Why Exoplanet Kepler 10c Was Downgraded From 'Mega-Earth'

An artist's conception of the Kepler-10 star system — An artist's conception of the Kepler 10 star system, located about 608 light-years away from Earth, near the Cygnus and Lyra constellations. NASA/Ames/JPL-Caltech/T. Pyle

Discovered by NASA's Kepler space telescope in 2011, Kepler 10c stands out as a unique member of the exoplanet family. (Exoplanets orbit stars outside the solar system and Kepler planets are those discovered by the Kepler space telescope.) It was originally hailed as a "mega-Earth" because of its size and the belief that it was solid. (In order to sustain life, a planet must be made of rocks, rather than gas). Although it seemed to defy conventional wisdom, it was later determined that this Earth-like planet wasn't so "Earth-like" after all.

When Was Kepler 10c First Discovered?

Astronomer Xavier Dumusque of the Harvard-Smithsonian Center for Astrophysics made the discovery in 2011. His team used the HARPS-North instrument on the Telescopio Nazionale Galileo in the Canary Islands to measure the mass of Kepler 10c and determined it to be a whopping 17 times the mass of Earth. This made it one of the heaviest rocky exoplanets ever discovered. Scientists had expected a planet of that size to become a Jupiter-like gas giant, so they were astounded that the planet was solid.

Dumusque presented his findings in 2014 at a meeting of the American Astronomical Society. There was a lot of excitement that old stars— like Kepler, which is 11 billion years old — could host rocky Earths, therefore increasing the chances of finding potentially habitable worlds. In fact scientists declared Kepler 10c to be the first in the new category of "mega-Earth" because it was even bigger than the "super-Earths" which average up to 10 times the size of Earth. (Super-Earths refers to the planet's size, not necessarily that the planet is Earth-like.)

However, further analysis has changed our view of Kepler 10c.

What Do We Know About Kepler 10c Today?

Kepler 10c is located approximately 608 light-years from Earth in the constellation Draco. Analysis in 2017 showed the planet was closer to about seven Earth masses, rather than the 17 previously thought. This revised mass is within the threshold where planets do not become gas giants. Kepler 10c was determined to be not a rocky planet, but rather a typical volatile-rich planet. This means it has life-essential elements, the main one being water, in this case.

Kepler 10c is in the middle of three stars that orbit the host star Kepler 10. The other two are Kepler 10b and Kepler 10d. Kepler 10b is a rocky planet with a mass three times that of Earth and an orbital period of less than one Earth day. Kepler 10d has an orbital period of 151 Earth days and a minimum mass about 13 times that of Earth. Kepler 10c orbits its parent star Kepler 10 every 45 Earth days.

The host or parent star, Kepler, is about 10 percent smaller than our sun, but older and cooler, at 11 billion years old and with a temperature of 9,668 degrees F (5,353 C). By contrast, the sun in our solar system has a surface temperature of 10,000 degrees F (5,500 degrees C) and 4.5 billion years old.

Is Kepler 10c Habitable?

Kepler 10c has an estimated temperature of 592 F (311 C), almost four times hotter than Jupiter and much too hot to sustain life. Lacking a solid surface would also be a strike against it being able to support life.

Further study on this planet and others in its system will be needed to gain insights into this complex exoplanet.