Giant Planets and Wonky Orbits

Kepler-108 is the first system observed with really wonky orbits, which may be a result of giant planets bumping into each other.

If you were anything like me as a kid, you bumped into a lot of stuff. Chairs. Tables. Other kids. There was no stopping me from bumping into something and (usually) falling.

It turns out that planets, like kids, also bump and shove into things. And sometimes they also fall. But instead of falling and scraping their knees, they fall into wonky orbits.

At least, that’s what Sean Mills and Daniel Fabrycky (both of University of Chicago) think happened to the two Saturn-mass planets orbiting the star Kepler-108. They found that the planets’ orbits have a “high mutual inclination,” meaning they’re very tilted with respect to each other. To understand why this could have happened, we have to go way back to when the planets were just kids — that is, still in their formation stages.

Planet Scattering

As a star forms, a disk of dust and gas grows around it. Clumps inside the disk eventually become planets. After the disk disappears, the planets continue on in their merry ways, usually all in the plane of the disk once occupied.

If the system has multiple planets, and most do, then the planets feel gravitational forces from the star and from each other. These forces change as the planets move in their orbits. If two planets have orbits that are really elongated (in science talk, have high eccentricities), then over time their paths may cross.

And by that I mean violently crash into one another.

Like two kids that collide, the planets scatter in different directions. They fall into new, very different orbits. “The natural result for this kind of scattering is a growth of eccentricity,” said Sourav Chatterjee (Northwestern University). The planets now have wonky orbits around the star.

The planets in Kepler-108 are not coplanar, but rather their orbits look more like this from our perspective (where the yellow circle is the star and, while red and blue circles are planets - orbits are to scale but sizes of the bodies are not). Sean Mills

The planets in Kepler-108 are not coplanar, but rather their orbits look more like this from our perspective (where the yellow circle is the star and, while red and blue circles are planets - orbits are to scale but sizes of the bodies are not).
Sean Mills

Kepler-108b and c’s Weird Orbits

This isn’t the first time we’ve observed off-kilter planetary orbits, but Kepler-108 is the first system that’s been well-characterized. The only two other planet pairs like this we’ve seen (Kepler-419 b and c, Upsilon Andromeda c and d) have modest inclinations, considering the planets’ high eccentricities.

The team realized the orbits of Kepler-108’s worlds veered away from each other by analyzing the planets’ transits. If planets move in a flat plane, then they will pass in front of the same part of the star every time they transit. Instead, the researchers found that each transit changed in width and depth by a small amount.

If two planets are not coplanar, they will interact with each other just as a top due to gravity on earth. Sean Mills

If two planets are not coplanar, they will interact with each other just as a top due to gravity on earth.
Click for animation.
Sean Mills

“If planets are not in a flat orientation. . . then their orbits will wobble around in a circle,” said Mills at a press conference at the 228th American Astronomical Society Meeting in San Diego, California. This is the same effect you see when you spin a top on a table. The handle will spin in a circle on one plane and the actual top will spin in a circle on a different plane. Same thing applies to planets.

We can’t know for sure if Kepler-108’s orbits happened because of planet scattering, but the research team thinks this discovery is important because we can use it as an example to test models that look at unstable systems from billions of years ago, when planets might have bumped into each other.

We should also note that the star is a wide binary, but we don’t have enough information to know what effect the secondary star may have on the planets’ orbits.

“This idea that planetary orbits get altered after their birth through dynamical processes is, I think, re-verified by this discovery,” says Chatterjee. “It is very possible that even more planetary systems will have high mutual inclinations.”

If more planets turn out to have wobbly, wonky, or non-coplanar orbits around their parenttars, then it’ll be a new challenge to observe them and to figure out how that happened. Maybe they did bump into each other. In which case, they’re really not much different from kids running around.

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