Although most of the planet candidates cataloged by NASA's Kepler space telescope have diameters in the range of 2 to 4 times that of the Earth, little Kepler-138 b is only about the size and mass of Mars.
Kepler-138 b is one of three planets orbiting the red dwarf star Kepler-138, which is located 200 light-years from Earth in the constellation Lyra. The gravity of Kepler-138's three planets pulls them into slightly different positions than they would occupy otherwise. This enables scientists to calculate the mass of the planets.
The innermost planet is Kepler-138 b, which has a density about the same as the rocky planet Mars. The middle planet, Kepler-138 c, has a density about the same as Earth, and the outermost planet (Kepler-138 d) is less than half as dense, indicating a large proportion of light materials such as water and hydrogen. [The Smallest Known Alien Planets in Pictures]
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Karl's association with Space.com goes back to 2000, when he was hired to produce interactive Flash graphics. From 2010 to 2016, Karl worked as an infographics specialist across all editorial properties of Purch (formerly known as TechMediaNetwork). Before joining Space.com, Karl spent 11 years at the New York headquarters of The Associated Press, creating news graphics for use around the world in newspapers and on the web. He has a degree in graphic design from Louisiana State University and now works as a freelance graphic designer in New York City.