There was probably a super Earth in our Solar System a long time ago, but the Sun pulled it in and ate it up a long time ago, say scientists from the University of Nevada, Las Vegas. Over a period of hundreds of millions of years, this super Earth would have swept up all the celestial bodies in its vicinity.
In a paper that has been accepted for publishing by the Astrophysical Journal (citation in ArXiv below), Rebecca G. Martin and Mario Livio explain that a Super Earth is a possible explanation for why there is nothing at all between the region of Mercury and the Sun.
For the moment, the researchers’ evidence is based purely on modelling and the fact that the region between the Sun and Mercury is so barren. The planet would have been larger than ours, but smaller than Neptune.
The area between Mercury and the Sun is empty, there is nothing there – not even a rock. Two scientists believe a super-Earth existed inside that region, swept up all the space debris in the vicinity, and was then pulled in by the Sun.
“Super Earths are exotic planets unlike any in our solar system. They are more massive than Earth yet lighter than gas giants like Neptune, and they can be made of gas, rock or a combination of both.”
Not even a rock between Mercury and the Sun
In an email to Discovery News, Dr. Rebecca Martin, who works at the University of Nevada’s Department of Physics & Astronomy, wrote:
“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. So they could have formed there sweeping up all of the solid material, but then later fell into the sun.”
Large celestial bodies, such as planets, are like gravitational vacuum cleaners, pulling in everything near their orbital path, until the area is swept clean.
According to observations of super-Earths outside our Solar System, they could have formed either in situ (exactly where they currently are), or further away from their parent star – over time they migrated.
For a super-earth to be formed in situ, it would have to slowly build up from debris in the ‘dead zone’ of a forming planetary system – its protoplanetary disc. This would only occur if there is a great deal of turbulence in the area, fuelled by magnetism of the surrounding area.
Dr. Martin explained:
“The size of the dead zone must be large enough that it lasts for the entire disc lifetime. 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.”
Dense and less dense super Earths
Of the super Earths that scientists have observed so far, the authors noted two distinct kinds, depending on their density. They believe that planets that were formed further out in the disc end up being less dense, because water and other volatiles freeze out in the lower temperatures of the outer disc. The closer ones are denser.
So, what might have happened in our own Solar System? The authors speculate that in our neighbourhood, super-Earth’s formed in situ and cleared out all the material inside Mercury’s orbit.
Dr. Martin said:
“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.”
She added that further research will be required to confirm this.
In an Abstract preceding the paper, the authors wrote:
“We speculate that super Earths could have formed in the inner parts of our solar system and cleared the material in the region inside of Mercury’s orbit. The super Earths could migrate through the gas disk and fall into the Sun if the disk was sufficiently cool during the final gas disk accretion process.”
“While it is definitely possible to meet all of these requirements, we don’t expect them to occur in all systems, which may explain why the solar system is somewhat special in its lack of super–Earths.”
The paper has not been published by the Astrophysical Journal, yet – the citation and hyperlink below are to Arxiv, where pre-print documents are registered.
Citation: “On the Formation of Super-Earths with Implications for the Solar System,” Rebecca G. Martin & Mario Livio. arXiv:1603.08145v1 . Draft Version, 29 March, 2016.
Video – What is a Super Earth?
The Universe is always presenting us with stuff we never imagined, or even thought possible.