Planetary Smash-Up May Have Produced This Distant Iron Exoplanet

Computer simulations suggest Kepler 107c could have been formed when two rocky planets collided, stripping it down to its metal core

Kepler 107c
That's so metal. Z. M. Leinhardt and T. Denman (Univ Bristol)

Since astronomers confirmed the first exoplanet in 1995, they’ve gone on to uncover almost 3,900 distant worlds, with thousands still awaiting analysis. Those planets come in all shapes and sizes: there are some that have swollen to be bigger than Jupiter and planets so hot they have skies made of vaporized metal that rain lava. Some are the right size and temperature to harbor liquid water and possibly life and one may possible be made mostly of diamond.

Now, reports Jeremy Rehm at Science News, we can add another space oddity to the list—a planet produced by a massive collision of two planets, creating a celestial body that is essentially a big ball of metal with a rocky crust.

The planet is one of four exoplanets discovered in 2014 orbiting a star called Kepler 107 about 1,670 light-years away. When researchers decided to calculate the size and mass of the planets, they discovered something unusual. Though the two innermost planets, Kepler 107b and Kepler 107c, are roughly the same size—about 1.5 times as big as Earth—their masses are quite different: Kepler 107c is three times as dense as its sister planet and 10 times as dense as Earth.

Furthermore, the Kepler twins don’t fit the normal pattern of planet formation. Typically, during the early years of a solar system there is an accretion disc made of gas and dust that spins around a star and planets condense from that material. Denser, rockier planets orbit closer to their stars since they are made of heavier elements while less dense planets orbit further away. That’s because these lightweight planets are usually made of elements, like hydrogen and helium, that would be stripped away by solar winds if they were closer to the star. But Kepler 107c breaks that rule, and orbits farther out than its lighter sister planet, Kepler 107b.

“It’s farther from its star [than Kepler 107b], but it’s more massive,” Eric Lopez, an astrophysicist at NASA’s Goddard Space Flight Center tells Rehm. “It’s kind of weird.”

So what made Kepler 107c such a metal head and why does it seem out of order? To probe this question, an international team of researchers collected over 100 spectroscopic measurements of the planets around Kepler 107 using the National Galileo Telescope in the Canary Islands and then fed the data into computer simulations, report Helen Briggs and Paul Rincon at the BBC.

They arrived at several possibilities that explain why Kepler 107c is so dense but located farther away from its star, which are explored in a paper in the journal Nature Astronomy. For starters, it could be that it formed closer to its sun then drifted away. It’s also possible that a bombardment of smaller objects hit a larger, earlier version of Kepler 107c and stripped away most of its rocky outer shell, leaving the dense metal core of the planet behind. But the most convincing scenario is a collision between two worlds.

If two rocky planets—each with about 10 times the mass of Earth and each with an iron core that made up about 30 percent of its mass—crashed into each other at high speed, it could rip away or vaporize most of the rocky material and produce a standalone planet with an oversized iron core. While other scenarios are possible, the collision theory is the idea that explains the data best.

While mega-impacts between planets and protoplanets are believed to be happen pretty often all around the universe, astronomers have never witnessed the event or found proof of the phenomenon outside our solar system. If Kepler 107c was created by a collision, it could help us understand more about planetary formation.

"Giant impacts are thought to have had a fundamental role in shaping our current solar system. The moon is most likely the result of such an impact, Mercury's high density may be also, and Pluto's large satellite Charon was likely captured after a giant impact,” co-author Zoe Leinhardt of the University of Bristol says in a press release. “But until now, we hadn’t found any evidence of giant impacts occurring in planetary systems outside of our own. If our hypothesis is correct, it would connect the general model we have for the formation of our solar system with a planetary system that is very different from our own."

While the hypothesis is fascinating, it will be difficult to prove. Cayman Unterborn, exogeologist at Arizona State University, tells Rehm at Science News that it’s an intriguing idea, but it’s not really possible to extrapolate data about Kepler 107c’s mantle and core simply from its density. There could be other things at play that we do not yet understand.

“Having the density of a planet, you can tell if it’s rocky-ish or watery-ish or gassy-ish,” he says. “But actually getting how big the mantle is versus the core is kind of tough,” though he hopes the paper “spurs some healthy debate about the origins of planets that are kind of weird.”

And it’s likely astronomers will soon be swimming in data about weird exoplanets. Just last month, researchers released the first drip of data from NASA’s next-gen, planet-hunting satellite TESS, which was launched last summer, and the dataset included 200 potential new worlds, including some that already seem as strange as Kepler 107c.

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