There are currently eight known planets in our solar system, but could there be more?
While there are dwarf planets such as Pluto and Eris, neither of them are considered true planets due to their small size. However, there is some evidence to suggest that there might be another planet — Planet 10 — lurking at the far edges of our solar system.
Anyone with an understanding of elementary-grade math will note that we skipped planet nine. That's because planet nine has long been speculated to be a Super-Earth. This theory was supported by the fact that the orbit of Trans-Neptunian Objects seemed to be clustered together. This led researchers to speculate there was a planet's gravity affecting them.
This is one of the main ways in which researchers discover new planets. It can be difficult to spot planets, but their existence can be inferred from the effect their gravity has on other celestial bodies — this is actually the way in which Neptune was discovered. Researchers noticed that Uranus' orbit didn't line up with Isaac Newton's laws, but it was theorized that the contradictions could be explained by the pull of another planet's gravity.
The above method is far from perfect. It was used as the basis for a theory of a planet beyond the orbit of Mercury; however, such a planet does not exist. Mercury's inconsistencies were eventually explained due to Albert Einstein's theory of relativity.
Recent data also suggest that the theory of a super-earth beyond Neptune is false.
While the current evidence rules out the existence of a large planet beyond Neptune, there is evidence that a smaller planet might exist. If so, this planet would be small and cold enough that it would be difficult to see. In order to find it, we would need to search for evidence that its gravity is affecting other planetary bodies and such evidence does exist.
Our solar system is gravitationally isolated. On average, the orbit of various bodies would follow along with the same plane — commonly referred to as the invariable plane. However, the orbit of the Trans-Neptunian Objects is tilted about 8 degrees from the invariable plane. This could be the result of a planet's gravity acting upon them.
The existence of Planet 10 is far from a sure thing, but we are getting closer to determining the number of planets in our solar system. We expect that researchers will have a final answer within the next decade.