In recent years, astronomers have examined many other star systems. They found ample evidence of “hot Jupiter,” those gas giants who are close to their parent star, and “super-Earth,” rocky worlds larger than Earth, but smaller than Neptune. But, in spite of all their searches and all discoveries, there is only one star system that includes both of them.
The system is called WASP-47 and actually has three planets close to the star: hot Jupiter, hot super-Earth and hot Neptune. At the same time, the other 100 or so hot ones found by Jupiter do not have any companions in the form (at least) of the super-Earth. What is wrong with them?
New research suggests that they are very difficult to find, because super-Earths are very easily destroyed.
“I argue that the absence of super-Earth companions for hot Jupiter is proof that most of these planets have experienced strong gravitational effects from other planets or stars in the past. They are sent to highly eccentric orbits, like comets, and then their orbits circulate over time, ”said lead author Alexander James Mustil, senior scientist for astronomy and physics from Lund University in Sweden, in an email to Discovery News. “During the high eccentricity phase, they destroy any super-earths that rotate close to a star, usually causing them to collide with a star or the planet itself.”
Most of Mustill's assumptions are made up of a simulation of how exoplanetary systems look and how planets move under the force of each other’s gravity. The calculations were compared with real systems of exoplanets to see if they reflect reality.
Part of his work related to what is happening with the planets is very difficult to understand. For example, planets whose orbits are located far from their stars and do not attract them so much (or how often they pass through them). These are the basic methods for detecting planets.
“I consider the systems of super-Earths at close range and ask what would happen if I added various additional bodies in wide orbits in the system. For example, such giant planets as Jupiter, or double stars, ”said Mastil.
“I believe that approximately 25% of these super-Earth systems will be destabilized and will begin to collide with each other. Responding to the inverse problem: Considering what we see in super-Earth systems, how do some of them manage to have some outer giant planets? This is a much more complicated issue, and the work continues to this day. ” This, of course, does not explain how WASP-47 managed to get both objects, but Mastil claims that in this case there was no high eccentricity of migration. Instead, the hot Jupiter hypothetically moved closer to the star due to the gravitational interaction with the disk that formed it. (These are two competing theories of how planets are formed, but still Mustil said that the rarity of WASP-47 suggests that high eccentricity of migration is the dominant mechanism).
Mastilla has a lot of ideas on what to study next. Some thoughts include modeling how planets form in a protoplanetary disk before performing the same simulation (which currently relies on fully formed planets), or to accurately simulate a reaction of how the planets bump into each other.
“They can beat each other so fast that they can tear each other apart and not merge into larger ones,” said Mustil. “It would be great to simulate the birth and death of planets from dust to dust!”.