In other solar systems, the presence of super-Earths, planets that typically have a mass higher than Earth’s, but below Uranus, which is 15 Earth masses. Planets above 10 Earth masses are usually termed mega-Earths. Some of which are even in habitable zones, have been discovered. So far, we have found thousands, although only a few are actually close enough and orbiting bright enough stars for astronomers to study, currently.
In fact, research suggests that while smaller planets are more common than the big ones, these super-Earths are actually the most common type of planet in the Universe.
And yet, there is no example of a super-Earth in our own Solar System.
“We are left with this situation where super-Earths appear to be the most common kind of exoplanet in the galaxy, but we don’t know what they’re made of,” Heather Knutson, assistant professor in the Division of Geological and Planetary Sciences at the California Institute of Technology, said in 2014.
The first super-Earths were discovered in 1992 with 2 outer planets that had masses approximately 4 times Earth. However, the first super-Earth discovered around a dwarf star occurred in 2005. In 2007, researchers discovered 2 super-Earths orbiting Gliese 581, both of which were on the edge of the habitable zone where liquid water may be possible on the surface.
As of December 2011, the Kepler Space Observatory Mission team had found 2,326 planetary candidates, 207 of which are of similar size to Earth, and 48 of which are in habitable zones of surveyed stars.
“Super-Earths are at the edge of what we can study right now,” Knutson said. “But super-Earths are a good consolation prize—they’re interesting in their own right, and they give us a chance to explore new kinds of worlds with no analog in our own solar system.”
So with astronomers finding Earth-like planets left and right in other solar systems, why doesn’t our Solar System have one?
There is a scenario, called the Nice model, that theorizes that Jupiter, Saturn, Uranus, and Neptune were originally found much more closely spaced and more compact than they currently are. However, their orbits moved outwards, and in doing so they continued to migrate the orbits of small, rock and ice planetesimals further outward. In fact, interaction with Jupiter’s immense gravity may have ejected some planetesimals right out of the Solar System.
Essentially, the thought is that we did once have super-Earths, but as our larger planets migrated out, they pushed those worlds out or may have even merged them. After moving out, the inner Solar System was left empty, and the worlds that grew inside it didn’t have the opportunity to grow larger than Earth and Venus.
It is theorized that had the movement of the gas giants not cleared out the outer Solar System and stunted the growth of the inner planets, there might be a super-Earth nearby and ripe for exploration.
Although the Nice model is not universally favored among planetary scientists, it is widely accepted as the current most realistic model of our Solar System’s early evolution.