In a paper recently published by a team of astronomers, the evidence is stamped for the idea that another large planet, nicknamed Planet Nine, orbits the Sun far beyond Neptune. What they found leaves the matter in doubt to assume that the planet is there. At the same time, they can not say that the evidence points to 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 large planets orbiting the sun, with Neptune at the farthest point at 4.5 billion kilometers, about 30 times farther from Earth from the sun. Outside Neptune there are various groups of icy bodies, some of which are quite large, such as Pluto almost 2400 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 up to 65 billion. This makes them very faint and hard to find. To date, only a few dozen of these extreme TNOs (or ETNOs) have been discovered.
A few years ago it was found that a handful of these (one that never even comes close to the sun to be affected by the gravity of Neptune) all resembled orbits that were oddly in line. We expect their orbits to be directed in any way, uncorrelated. But it is not. They seem to have similar orientations, as if there is something that aligns it.
It could be another planet, more massive than Earth and very far out, which over time interacts heavily with them and organizes their orbits. Its position in the air can be very roughly calculated using the orbits of those ETNOs as a guideline, but so far nothing has been found.
The point is that it is possible that these ETNO observations suffer from the so-called a selection bias. The recordings that these objects find tend to look only at certain times of the year in certain parts of the sky and find it easier when they are closer to the sun and therefore brighter. This means that it is possible that the recordings tend to find with their orbits, and it selects from a much larger number of objects that are really randomly aimed.
If so, the reason for looking for Planet Nine in the first place disappears. Clearly, it is important to find out how important this prejudice can be.
So this is what the team of astronomers did. They examined the surveys in question and noticed how these observatories looked through the sky. Thereafter, they simulated a large number of objects outside Neptune using realistic features, asking how many of them would miss the recordings, and whether they would find an orbit that brought them into line.
What they found is that the observations of these ETNOs are statistically speaking consistent with them from a larger population of objects with random orbits. It is therefore possible that ETNOs are uniformly distributed over the sun, and the effects of Planet Nine are an illusion. We just have think we see the consequences because of the manner we observe these objects.
However, this does not conclusively prove that this is the case! You can fit the data into a random population, but you can also fit them into a population affected by Planet Nine. What they essentially find is that the latter is much less likely.
But they can not rule it out. Even if the distribution is random, it does not prove that Planet Nine does not exist. It may still be there, but just one of the initial reasons to assume it has weakened. I will note that the astronomers who seek it have other reasons to think that it exists, because of the way it can affect other objects in the outer solar system.
I would also add that this whole thing is severely limited by the very small number of objects discovered. This is all based on a few dozen of them, and you need to be extremely careful when working with statistics on small numbers. It’s like tossing a coin four times and tossing it four times in the same way. Is the currency fair? It can be; there is a 1 in 8 chance that this will happen randomly. You have to turn it over many times before the chance happens to play a small enough role to be more sure of the coin itself.
The best thing to do here is to get more observations. We need to find many more of these extreme objects and see what their orbits are like. New telescopes that will be online soon, such as the Vera Rubin Observatory, will hopefully do just that, and other surveys will be done.
Scanning the sky to search for Planet Nine in its most likely 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 many purposes, and if Planet Nine is not found, we’re still learning there.
I will admit that I want the planet to be there because it will be extremely nice, and we will learn a lot from it about the history of the solar system. But because I lean so much, I myself have to be skeptical about claims about it and critically examine it. I have my own prejudices*, just like we all do. We must remember not to over-interpret the results, and not to draw firm conclusions based on statistical data.
Hopefully we will have a better idea about Planet Nine soon enough.
*Mike Brown, one of the astronomers actively searching for Planet Nine, is an old friend of mine.