Astronomers using data from NASA’s Kepler mission have determined the sizes and masses of three planets orbiting a star much fainter and cooler than the Sun, known as Kepler-138, including the Mars-size exoplanet Kepler-138b.
The artistic concept shows the planetary system Kepler-138 harboring the Mars-size planet Kepler-138b Kepler-138b. The sizes of the planets relative to the star have been exaggerated. Image credit: SETI Institute / Danielle Futselaar.
Determining the size of an Earth-size exoplanet by the amount of starlight it blocks hundreds of light-years away once was the realm of science fiction.
Measuring the mass of such a small planet based on its gravity was another level entirely, but a team of astronomers led by Dr Daniel Jontof-Hutter of Pennsylvania State University has done just that for an exoplanet 50% the size of Earth.
Called Kepler-138b, it is the first exoplanet smaller than Earth to have both its mass and size measured.
This exoplanet is the innermost of three planets that orbit Kepler-138 (also known as KOI-304), located in the constellation Lyra, approximately 200 light-years away.
Dr Jontof-Hutter and co-authors determined that the mass of Kepler-138b is 0.066 Earth masses and its density is 2.6 grams per cm3.
The outer two planets, Kepler-138c and Kepler-138d, are both slightly larger than Earth.
Kepler-138c’s density (6.2 grams per cm3) is similar to that of Earth, and Kepler-138d is less than half as dense at 2.1 grams per cm3, implying that it contains a greater portion of low-density components such as water and hydrogen.
“The substantial difference between the densities of the two larger planets tells us that not all planets similar to Earth in size are rocky,” said Dr Jack Lissauer of NASA’s Ames Research Center, a co-author on the paper published in Nature.
“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,” he said.
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Daniel Jontof-Hutter et al. 2015. The mass of the Mars-sized exoplanet Kepler-138 b from transit timing. Nature 522, 321–323; doi: 10.1038/nature14494
