Forget Planet Nine (well, don’t actually forget about it). A new study suggests that at least one super-Earth — a planet that is up to 10 times larger than Earth, but smaller than Neptune — may have formed very close to the sun, a very long time ago ... and then the sun ate it.
It turns out our solar system is a bit unusual because it doesn’t contain a super-Earth — they are actually very common throughout our Milky Way galaxy.
Now, if the researchers are right, it could explain why the region of space within the orbit of Mercury — the planet closest to the sun — is so bare. Over time, this hypothetical super-Earth would have collected all the space debris in the area, before surrendering to the sun’s gravity and vaporizing.
“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,” said lead author Rebecca Martin, an assistant professor at the University of Nevada, Las Vegas, in an email to Discovery News. “So they could have formed there sweeping up all of the solid material, but then later fell into the sun.”
According to observations of super-Earth exoplanets outside our solar system, they could have formed in just two locations: in situ (where we see them today) or farther out from their observed locations, migrating over time.
To form in situ, the super-Earths would have to slowly build up from debris in the “dead zone,” or region of low turbulence, of a forming planetary system known as a protoplanetary disc. Material within this region can become self-gravitating, resulting in the magnetism and accumulation of the surrounding material, eventually forming a planet.
But if the final density of the planet is too large, it will begin to migrate toward its star, which is what may have happened to our hypothetical super-Earth.
“The size of the dead zone must be large enough that it lasts for the entire disc lifetime,” Martin explained. “Since different systems may have different dead zone sizes, formation in the inner parts may not be possible in all systems and thus both formation locations may be operating.”
However, the researchers concluded that planets that materialized farther out in the protoplanetary disc would be less dense since water and other volatiles freeze out in the colder outer parts of the disc, meaning that a migration through the dead zone, followed by the migration towards the star was unlikely within our solar system.
So what about our own solar system? The researchers stated that the hypothetical super-Earths had to have formed in situ and gathered up all the material inside of Mercury’s orbit. “If the disc is sufficiently cool, the migration timescale for them to fall into the sun is short enough for this to happen in the lifetime of the disc,” Martin added.
However, more research is needed to confirm this.
The research was accepted for publication in The Astrophysical Journal, and is currently available online on arxiv.org.