An article just published by a team of astronomers examines the evidence for the idea that another large planet, nicknamed Planet Nine, orbits the sun far beyond Neptune. What they found casts doubt on the assumption that the planet is out there. At the same time, they cannot say that the evidence points in the direction of the planet not existing.
We know eight large planets orbiting the sun, with Neptune farthest away at 4.5 billion kilometers, about 30 times as far as Earth from the sun. Outside Neptune are several groups of icy bodies, some of which are quite large, such as Pluto with a width of almost 2400 km. These are collectively referred to as Trans-Neptunian Objects or TNOs.
Some of these objects are extreme far away, such as 2012 VP113, which never gets closer to the sun than 12 billion km, and even reaches 65 billion km. This makes them very weak and difficult to find. Only a few dozen of these extreme TNOs (or ETNOs) have been discovered to date.
Several years ago, it was discovered that a handful of these (those who never even got close enough to the sun to be affected by Neptune’s gravity) all had orbits that were strangely aligned. We expect their jobs to be omnidirectional, not correlated. But they are not. They seem to have similar characteristic orientations, as if there is something that aligns them.
That could be another planet, heavier than Earth and very distant, that gravity interacts with them over time and organizes their orbits. Its position in the sky can be calculated very roughly using the orbits of those ETNOs as a guide, but nothing has been found so far.
The thing is, it’s possible that these ETNO observations are suffering from what’s called a selection biasThe surveys that find these objects usually only look into certain areas of the sky at certain times of the year, finding them more easily when they are closer to the sun and thus brighter. That means it is possible that the surveys tend to find those with their orbits aligned this way, selecting from a much larger population of objects that are truly randomly oriented.
If so, then the reason for looking for Planet Nine in the first place disappears. Obviously, it is important to understand how important a role this bias can play.
So that’s what the team of astronomers did. They examined the studies in question and noted how these observatories scan the sky. They then simulated a large population of objects beyond Neptune using realistic features, asking how many of them would miss the studies and if they would find one with aligned orbits.
What they found is that, statistically, the observations are from these ETNOs consistent coming from a larger population of objects with randomly oriented orbits. So it is possible that ETNOs are really uniformly distributed around the sun, and the effects of Planet Nine are an illusionWe just have to think we see its effects because of the way we observe these objects.
However, this does not definitively prove that this is the case! You can fit the data to any population, but you can also match them with a population affected by Planet NineEssentially, what they think is that the latter is much less likely.
But they cannot rule it out. Even if the distribution is random, it doesn’t prove Planet Nine doesn’t exist. It may still be there, it’s just that one of the first reasons to believe it’s weakened. I will note that the astronomers who search for it have other reasons to think it exists, because of the way it affects other objects in the outer solar system.
I will also add that this whole thing is severely limited by the very small number of these discovered objects. All of this is based on a few dozen of them, and you have to be extremely careful when dealing with metrics with a small number. It’s like flipping a coin four times and it pops up the same way four times. Is the coin fair? It could be; there is a 1 in 8 chance that this will happen randomly. You have to turn it over a lot more times before random probability comes into play that is small enough to be 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 look like. New telescopes coming online soon, like the Vera Rubin Observatory, will hopefully do just that, and other studies are being done as well.
Anyway, it is still a good idea to scan the sky to find planet Nine in the most likely locations. At best, it finds the planet. Yes! At worst, it’s more data that can be used for many purposes, and if Planet Nine isn’t found, we’ll learn something there too.
I admit I want the planet to be out there, because that would be extremely cool, and we would learn a huge amount about the history of the solar system. But because I lean in that way, I myself have to be skeptical of the claims made about it and scrutinize them. I have my own prejudices, just as we all do. In any case, we should not forget not to overinterpret the results and not to draw 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.