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Astronomers discover new mega-Earth planet

An artist concept shows the Kepler-10 system, home to two rocky planets.
An artist concept shows the Kepler-10 system, home to two rocky planets.
Harvard-Smithsonian Center for Astrophysics/David Aguilar

On June 2, astronomers announced the discovery of a new type of planet: a mega-Earth.

This type of planet is a terrestrial planet that is larger than the super-Earth planets which have already been found. Scientists believed that such a world would attract hydrogen and become a gas giant like Jupiter or Saturn, but Kepler-10c appears to have disconfirmed this hypothesis.

“We were very surprised when we realized what we had found,” says astronomer Xavier Dumusque of the Harvard-Smithsonian Center for Astrophysics (CfA), who led the data analysis and made the discovery.

“This is the Godzilla of Earths!,” adds CfA researcher Dimitar Sasselov, director of the Harvard Origins of Life Initiative. “But unlike the movie monster, Kepler-10c has positive implications for life.”

Kepler-10 is a star that is 173 parsecs away from Earth in the constellation Draco that has 0.900 times the mass of the Sun and 1.060 times the radius of the Sun. Its mean surface temperature is estimated to be 5,627 K, while the mean surface temperature of the Sun is 5,778 K. The spectral classification of Kepler-10 is G5V, which is barely dimmer and redder than the G2V spectral classification of the Sun. The Kepler-10 system is about 11 billion years old, compared to the Sun's age of about 4.6 billion years.

The two planets that have been discovered have been given the names Kepler-10b and Kepler-10c. Kepler-10b is notable for being the first terrestrial planet discovered in the Kepler data.

Kepler-10c, the first mega-Earth discovered, has an estimated mass of 17 Earth masses and an estimated radius of 2.227 Earth radii, giving it an estimated density of 8.5 g/cm^3, which is 54 percent more dense than Earth, the densest planet in our solar system. These values give Kepler-10c a surface gravity of about 34 m/s^2, so a person who weighs 200 pounds on Earth would weigh 686 pounds on Kepler-10c. It is 0.2407 AU away from its star, and completes an orbit of Kepler-10 once every 45.29485 Earth days. For comparison, Mercury is 0.387 AU away from the Sun on average, and orbits the Sun once every 87.969 Earth days. The black-body temperature of Kepler-10c is 485 K or 212°C, which consistent with a high baking temperature inside an oven. High pressure could make life as we know it possible on this planet, as liquid water can exist at temperatures up to 647 K under pressure, but there would be no liquid water present with Earth-like atmospheric pressure. The actual temperature of any planet will be warmer than its black-body temperature due to albedo, atmosphere, and internal heating effects.

Until recently, the radius of Kepler-10c was known but its mass was unknown. This is because NASA's Kepler spacecraft finds planets using the transit method, in which the dimming of a star's light as one of its planets passes between it and Kepler is measured. By measuring the amount of dimming and the time of transit, astronomers can calculate the planet's radius. However, this method cannot determine the mass of a planet, meaning that astronomers cannot tell what type of planet they have found by the transit method alone.

The radial velocity method is necessary to determine the mass of a planet. The researchers used the HARPS-North instrument on the Telescopio Nazionale Galileo in the Canary Islands to do this with Kepler-10c. The result showed that Kepler-10c must be a rocky planet rather than a gaseous planet, despite its large size.

“Kepler-10c didn't lose its atmosphere over time. It's massive enough to have held onto one if it ever had it,” explains Dumusque. “It must have formed the way we see it now.”

Lars A. Buchhave, another CfA astronomer, found a correlation between the period of a planet and the size boundary between terrestrial and gaseous planets. The farther away a planet is from its star, the larger it can grow before becoming a gaseous planet. This means that as astronomers search for longer-period exoplanets, they are likely to find more planets like Kepler-10c.

Kepler-10c also alters theories of planet formation because of its age, only 2.7 billion years younger than the Big Bang. At that time, the universe had more hydrogen and helium with less heavier elements that are necessary for forming terrestrial planets. This suggests that heavier elements were either more common in the early universe than previously thought, or that terrestrial planets can form more readily than previously thought.

“Finding Kepler-10c tells us that rocky planets could form much earlier than we thought. And if you can make rocks, you can make life,” says Sasselov.

“Just when you think you've got it all figured out, nature gives you a huge surprise – in this case, literally,” said Natalie Batalha, Kepler mission scientist at NASA's Ames Research Center in Moffett Field, California. “Isn't science marvelous?”

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