We’ve learned a thing or two about exoplanets over the last few years. One of the more surprising discoveries is that our solar system is quite unusual. The Sun’s worlds can be easily divided into small rocky planets and large gas giants. Exoplanets are much more diverse, both in size and composition.
The Sun’s planets can be divided into gas giants such as Jupiter and Saturn, gas dwarfs such as Neptune and Uranus, and terrestrial worlds such as Earth and Mars. But many exoplanets fall into a new category called super-Earths.
Super-Earth fills the gap between Earth and Uranus. They vary in size from 1 – 4 times that of the earth and are between 1 – 15 masses of earth. Because our solar system does not have such a planet, we are not quite sure what it looks like. Planets that are close to Earth are probably rocky worlds, while the largest super-Earth are probably gas planets. But between these extremes, it’s harder to see. Is there a dividing line between rocky and gaseous planets?
Most super-Earths we have discovered revolve around their star. This is mostly because the easiest find large planets orbiting near their star. But the fact that we found so much super-Earth around us led to the idea that it could be gas dwarf planets removed from their atmosphere. Super-Earths and worlds like Uranus and Neptune thus form in similar ways, and only those that are close to a star become super-Earths. But a new study contradicts this idea.
The team noted that these mediocre planets tend to merge into two groups: worlds that are less than 1.7 times the size of the earth, and worlds that are more than twice the size of the earth. The first group is probably rocky planets, while the second group is probably gas planets. The team therefore looked at computer models to understand why these groups are so distinguished. They found that super-Earths apparently do not form like small gas planets and are later just stripped of their atmosphere. Instead, these medium-sized worlds can form in two different ways. One way follows the evolution of terrestrial worlds, where a rocky protoplanet forms, which later acquires atmosphere. The other side follows the gas dwarf path, where a planet forms a thick atmosphere early in its evolution.
This research suggests that there is a rocky gorge in planet formation. Small super-Earths are part of the terrestrial family, while the larger super-Earths are really small host dwarfs.
Reference: Eve J. Lee and Nicholas J. Connors. “Excessive radius gap and potential broad nuclear mass distributions of Super-Earth and Sub-Neptunes.” The Astrophysical Journal 908.1 (2021): 32.