First smaller-than-Earth exoplanet discovered

Chuck Bednar for – @BednarChuck

A planet located approximately 200 light-years from our solar system has been identified as the first alien world to be smaller than Earth in terms of both measured mass and size, according to a study published in a recent edition of the journal Nature.

The discovery was made by researchers from the Pennsylvania State University Department of Astronomy, the NASA Ames Research Center, the SETI Institute, and the University of Chicago Department of Astronomy and Astrophysics. They measured the size and mass of the Mars-sized planet Kepler-138b, an extrasolar planet in orbit around a red dwarf star.

According to, since Kepler 138b is about the same size as Mars, and Mars is just 53 percent as big as Earth, the new planet must be smaller than our home world. Furthermore, they found it to have a mass of about 6.7 percent that of Earth and two-thirds that of Mars, and is also the smallest exoplanet ever to have its density measured.

More about Kepler-138b and its sister planets

The new study, led by Penn State University astronomer Daniel Jontof-Hutter, looked at a total of three planets in orbit around a cold red dwarf, Kepler-138. It’s a cold, dim star located in the constellation Lyra, and is located roughly 10 million times further away from Earth than our sun, Jontof-Hutter said.

Two of the planets orbiting Kepler-138, Kepler-138c and Kepler-138d, are about 1.2 times the width of Earth, while Kepler-138b is slightly more than half the width of Earth. All three exoplanets orbit their star closely. Kepler-138b takes a little more than 10 days to complete one orbit, while Kepler-138c needs nearly 14 days and Kepler-138d requires approximately 23 days.

Using the NASA Kepler spacecraft, they were able to examine the relationship between gravity and the length of their orbits, and since they knew the strength of a planet’s gravitational pull is directly related to its mass, they were able to determine each planet’s size, the website explained. In addition, after Kepler-138b’s mass and width, they were able to determine its density, which is approximately two-thirds that of Mars and indicates that it is a rocky planet.

Because of its proximity to its host star, Kepler-138b is believed to be too hot to retain liquid water, as are its sister planets. In fact, Jontof-Hutter said the outermost of the three worlds could experience surface temperatures of 250 degrees Fahrenheit (120 degrees Celsius), while the innermost planet likely sees temperatures of up to 610 degrees F (320 degrees C).


Follow redOrbit on Twitter, Facebook, Google+, Instagram and Pinterest.