured the mass and size of an exoplanet smaller than Earth for the first time.

Called Kepler-138b, the Mars-size exoplanet is about one tenth the mass of Earth and 50 per cent the size.

Normally, the mass of a planet is measured by observing the star that it orbits and watching for tiny movements as that planet’s gravitational pull causes it to wobble.

However, for such a small exoplanet like Kepler-138b, a different method was required.

Kepler-138b is one of three planets observed orbiting star Kepler-138.

A study to measure the mass of all three was led by Daniel Jontof-Hutter, research associate at the Pennsylvania State University’s Center for Exoplanets and Habitable Worlds.

The masses were determined by observing the planets as they passed in front of, or transited, the star. This is done by measuring the change in speed as each planet slows down and accelerates due to the gravitational tug of its neighbours.

The size of each planet is then measured by the transit method, whereby the team observes how much of the star’s light is blocked as one of the orbiting planets passes by.

Having measured both the mass and the size, the team was then able to calculate the density and bulk composition and infer whether each planet was made mostly of rock, water or gas.

The team found Kepler-138b’s density to be consistent with a rocky planet like Earth or Mars, but further observations will be required in order to confirm this.

Star Kepler-138 is less than half the size of the Sun and about 30 per cent cooler.

The Kepler-138 system is found about 200 lightyears from Earth in the direction of the constellation Lyra.

Its outer planets, Kepler-138c and Kepler-138d, are about the size of Earth.

It is thought that Kepler-138c is rocky, whereas Kepler-138d is less dense and so does probably not have the same composition as Earth.

But researchers say all three are orbiting too close to the star for liquid water to exist on the surface and support life.

“The substantial difference between the densities of the two larger planets tells us that not all planets similar to Earth in size are rocky,” says Jack Lissauer, co-author and planetary scientist at NASA’s Ames Research Center in Moffett Field, California.

“Further study of small planets will help provide more understanding of the diversity that exists in nature, and will help determine if rocky planets like Earth are common or rare.”

Data from the study will contribute towards NASA’s future Transiting Exoplanet Survey Satellite, which will seek to identify patterns in the relationship between the mass and size of exoplanets.


This, in turn, will contribute to astronomers’ understanding of the history of Earth and other planets in the Solar System and provide a basis for more far-reaching searches in future.