There are eight known planets in the solar system (since Pluto was booted out of the club), but there has been evidence for some time that there might be another one. A hypothetical planet 9 lurks on the outer edge of our solar system. So far, this world has eluded discovery, but a new study has pinpointed where it should be.
The evidence for Planet 9 comes from its attraction to other bodies. If the planet exists, its gravity affects the orbits of other planets. So if something seems to be tugging at a planet, just do some research to find the source. This is how Neptune was discovered when John Couch Adams and Urbain Le Verrier independently noticed that Uranus appeared to be being pulled from an invisible planet.
In the case of planet 9, we have no gravitational effect on a planet. What we see is a strange collection of tiny icy bodies in the outer solar system known as Kuiper Belt Objects (KBOs). If there were no planet beyond the Kuiper Belt, you would expect the orbits of KBOs to be randomly oriented within the solar system’s orbital plane. But instead we see that many KBO orbits are grouped in the same orientation. It is possible that this was just a fluke, but it is unlikely.
The possible orbit of Planet Nine. Photo credit: CalTech / R. Injured (IPAC)
As early as 2016, the authors examined the statistical distribution of KBOs and came to the conclusion that the accumulation was caused by an undiscovered outer planet. According to their calculations, this world has a mass of 5 earths and is about 10 times further from the sun than Neptune. The paper even calculated a vast region of the sky in which the planet could be. But research did not reveal anything. This led some to conclude that the planet does not exist. Orbital strangeness does not prove that a planet exists. Just ask Planet Vulkan. Others even went so far as to argue that Planet 9 exists, but we cannot see it because it is a primeval black hole.
This new study examines the original work in the light of some of the criticism it has received. One major criticism is that the bodies of the outer solar system are hard to find, so let’s look for them where it’s convenient. The cluster effect that we are seeing could only be due to skewed data. Taking into account the observation bias, the authors find that the clustering is still statistically unusual. The probability that it is a coincidence is only 0.4%. When they recalculated Planet 9’s likely orbit, they were able to better pinpoint where to look.
An interesting aspect of the study is that the recalculated orbit brings Planet 9 closer to the Sun than originally thought. This is strange because by the time it’s closer we would have already found it. The authors argue that observations so far have ruled out the next options for Planet 9, which helps narrow down its possible location even further. If the planet exists, it should be detectable from the Vera Rubin Observatory in the near future.
This study is inconclusive and many astronomers still argue that Planet 9 does not exist. But this study makes it clear that we don’t have to argue about it much longer. Either it will be discovered soon or observations rule it out as an explanation for the KBO clustering effect.
Reference: Michael E. Brown, Konstantin Batygin. “The orbit of Planet Nine.” ArXiv preprint arXiv: 2108.09868 (2021).
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