Why Don't Super-Earths and Hot-Jupiters Like Each Other?

Astronomers have looked at a lot of other star systems in recent years. They’ve found ample evidence of “hot Jupiters,” those gas giants that are close to their parent star, and “super-Earths,” those probably rocky worlds, larger than Earth but smaller than Neptune. But in all their searching, all their discoveries, there is only one star system that has them both.

The system is called WASP-47 and actually has three planets close in to the star: a hot Jupiter, a hot super-Earth and a hot Neptune. Meanwhile, the other 100-or-so hot Jupiters found don’t have any (detected, at least) super-Earth companions. What is up with that?

A new study proposes that it’s very hard to find both in the same system because super-Earths get very easily destroyed.

“I argue that the lack of super-Earth companions to hot Jupiters is evidence that most of these planets have experienced strong gravitational effects from other planets or stars in the past: they are sent onto highly eccentric orbits like those of comets, and then their orbits circularize over time,” said lead author Alexander James Mustill, a senior astronomy and physics research fellow at Lund University in Sweden, in an email to Discovery News.

“During the phase of high eccentricity, they destroy any super-Earths orbiting close to the star, usually by forcing them to collide with the star or with the planet itself.”

Most of Mustill’s paper deals with modelling what exoplanetary systems look like and then viewing how the planets move under the influence of each other’s gravity. The simulations were then compared to actual exoplanet systems to see if they reflected reality.

A part of his paper, for example, looked at what happens with planets that are very hard to see — that is, the planets that orbit far away from their star and don’t tug on it as strongly (or pass across it as frequently.) These are the main techniques for detecting planets.

“I take the close-in systems of super-Earths and ask what would happen if I add various extra bodies on wide orbits to the system, such as giant planets like Jupiter, or binary stars,” Mustill said.

“I find that in about 25 percent of these systems, the super-Earths will be destabilized and begin colliding with each other. Answering the inverse problem: given what we observe about the super-Earth systems, how many have some outer giant planets, is a much trickier question, and the work is still ongoing.”

This of course does not explain how WASP-47 came to be, but Mustill argues that in this case there was no high-eccentricity migration. Instead, the hot Jupiter is hypothesized to have moved closer to the star due to gravitational interactions with the disc that formed it. (These are two competing theories for how planets formed, but Mustill said the rarity of WASP-47 is evidence that high-eccentricity migration is the dominant mechanism.)

Read more at Discovery News