A paper just published by a team of astronomers looks at the evidence for the idea that another major planet, nicknamed Planet Nine, orbits the Sun far beyond Neptune. What they found casts doubt on the case to suppose the planet is out there. At the same time, they cannot say the evidence points toward the planet not existing.
[UPDATE (Feb. 16, 2021 at 20:30 UTC): Mike Brown, one of the astronomers looking for Planet Nine, weighed in on this paper not long after I posted this article. He shows that, broadly speaking, the paper is correct in its methodology, but (as I also say below) cannot disprove the orbital alignments of the objects observed. It adds some uncertainty to the observations but in fact the observations are still wholly consistent with the existence of Planet Nine.]
We know of eight major planets orbiting the Sun, with Neptune being the farthest out at 4.5 billion kilometers, roughly 30 times more distant than the Earth from the Sun. Out beyond Neptune are several groups of icy bodies, some of them decently sized, such as Pluto at nearly 2,400 km wide. These are collectively called Trans-Neptunian Objects, or TNOs.
Some of these objects are extremely far away, like 2012 VP113, which never gets closer to the Sun than 12 billion km, and gets as far as 65 billion. This makes them very faint and difficult to find. To date, only a few dozen of these extreme TNOs (or ETNOs) have been discovered.
Some years ago, it was found that a handful of these (ones that never even come close enough to the Sun to be affected by Neptune's gravity) all seemed to have orbits that were strangely aligned. We'd expect their orbits to be oriented every which way, uncorrelated. But they're not. They seem to have similar characteristic orientations, as if something is out there aligning them.
That could be another planet, more massive than Earth and very far out, that interacts with them gravitationally over time, organizing their orbits. Its position in the sky can be very roughly calculated using the orbits of those ETNOs as a guide, but so far nothing has been found.
The thing is, it's possible that these ETNO observations are suffering from what's called a selection bias. The surveys that find these objects tend to only look in certain parts of the sky at certain times of the year, and find them more easily when they're closer to the Sun and therefore brighter. That means it's possible the surveys tend to find ones with their orbits aligned this way, selecting them out of a much larger population of objects that really are oriented randomly.
If that's the case, then the reason to look for Planet Nine in the first place goes away. Obviously, it's important to figure out how important a role this bias may play.
So that's what the team of astronomers did. They examined the surveys in question, noting how these observatories scan the sky. They then simulated a big population of objects out past Neptune using realistic characteristics, and asked how many of them the surveys would miss, and if they would find ones that had aligned orbits.
What they found is that, statistically speaking, the observations of these ETNOs are consistent with them being from a larger population of objects with randomly oriented orbits. So it's possible that really, ETNOs are uniformly distributed around the Sun, and the effects of Planet Nine are an illusion. We just think we see its effects because of the way we observe these objects.
However, this doesn't conclusively prove that's the case! You can fit the data with a random population, but you can also fit them with a population that is affected by Planet Nine. What they find, essentially, is that the latter is much less likely.
But they can't rule it out. Also, even if the distribution is random, that doesn't prove Planet Nine doesn't exist. It could still be out there, it's just that one of the initial reasons to suppose it is has been weakened. I'll note that the astronomers who are looking for it have other reasons to think it exists, because of the way it may be affecting other objects in the outer solar system.
I'll add too that this whole thing is severely limited by the very small number of these objects discovered. All of this is based on a couple of dozen of them, and you have to be extremely careful when dealing with small number statistics. It's like flipping a coin four times and having it come up the same way all four times. Is the coin fair? It might be; there's a 1 in 8 chance of this happening randomly. You need to flip it a lot more times before random chance plays a small enough role that you can be more sure of the coin itself.
The best thing to do here is get more observations. We need to find a lot more of these extreme objects and see what their orbits are like. New telescopes coming online soon, like the Vera Rubin Observatory, will hopefully do just that, and other surveys are being done as well.
Scanning the skies to look for Planet Nine in its likeliest locations is still a good idea anyway. At best it finds the planet. Yay! At worst it's more data that can be used for lots of purposes, and if Planet Nine isn't found, we still learn something there, too.
I'll admit I want the planet to be out there, because that would be extremely cool, and we'd learn a huge amount about the history of the solar system from it. But because I lean that way I myself have to be skeptical of claims made about it and examine them critically. I have my own biases*, just as we all do. We have to remember not to over-interpret results either way, and not make firm conclusions based on statistical data.
Hopefully, we'll have a much better idea about Planet Nine soon enough.
*Mike Brown, one of the astronomers actively looking for Planet Nine, is an old friend of mine.