There may be a 9th planet in the solar system (and it’s not Pluto)

Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. If you were an old-school student (now called Elementary School) through the 1990s (and early 2000s) you would certainly amend Pluto in this memorized list of planets in the solar system. The celestial body was no longer recognized as a planet in 2006, but to this day it still has a “legion of non-conforming fans”.

Perhaps it can comfort some of these people to know that, yes, the solar system is likely to have nine planets. But, no, the “extra” is not about Pluto.


Years after Pluto’s downgrade, do we have a new ninth planet in the solar system? Image: Withan Tor – Shutterstock

We can say that the hypothetical Planet 9, currently, is “neither there nor here”. That is, it may be located lurking on the edge of the solar system, this “new world” has been the target of speculation for some years now, trying to discover three things, basically: first, if it really exists; second, if it can even be considered a planet and, finally, of course, if it is part of our gang.

Where did the idea of ​​a possible Planet 9 come from?

According to the Phys website, the evidence for Planet 9 would come from its gravitational relationship with other bodies. If the planet exists, its gravity must affect the orbits of other planets.

Possible orbit of Planet Nine. Credit: CalTech / R. Wound (IPAC)

So if something seems to be pulling a planet, it takes a little math to find the source. This is how Neptune, for example, was discovered, when John Couch Adams and Urbain Le Verrier noticed that Uranus appeared to be pulled by an invisible planet.

In the case of Planet 9, we have no gravitational effect on a planet. What we see is a strange grouping of small icy bodies in the outer solar system, known as Kuiper belt objects (KBOs).

If there were no planet beyond the Kuiper belt, KBOs’ orbits would be expected to be oriented randomly within the solar system’s orbital plane. But instead, we see many orbits grouped in the same orientation. It is possible that this is just due to chance, but it is quite unlikely.

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In 2016, scholars analyzed the statistical distribution of KBOs and concluded that the clustering was caused by an undetected external planet. Based on his calculations, such a world would have a mass of five Earths and would be about 10 times farther from the Sun than Neptune.

The research even calculated a wide region of the sky where the possible planet would be. But this has come to nothing, leading many to conclude that the planet does not exist.

Orbital weirdness doesn’t prove a planet exists

Some have argued that Planet 9 exists, but we cannot see it because it would actually be a primordial black hole.

Now, a new study re-examines the original work in light of some of the criticism it has received. One is that bodies in the outer solar system are hard to find, so we look for them wherever it’s convenient.

The clustering effect seen by scientists may just be due to biased data. Taking observational bias into account, the authors of the current study found that clustering is still statistically unusual.

According to them, there is only a 0.4% chance of being a fluke. When Planet 9’s probable orbit was recalculated, they were able to better understand where to look.

After all, if so, is Planet 9 in the solar system or not?

An interesting aspect of the study is that the new calculated orbit places planet 9 closer to the sun than originally thought. That’s weird, because if it’s closer, it should have been found by now.

The authors argue that observations so far have ruled out the closest options for Planet 9, which helps to further narrow its possible location. If the planet exists, it must be detected by the Vera Rubin Observatory in the near future.

But be calm: this study is not conclusive and many astronomers still argue that Planet 9 does not exist. However, it looks like we won’t have to argue about this for much longer. That’s because scholars guarantee that one of the two: either it will be discovered soon or the observations will dismiss it as an explanation for the clustering effect of KBOs.

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