A recent study by two UNLV astrophysicists suggests that a super-Earth — a planet larger than Earth but smaller than Neptune — could have formed in the early days of our solar system before being pulled in by the sun and destroyed.

Professors Rebecca Martin and Mario Livio’s new work explains how super-Earths form — as well as the differences between our solar system and others, called exoplanetary systems. “The lack of super-Earths in our solar system is somewhat surprising given that more than half of observed exoplanetary systems contain one,” Martin said. “There is no planet in the intermediate mass range between the Earth and giant planets such as Neptune and Uranus.”

Another difference in our solar system is that there are no planets in the region inside Mercury’s orbit. Other exoplanetary systems can have several planets in that region, but we don’t have anything, which is puzzling to the research team.

Formation of Super-Earths

According to Martin, there are two theories as to how super-Earths form. One is that super-Earths form in situ (where they are currently observed), which would have to be close to the sun.

For them to form in situ requires a massive disk and lots of material in that region. A star forms from the collapse of a big cloud of gas, and as it collapses it has to conserve angular momentum or spin. So instead of just forming a star, it forms a disk of material around the star. It is from these disks that all planets form.

These protoplanetary disks are thought to have “dead zones,” meaning regions that are too cold to be viscous. The dead zone is a likely formation site for planets because there is no turbulence in this region, allowing solids to settle to form a planet.

The second theory is that super-Earths form further out where there is more solid material and then migrate inward through the disk to where we see them today.

Martin and Livio’s work combines these two ideas, speculating that both processes could be occurring, depending on the size of the dead zone region in the disk. If the dead zone is large, a super-Earth could form close to the sun. But if the dead zone is too small, then the super-Earths form further out, where there is more solid material, and then move inward.

“We think that the reason we don’t have any super-Earths is that they formed in this inner part of our solar system where there is now nothing, clearing out all of the solid material before falling into the sun,” Martin said.

Martin and Livio’s research, which has been accepted for publication in the Astrophysical Journal, was done using computer modeling and numerical simulations with information from the Kepler telescope. The pair hope further research will allow them to better understand how Earth would be affected if there was a super-Earth in the inner parts of our solar system.