Discovery of Mega-Earth Exoplanet!

Kepler-10c, also known as Mega-Earth, is 18,000 miles in diameter and 2.3 times as large as Earth. It appears to be as solid as the planet beneath our feet.
Kepler-10c dominates the foreground in this artist’s conception. Its sibling, the lava world Kepler-10b, is in the background. Both orbit a sunlike star. Kepler-10c, also known as Mega-Earth, is 18,000 miles in diameter and 2.3 times as large as Earth. (Image Credit: Harvard-Smithsonian Center for Astrophysics/David Aguilar)

Move over, Kepler-186f! A new exoplanet has stolen the spotlight! Dubbed the “Mega-Earth,” Kepler-10c is a rocky planet similar to Earth but 17 times more massive. The finding was presented in a press conference today at the 224th meeting of the American Astronomical Society. Dimitar Sasselov, director of the Harvard Origins of Life Initiative, in a written statement said, “This is the Godzilla of Earths!”

Kepler-10c was discovered in 2011 by NASA’s Kepler Space Telescope. Kepler finds planets by using the transit method, which involves looking for stars that dim when planets pass in front of them. By measuring the amount of dimming, astronomers can calculate the planet’s physical size or diameter. Kepler, however, is unable to determine a planet’s mass or composition.

Scientists at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts have now pinned down the mass of Kepler-10c, which is 17 times that of the Earth. The exoplanet orbits a sunlike star that is about 560 light years away from Earth in the constellation Draco. Its sister planet, Kepler-10b, was the first rocky planet found beyond the solar system and has a remarkably fast, 20-hour orbit around its star.

Computer models predicted that the atmospheres of such massive planets would resemble those of Jupiter, Neptune, and other gas giants because these types of planets would have enough gravity to collect vast amounts of hydrogen and helium. However, upon closer inspection, Kepler-10c has been found to be a rocky planet. “Kepler-10c is a big problem for the theory,” Sasselov told Discovery News.

So why exactly is Kepler-10c a terrestrial planet rather than a gas giant? This amateur scientist took to Twitter to find out!

Recent research conducted by astronomer Lars A. Buchhave (also presenting at AAS) and published in Nature proposes a correlation between the period of a planet (the length of time it takes to orbit its star) and the size at which a planet transitions from rocky to gaseous. This finding suggests that more mega-Earths will be found as astronomers extend their data to longer-period orbits. Additionally, Buchave’s data shows that the farther a planet is from its star, the larger it can grow before accumulating a thick atmosphere and turning into a gas dwarf. Kepler-10c orbits its host star every forty-five days at a quarter of the average distance between the Sun and Earth, both of which may contribute to its unique size and composition. Furthermore, since a star and its planets form from the same disk of material, the metallicity, or metal content, of a star reflects the composition of its planets. Buchave explains,

“It seems that there is a ‘sweet spot’ of metallicity to get Earth-size planets, and it’s about the same as the Sun. That makes sense because at lower metallicities you have fewer of the building blocks for planets, and at higher metallicities you tend to make gas giants instead.”

The discovery of Kepler-10c also has profound implications for the history of the universe and the search for extraterrestrial life. The Kepler-10 system is about 11 billion years old, which means it formed less than 3 billion years after the Big Bang. The early universe only contained hydrogen and helium. Heavier elements needed to make rocky planets, like silicon and iron, had to be created in the first generations of stars. When those stars exploded, they scattered these crucial ingredients through space, which then could be incorporated into later generations of stars and planets.

This process was predicted to have taken billions of years. However, Kepler-10c shows that the universe was able to form such huge rocks even during the time when heavy elements were scarce. Therefore, very old star systems should not be dismissed in the search for habitable planets. Sasselov says,

“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.”


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