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SYFY WIRE Bad Astronomy

Kepler 107c: A massive collision leaves behind a superdense exoplanet

By Phil Plait
Dramatic artwork depecting the aftermath of the collision of two planets. Credit: Dana Berry/SwRI

A long time ago, something very disastrous happened in the Kepler-107 system.

Kepler-107 is a star about 1,700 light years away. It's somewhat Sun-like, though slightly more massive (1.2X), bigger (1.45X), and warmer (by about 80° C) than our own star. It's just a wee bit younger, at an age of 4.3 billion years, compared to 4.6 for the Sun.

It has planets, too, at least four of them. These were discovered by the Kepler satellite, which looked for dips in starlight if a planet happens to pass in front of the star once per orbit as seen from Earth (we call this a transit, and I've written about these extensively, as well as discussed it in my episode of Crash Course Astronomy: Exoplanets).

This is where similarities to our own solar system end. The four known exoplanets orbiting Kepler-107 are very close to their star compared to our planets. Called Kepler 107-b through e (in order out from the star), they are all far closer to the star than Mercury is to the Sun. The closest is just under 7 million kilometers out, giving it a scorching surface temperature of something like 1,300° C, hot enough to melt iron! Not exactly a garden spot.

The sizes of the planets can be determined by how much the star’s light dips during each transit. The outermost one, 107e, is the biggest, at about 3 times the Earth's diameter. 107 d is the smallest, at 0.9 Earths. 107 b and c are roughly the same size, at 1.54 and 1.6 times our own home world.

What about their masses? Well, this is where things get weird.

The best way to get the mass of a planet is to see how hard it pulls on its star as it orbits. The star has lots of mass, which means lots of gravity to pull on the planet. But planets have mass too, so they pull on the star as well. As the planet makes a big circle (or ellipse) around the star, the star makes a much smaller one as well — technically, they both move around their center of mass, called the barycenter. If you know the mass of the star, as well as how much the planet is pulling on it, you can get the planet's mass (again, I talk about this in the Crash Course Astronomy episode at 1:29 in).

Astronomers used a very sophisticated camera to do just this, and what they got was a big surprise. Although the two innermost planets, Kepler 107b and c, are about the same size, 107c is far more massive. 107b has a mass of about 3.5 times the Earth, but 107c has a mass 9.4 times Earth’s!

If it has more mass but the same size, it must be far more dense. While 107b has a density roughly equal to Earth’s (about 5.3 grams per cubic centimeter, also equal to 5.3 times the density of water), 107c has a density over twice that at 12.7 grams per cc! This makes it one of the densest planets found so far.

Given what we know about planetary formation, it's likely 107c has an iron core, like Earth does, but the core is so huge it accounts for 70% of the planet’s total mass! That’s more than twice as much as Earth's core mass relative to the whole Earth. Surrounding 107c's core is possibly a rocky mantle and crust that makes up the remaining 30%.

So how did it get so dense?

Sometimes, when planets orbit really close to their star, the heat and light from the star can literally blow the atmosphere of a planet away. If you start with a gas giant, then after a while the lower density atmosphere is stripped away, leaving behind the denser core. However, you'd expect this to be a more serious issue for the innermost planet, and 107b is less dense than 107c. So that can't be it.

Enter planetary apocalypse.

Artist's depiction of two planets colliding. Credit: NASA/JPL-Caltech 

I'm not exaggerating. Another way to strip off the lighter stuff from a planet is to, well, smack it with another planet.

The astronomers investigating this ran some computer models of what happens when planets collide, and found that the density and inferred internal structure of 107c can be explained if two planets each with about ten times the mass of the Earth collided head-on billions of years ago.

You know, let me rephrase that: The astronomers investigating this ran some computer models of what happens when planets collide, and found that the density and inferred internal structure of 107c can be explained IF TWO PLANETS EACH WITH ABOUT TEN TIMES THE MASS OF THE EARTH COLLIDED HEAD-ON BILLIONS OF YEARS AGO AIIEEEE AIIIEEEEEE!!

That’s a big impact. Like, literally planet shattering. More than half the mass of the two planets would shatter or vaporize from the energy of the impact and be flung away — much of this consisting of the lower density rocky material — leaving the higher density stuff like iron behind to coalesce and form the new, more dense planet.


This idea isn't unprecedented. In our own solar system, Mercury has an unusually large core, and may have once been a bigger planet that suffered a similar collision. We think the Earth did, too, and the debris from that impact formed the Moon.

However, given 107c's huge size and mass, this was one helluva wallop, far larger than what whacked Earth or Mercury.

Now, this isn't written in stone (nor is it iron clad). But, as I pointed out, collisions happen, and pretty often, so I think a massive planetary catastrophe is the way to bet here.

And this makes me wonder just how often and how violent these collisions get. The only way to find out is to keep finding more planets, more planetary systems, and see how they behave. It's incredible to me that this is where we are in exoplanet science, given that the first one was found less than 30 years ago! And now we've found so many we can start looking for trends, histories, and similarities and differences with our own solar system.

Over and again we find that we're the weird ones here. What's funny to me is that the more planets we find, the more I wonder why we ever thought differently.