Astronomers find a new type of planet: The 'mega-Earth'

Astronomers find a new type of planet: The 'mega-Earth'
The newly discovered 'mega-Earth' 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 has a diameter of about 18,000 miles, 2.3 times as large as Earth, and weighs 17 times as much. Therefore it is all solids, although it may possess a thin atmosphere shown here as wispy clouds. Credit: David A. Aguilar (CfA)

Astronomers announced today that they have discovered a new type of planet - a rocky world weighing 17 times as much as Earth. Theorists believed such a world couldn't form because anything so hefty would grab hydrogen gas as it grew and become a Jupiter-like gas giant. This planet, though, is all solids and much bigger than previously discovered "super-Earths," making it a "mega-Earth."

"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."

The team's finding was presented today in a press conference at a meeting of the American Astronomical Society (AAS).

The newfound mega-Earth, Kepler-10c, circles a sunlike star once every 45 days. It is located about 560 light-years from Earth in the constellation Draco. The system also hosts a 3-Earth-mass "lava world," Kepler-10b, in a remarkably fast, 20-hour orbit.

Kepler-10c was originally spotted by NASA's Kepler spacecraft. Kepler finds using the transit method, looking for a star that dims when a planet passes in front of it. By measuring the amount of dimming, astronomers can calculate the planet's physical size or diameter. However, Kepler can't tell whether a planet is rocky or gassy.

Kepler-10c was known to have a diameter of about 18,000 miles, 2.3 times as large as Earth. This suggested it fell into a category of planets known as mini-Neptunes, which have thick, gaseous envelopes.

The team used the HARPS-North instrument on the Telescopio Nazionale Galileo (TNG) in the Canary Islands to measure the mass of Kepler-10c. They found that it weighed 17 times as much as Earth - far more than expected. This showed that Kepler-10c must have a dense composition of rocks and other solids.

"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."

Planet formation theories have a difficult time explaining how such a large, rocky world could develop. However, a new observational study suggests that it is not alone.

Also presenting at AAS, CfA astronomer Lars A. Buchhave found a correlation between the period of a planet (how long it takes to orbit its star) and the size at which a planet transitions from rocky to gaseous. This suggests that more mega-Earths will be found as planet hunters extend their data to longer-period orbits.

The discovery that Kepler-10c is a mega-Earth also has profound implications for the history of the universe and the possibility of 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 contained only hydrogen and helium. Heavier elements needed to make , 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 should 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.

"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.

This research implies that astronomers shouldn't rule out old stars when they search for Earth-like planets. And if old stars can host rocky Earths too, then we have a better chance of locating potentially habitable worlds in our cosmic neighborhood.

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Jun 02, 2014
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Jun 02, 2014
~3.2 Earth Surface gravities. It's a tad denser than Earth on the whole...

Haha. Would be hell to live on that thing. Men typical would weigh 500 to 700 lbs.

Moral of the story is if there is enough material around, you can apparently get a planet of any size and composition conceivable to those conditions.

I wonder if we'll ever discover a "Rock Star"...

Get it? Rock Star.

I'd say that would be a star-sized object formed of rocky-metallic remnants of another super-nova, but not large enough to form a neutron star from those remnants...

What would it look like in a telescope? A red dwarf? Would it be capable of sustaining fusion of heavy elements?

Jun 02, 2014
Yeah, it's actually ~40% denser than Earth's average density, which suggests an enormous amount of iron or nickel, or maybe even Lead in the interior.

Whatever it is, it's much denser on average than Earth's mantle and crust.

Jun 02, 2014
Why must a planet have been formed at the same time as the star? According to other articles I've read here, there are 100,000 planets thrown out for each star formed in the galaxy. Some of these must occasionally come close enough to a star to be captured. System gases would make up the atmosphere after a few billion years from bombardment of local asteroids and comets and would cover much of the surface after a time. This would end up making the original composition and origin hard to determine from a distance.

Jun 02, 2014
I find the most interesting part of this article is the age of the system. Indeed 3 billion years isn't a long time for nucleosynthesis to give us enough material to form something like the moon, much less this monster.

Then again, on cosmic scales, the thing is insignificant in size.

I guess I just never would have expected a planet like this at all...much less in a system with a (population II?) star....

Jun 02, 2014
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Jun 02, 2014
So it's a sun-like star 11 bn years old... That's not much sun-like...

Jun 02, 2014

Too complicated/not enough information to model a "capture" theory over cosmic time scales, so they are just sticking with "standard" solar system formation concepts.

A planet could easily be older than it's host star given the insane time scales of billions of years. I think you would expect such a planet to have collected a tremendous amount of material and possibly show evidence in it's isotopic ratios of having experienced accretion in more than one stellar environment, which of course would be the smoking gun of a "captured" planet..

Not impossible at all, but probably beyond the limits of existing technology to detect or prove that in most scenarios.

Jun 02, 2014
It is nice to immediately see exceptions from the new rule observations gave us the other day(habitables are 1-1.5 Earth radius, neptunes above).

@Jeffhans1: Good question!

It is a numbers game mostly. A planet formed in the system is much more likely, having ~1 ejected planet/star (from microlensing surveys) but often (and explicitly as here) several planets in the system.

Another check that tells of non-capture is when the orbit is corotational with the star. One can tell star rotation from Doppler measurements of the edges. That means the planet is a member of the original disk the system formed out of. (If the orbit planes differs, it still doesn't need to be capture of course, since there are other ways that can happen.)

Jun 02, 2014
There can be more simple explanations. It could be formed by just the collision of different rocky planets. The Earth itself experienced such kind of collision, so it seems to be a probable event (in our case part of it formed the moon, but other trajectories and conditions would result in a complete merge of both bodies, also altering the orbit to cross that of other planets in a domino effect ).

Jun 02, 2014
Looks like they finally found E. E. Doc Smith's Valeria..or close enough. Dutch Space Marines next?

Jun 02, 2014
It is interesting to me that this finding goes against at least two astronomical theories yet they are confident that they can accurately deduce the size, weight, and composition of something so far away they cannot even see it, based on astronomical theories.

Jun 03, 2014
Perhaps there is a state of object less than a brown dwarf and that is what has been detected?

Now for some completely wild crank talk, perhaps the planet came out of a wormhole/blackhole?
Perhaps dark matter can coalesce into normal matter.

Try not to be too harsh on my crank, it is within Crank Quotes!!! :-)

Jun 03, 2014
Nice, more bad theories put to rest. Been a heck of a year so far on killing some theories.

Jun 03, 2014
There can be more simple explanations. It could be formed by just the collision of different rocky planets.

This is certainly true, but to put this in perspective, if you took all the rocky planets, asteroids, and rocky moons in our own solar system and put them together, you wouldn't even get an object a quarter of this mass; Earth and Venus are the vast majority of the rocky material in the Solar System. Mars is about 1/9th of Earth, and the rocky moons are about 1/80 to 1/100 of Earth mass, so basically insignificant.

So basically there's about 3 (maybe 4) Earth Masses worth of rocky material in our solar system. Maybe there's a bunch of metallic dust and sand sized particles floating around too in trace amounts, but it wouldn't be enough to even matter compared to 17 Earth masses.

The Sun actually contains a LOT of heavy metal material, so it's possible that less/slower stellar accretion could provide more time for metals to form into a planet in a stable orbit...

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