A paper just published by a team of astronomers analyzes evidence for the idea that another important planet, nicknamed the New Planet, orbits the Sun far beyond Neptune. What they found calls into question the case for assuming the planet is there. At the same time, they cannot say that the evidence points to the planet no 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 eight major planets orbiting the Sun, with Neptune the furthest at 4.5 billion kilometers, about 30 times farther away than the Earth from the Sun. Beyond Neptune are several groups of icy bodies, some of them of decent size, such as Pluto about 2,400 km wide. These are collectively called trans-Neptunian objects or TNOs.
Some of these objects are extremely very far away, like the 2012 VP113, which never approaches the Sun more than 12 billion km and reaches 65 billion. This makes them very weak and hard to find. So far, 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 (which are not even close enough to the Sun to be affected by the gravity of Neptune) appeared to have strangely aligned orbits. We would expect their orbits to be oriented anyway, with no correlations. But they are not. They appear to have similar characteristic orientations, as if there were something aligning them.
It could be another planet, more massive than Earth and very far away, that interacts with them gravitationally over time, organizing their orbits. Their position in the sky can be roughly calculated using the orbits of these ETNOs as a guide, but so far nothing has been found.
The fact is that it is possible that these ETNO observations suffer from what is called a selection bias. Surveys that find these objects tend to look only at certain parts of the sky at certain times of the year, and find them more easily when they are closer to the Sun and therefore brighter. This means that it is possible that surveys tend to find others with their orbits aligned in this way, selecting them from a much larger population of objects that are actually randomly oriented.
If this is the case, the reason for searching for New Planet disappears. Viously, obviously, it is important to find out the importance of a role that this bias can play.
So that made the team of astronomers. They examined the surveys in question, observing how these observatories scan the sky. They then simulated a large population of objects from the past Neptune using realistic features and asked how many of them would be missing from the surveys and if they would find others that had aligned orbits.
What they found is that, statistically speaking, the observations of these ETNOs are coherent with a larger population of objects with randomly oriented orbits. Therefore, it is possible that ETNOs are actually evenly distributed around the Sun, and the effects of the New Planet are an illusion. We only to think we see its effects because of the way we observe these objects.
However, this does not conclusively prove that this is the case. You can adjust the data with a random population, however you can also adapt them to a population affected by the New Planet. 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, this does not prove that the New Planet does not exist. It could still be out there, is that one of the initial reasons for assuming it has been weakened. I will note that astronomers looking for it have other reasons to think it exists, due to the way it can affect other objects in the outer solar system.
I will also add that all this is severely limited by the small number of these objects discovered. All of this is based on a couple of dozen of them, and you have to be very careful when it comes to small number statistics. It’s like flipping a coin four times and making it appear the same way all four times. Is the currency fair? It could be; there is 1 in 8 chance that this will happen at random. You need to turn it over many times before chance plays a small enough role to make sure the coin.
The best thing you can do here is get more feedback. We need to find much 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, are expected to do so, and other surveys are also being conducted.
Scanning the sky for the new planet in its most likely places is still a good idea. At best he finds the planet. Yay! In the worst case, this is more data that can be used for many purposes, and if New Planet is not found, we also learn something from it.
I admit I want the planet to be out there, because it would be a lot of fun and we would learn a lot about the history of the solar system. But because I lean in this way, I have to be skeptical of the claims that are made of it and examine them critically. I have my own biases*, as we all do. We must remember not to over-interpret the results in any way or draw firm conclusions based on statistical data.
Hopefully, we will have a much better idea about the new planet soon enough.
*Mike Brown, one of the astronomers actively searching for the New Planet, is an old friend of mine.