Kepler 10c: An Unexpected Heavyweight Earth

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Artist concept of exoplanet Kepler 10c (David Aguilar/Harvard-Smithsonian Center for Astrophysics)
Artist concept of exoplanet Kepler 10c (David Aguilar/Harvard-Smithsonian Center for Astrophysics)

How big can an Earth-like planet be? Astronomers thought they had a pretty good handle on this question but have just been given a fresh example for how nature never ceases to outpace our imaginations and show us something unexpected.

That example is Kepler 10c, an extrasolar planet astronomers didn't think could exist: a heavyweight "Earth" two-and-a-half times larger and 17 times more massive than our own welterweight home world.

Kepler 10c was originally spotted in the data from NASA's Kepler spacecraft, the most productive extrasolar planet hunter to date. Its diameter was measured to be 2.3 times that of Earth's, but at the time its mass was unknown. Common wisdom in the planetary formation community was confounded when later observations with the HARPS-North instrument at the Telescopio Nazionale Galileo on the Canary Islands' La Palma discovered that Kepler 10c weighs in at 17 times the Earth's mass.

Before this discovery, planets with diameters between 1.7 and 3.9 that of Earth were classified as "gas dwarfs": planets expected to have a heavy rocky core surrounded by an accumulated thick atmospheric envelope, more like a mini-Neptune than a maxi-Earth. But Kepler 10c's calculated density pegs it as a rocky world like Earth: mostly solid, perhaps with a thin coating of atmosphere.

It was believed that such a massive solid planetary body would have developed a very thick sheath of gases during its formation, gravitationally snowballing to become a Neptune or even Jupiter-sized gas giant.


Beyond the commotion of the upset of conventional planetary formation theory, this heavyweight Earth opens up a lot of possibilities to the imagination. Science fiction stories have mused about the idea of high-gravity planets where its characters strain under their own weight just to move around.

A quick high school physics calculation shows that the surface gravity of Kepler 10c would be about 3.2 times what we're used to. Imagine the exercise you would get just walking around: myself, I would be lugging around almost 700 pounds!

Kepler 10c also has over five times the real estate of Earth, even when counting Earth's solid surface and oceans--a lot more room for people to spread out in. Land might be a lot cheaper.

But there's a hitch to anyone thinking of opening a gym or flipping real estate: Kepler 10c is very close to its star, making a complete orbit in only 45 days. This means it is a hot, giant heavyweight world: almost 1400 degrees Farenheit! (Might be a good place to open a health spa.)

So, Kepler 10c is definitely a horse of a different color. To date, 1,794 exoplanets have been confirmed to exist, most of which fall into the larger "ice giant" (like Uranus and Neptune) or "gas giant" (Jupiter, Saturn) categories. With more recent discoveries of smaller planets that fall into categories like gas dwarf, super-Earth, Earth and sub-Earth sized, we may find that the possible characteristics of planets is even more diverse than what Kepler 10c has pushed us to imagine.

Right now the scientific puzzle astronomers have to solve is how Kepler 10c developed into what it is today: new heavyweight record-breaking rocky planet champion of all time -- for now….