There are eight known planets in the Solar System, since Pluto was excluded from the planetary 'club'. But for now, there is some evidence that there may be another planet to replace Pluto, namely Planet Nine or Planet Ninth.
According to the hypothesis, Planet Nine is hiding at the outer edge of our Solar System. So far, the planet seems to have eluded discovery. But a new study has pinpointed where he should be.
Quoted from Science Alert, evidence for the existence of Planet Ninth comes from the existence of its gravitational pull on other objects. If that planet existed, its gravity would affect the orbits of other planets.
So, if there is something interesting on a planet, scientists will do some math to find its source. This is how Neptune was discovered. John Couch Adams and Urbain Le Verrier then noticed independently that Uranus seemed to be being pulled by an invisible planet.
In the case of Planet Ninth, scientists found no effect of gravity on a planet. What is visible is a strange grouping of small icy bodies in the outer Solar System known as Kuiper belt objects (KBOs).
If there were no planets outside the Kuiper belt, scientists would expect KBO's orbit to be randomly oriented within the plane of the Solar System's orbit. On the other hand, there are many KBO orbits clustered in the same orientation. Maybe this is just a coincidence, but the team of scientists say it's impossible.
Ninth Planet Quest
Back in 2016, researchers looked at the statistical distribution of KBOs and concluded that the clustering was caused by an undetected outer planet. Based on their calculations, the world has the mass of five Earths and is about 10 times farther from the Sun than Neptune.
The paper even calculated the vast region of the sky where the planet is located. Unfortunately, the search turned up nothing. This has led some to conclude that the planet does not exist. The reason, the oddity of the orbital does not prove a planet exists.
Other studies have gone so far as to argue that Planet Nine does exist, but we can't see it because it's a primordial black hole.
Well, this latest study reviews the original work by considering some of the criticisms it received. One big criticism is that outer Solar System objects are hard to find, so we look for them in a convenient place.
The clustering effect we see could be simply due to biased data. By applying an observational bias, the authors found the clustering was still statistically unusual. There is only a 0.4% chance of it being a coincidence. When they recalculated the possible orbits of Planet Ninth, they were able to better localize where to look.
One interesting aspect of the study is that the newly calculated orbit places Planet Ninth closer to the Sun than originally thought. It's weird, because if it was closer, then we should have found it.
The authors argue that observations so far have ruled out the closest option for Planet Ninth, helping to narrow down its possible location even further. If such a planet exists, Planet Nin should be detected by the Vera Rubin Observatory in the near future.
These studies are not conclusive, and many astronomers still argue that Planet Nine does not exist. However, this research makes it clear that we don't need to debate it any longer. Either Planet Nin is about to be discovered, or observations will rule it out as an explanation for the KBO clustering effect.