How violent are the early histories of solar systems? Planets are built by the collisions of smaller bodies, so a certain amount of violence is probably unavoidable. Our own Earth-Moon system seems to have been formed by a smash-up of two planets, while Uranus seems to have been flipped on its side by a collision, and Mercury seems to have lost a lot of its material early in its history. Is this sort of history common as planets form?
Answering these questions requires a detailed understanding of the planets themselves, knowledge difficult to attain for any solar system but our own. But now, following up on observations made with the Kepler space telescope, researchers are suggesting they've found evidence of a smash-up in an exosolar system about 1,750 light years from Earth.
That's dense
Kepler-107 has a Sun-like star orbited by at least four planets. The planets are tightly packed around the star, with orbital periods ranging from three to 14 days. The lengths of the orbits of neighboring planets can be expressed as simple ratios of integers (5:2, 3:1, and so on). This creates what are called "resonant orbits," where the periodic alignment of the bodies helps stabilize and reinforce the orbits. Generally, this is thought to occur when planets that form farther from the star are migrating inward toward it; the resonances help balance things out and keep the planets from continuing on into the star.
While Kepler data allows us to identify the size of the exoplanets, it doesn't tell us anything more than that. To go deeper, we have to determine the planets' masses. Combined with their size, this tells us something about their density; we can use the density to infer something about their composition.
An exoplanet's mass can be determined by examining its gravitational influence on the star it orbits. When the planet's orbit takes it in the direction of Earth, it pulls the star slightly toward us, causing a small blue-shift of the star's light. At the opposite side of its orbit, the star's light is red-shifted. The degree of these shifts is dependent on the planet's distance from the star (which we know from the orbit) and the planet's mass.
A team of researchers obtained data on the red and blue shifts of Kepler-107 and used that to determine the mass of each planet. Combined with the information on their sizes generated by Kepler, this information could provide a sense of what the planets look like.
The data on Kepler-107d isn't very well constrained, but it is consistent with it being rocky (it's a bit smaller than Earth). Planet 107e is a mini-Neptune with a 15-day orbit. The two inner planets, 107b and 107c, are, well, a bit odd.
Inside-out
Planets that are closer to a star tend to have a greater abundance of heavier, rockier materials. Part of that is because the higher temperatures near the star keep volatile materials from condensing on bodies in that region; another reason is that flares of star activity can boil off atmospheres (and more than atmospheres) after planets form. As a result, you'd expect planets to show a drop in density as you move out from the star—at least until you get far enough out that things like methane and nitrogen start to freeze solid.
And that's not what they see here. Kepler-107b, the planet closest to the star, has a density similar to Earth's. But Kepler-107c, the next planet out, is likely to be more than twice as dense. The easiest way to get a disparity like that is to have the planet be roughly 70-percent iron at the core. And, again, if there were that much iron floating around the inner part of Kepler-107's planet-forming disk, then more of it should have ended up in the neighboring planet.
So why is there so much iron? The researchers would have you ask the opposite question instead: why is there so little rock? They proposed that 107c started as a much larger planet and had already differentiated into a metallic core and rocky surface. Then a head-on collision blasted much of the rocky material off the planet, leaving the metal-rich core present now. The team simulated these collisions and found that head-on ones produced the right ratio of metal to rock, although a series of smaller collisions would also work.
Conveniently, there is a precedent for this scenario in our own Solar System: Mercury. Roughly 80 percent of that planet is a metallic core, making it extremely dense. And that high metal content is also thought to have come as the result of a massive collision.
We're still a long way off from generating the kind and amount of evidence that has told us about the violent history of our own Solar System. We haven't clearly detected any exomoons yet, nor do we have the imaging technology needed to figure out the rotation of planets outside the Solar System. But the new paper suggests that, at least in some cases, we already have the tech we need to spot the aftermath of planet-shattering collisions.
Nature Astronomy, 2019. DOI: 10.1038/s41550-018-0684-9 (About DOIs).
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