Thanks to the Kepler spacecraft, we know that at least half the Sun-like stars in the galaxy host at least one planet, and for three out of ten stars that planet is a super-Earth. But the existence of these planets, bigger than Earth but still rocky, has complicated our ideas of how planets form. Now, a new model provides a solution to this puzzle.
The size of super-Earths, a mass between 2 to 20 Earth masses and 1 to 4 times Earth’s radii, means that they are on the borderline of becoming gas giants. Exactly why they dont become worlds like Jupiter, though, is a bit of a mystery.
According to a paper in the Astrophysical Journal from researchers Eve Lee and Eugene Chiang at the University of California, Berkeley, the answer may lie in them forming later than expected. In their model, they found the planets are likely to form when the disk of dust and gas that surrounds a young star has lost most of its gas.
Forming late may be the necessary mechanism that stops them entering a runaway gas accretion event, ensuring that they become large rocky worlds, rather than gaseous ones. An alternative explanation was that they formed by slowly gathering material in a gas-rich, dusty, and cool planetary nebula, but the researchers showed that super-Earths would become gas giants in that scenario, favouring their other hypothesis.
“The riddle posed by super-Earths is that they are not Jupiters: their core masses are large enough to trigger runaway gas accretion, yet somehow super-Earths accreted atmospheres that weigh only a few percent of their total mass,” the researchers wrote in their paper. “We show that this puzzle is solved if super-Earths formed late, as the last vestiges of their parent gas disks were about to clear.”
Their study also focuses on another type of unusual world, super-puffs, which are planets that have a larger than usual atmosphere for their size. These objects have large radii (4 to 10 times that of Earth) but small masses (no more than 6 times Earth’s), with their atmospheres weighing more than one-fifth of the planet’s core.
Super-puffs are quite uncommon among the planets discovered by the Kepler spacecraft, but their existence is just as puzzling as super-Earths. Applying the model to these objects, Lee and Chiang suggest that the super-puffs have migrated inwards after forming at larger distances (further than Earth is from the Sun) in a dust-free environment. There the gas is cooler, which allows for denser atmospheres to form more quickly and then, through gravitational interactions with other planets, they move forward.
In this research, the astronomers manage to integrate the formation scenarios of super-Earths and super-puffs into existing models for planetary formation. Future observations will provide more insights on this proposed picture and, hopefully, answer once and for all how planets form.