As we continue to observe other planetary systems, one thing in particular is puzzling astronomers. Why do many other systems appear to have large super-Earths, whereas ours has none?

In a new paper published on Arxiv, Rebecca Martin and Mario Livio from the University of Nevada provide an answer. They suggest that we may actually have had one or more super-Earths, but they were swallowed by the Sun earlier in its life.

The researchers point to two possible ways a super-Earth can form in a planetary system: either in situ, where it forms in the system itself, or migration, when they form further out before swinging inwards.

In the case of our own Solar System, there is no direct evidence for a super-Earth forming via either scenario. But the fact that there is nothing between Mercury and the Sun hints at the possibility of one existing in the early life of our Solar System via the in situ method.

“The only (physical) evidence that super-Earths could have formed in our Solar System is the lack of anything in that region, not even a rock,” Martin told Elizabeth Howell at Discovery News. ”So they could have formed there sweeping up all of the solid material, but then later fell into the Sun.”

^{An artist's depiction of super-Earth Kepler-62f. NASA/Ames/JPL-Caltech}

Via the in situ method, a super-Earth would sweep up material in the “dead zone” of the disk of dust and debris that encircles a young star, where the debris is left to sit without much hassle from other bodies. This enables super-Earths to grow to their huge sizes, without much interference from other objects. Did this happen around our Sun?

“The lack of super–Earths in our Solar System is somewhat puzzling given that more than half of observed exoplanetary systems contain one,” the researchers note in their paper. “However, the fact that there is nothing inside of Mercury’s orbit may not be a coincidence.”

If we did once have a super-Earth, the researchers rule out the idea that it was ejected from the Solar System by some other means. “Given that the orbits of the planets in the Solar System are coplanar [all on the same plane] and not very eccentric, planet–planet scattering does not seem to be a likely ejection mechanism,” they note, possibly ruling out the chance of a world like Planet Nine being an ejected super-Earth from this research. “Thus, if super–Earths formed, they most likely fell into the Sun.”

The researchers don’t provide exact dates for when this might have occurred, nor do they attempt to guess at what the size or orbit of such a super-Earth might have been. But as our Solar System continues to seem more and more unique as we discover other planetary systems, perhaps this research suggests it was not always one-of-a-kind.