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An article just published by a team of astronomers examines the evidence for the idea that another major planet, dubbed Planet Nine, revolves around the Sun far beyond Neptune. What they found casts doubt on the case to assume the planet is out there. At the same time, they cannot say that the evidence points to the planet do 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 the furthest away at 4.5 billion kilometers, or about 30 times farther than Earth from the Sun. Beyond Neptune are several clusters of frozen bodies, some of decent size, like Pluto nearly 2,400 km wide. These are collectively referred to as Trans-Neptunian Objects, or TNO.
Some of these objects are extremely far, like the 2012 VP113, which never gets closer to the Sun more than 12 billion km, and goes up to 65 billion. This makes them very weak 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 discovered that a handful of them (those that never even get close enough to the Sun to be affected by Neptune’s gravity) all appear to have oddly aligned orbits. We would expect their orbits to be oriented all over the place, uncorrelated. But they are not. They appear to have similar characteristic orientations, as if something is aligned.
It could be another planet, more massive than Earth and far away, interacting with them by gravity over time, organizing their orbits. Its position in the sky can be calculated very roughly using the orbits of these ETNOs as a guide, but so far nothing has been found.
The point is that it is possible that these ETNO sightings suffer from what is called a selection bias. Surveys that find these objects tend to only look 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 the polls tend to find some with their orbits aligned this way, selecting them from a much larger population of objects that are really oriented at random.
If that’s the case, then the reason to search for Planet Nine in the first place disappears. Obviously, it is important to determine how important this bias can play.
So that’s what the team of astronomers did. They examined the polls in question, noting how these observatories sweep the skies. They then simulated a large population of objects beyond Neptune using realistic characteristics, and asked how many of them the polls would be missing, and if they would find those with aligned orbits.
What they found was that, statistically speaking, the observations of these ETNOs are coherent with them being a larger population of objects with randomly oriented orbits. It is therefore possible that in reality, ETNOs are uniformly distributed around the Sun, and the effects of Planet Nine are an illusion. We come thought we see its effects because of the path we observe these objects.
However, this does not conclusively prove that this is the case! You can fit the data with a random population, but you can also integrate them into a population 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 though the distribution is random, that doesn’t prove that Planet Nine doesn’t exist. It might still be out there, it’s just one of the first reasons to assume it’s has been weakened. I will note that astronomers who research it have other reasons to believe it exists, because of how it can affect other objects in the outer solar system.
I will also add that all this is severely limited by the very small number of these objects discovered. This is all based on a few dozen of them, and you need to be extremely careful when dealing with low count stats. It’s like flipping a coin four times and winding it up in the same way four times. Is the room fair? It could be; there is a 1 in 8 chance that this will happen at random. You have to flip it a lot more times before chance plays a role small enough that you can be more sure of the coin itself.
The best thing to do here is to get more observations. We need to find a lot more of these extreme objects and see what their orbits look like. We hope that new telescopes coming online soon, like the Vera Rubin Observatory, will do just that, and more investigations are also underway.
Scanning the sky for Planet Nine in its most likely locations is always a good idea anyway. At best, he finds the planet. Yay! At worst, it’s more data that can be used for many purposes, and if Planet Nine isn’t found, we’re still learning something there too.
I admit I want the planet to be there, because that would be extremely cool, and we would learn a lot about the history of the solar system. But because I lean in this way, I myself have to be skeptical of claims made about it and examine them critically. I have my own prejudices*, as we all do. We must remember not to overinterpret the results in either case, and not to draw definitive conclusions based on statistical data.
Hope we get a much better idea of Planet Nine soon.
*Mike Brown, one of Planet Nine’s actively researching astronomers, is an old friend of mine.
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