The idea of a 9th large planet possibly existing on the outskirts of our solar system made quite the splash in the media when it was re-suggested earlier this year, but I have to emphasize that it hasn’t been directly detected – yet, in any case.
In late January, Konstantin Batygin and Michael Brown published a paper in the Astronomical Journal suggesting that the most plausible explanation they could think of for the data they were looking at was the existence of an unseen, very distant planet, which was almost immediately dubbed Planet Nine. This wasn’t the first suggestion of a ninth planet way out in the solar system – that honor goes to Chadwick Trujillo & Scott Sheppard, who published a short letter in Nature in 2014, making the same suggestion.
So what’s the data they’re looking at that suggests Planet Nine should exist? There’s a population of objects in our Solar System which orbit at very large distances from the sun – well past Neptune. Now, Neptune is a bit of a heavyweight in the outer solar system, and if objects get close to Neptune, Neptune can dramatically alter the way they orbit the Sun. But if you keep going farther out, you’ll find a set of objects which never come close enough to the Sun to be perturbed by Neptune. Neptune orbits our Sun 30 times farther out than the Earth does, so these are very, very distant objects.
These distant objects, unperturbed by Neptune, have weird orbits. They’re not a funny shape or anything, but the objects that have been found out there so far seems to have oddly clustered orbits. They’re all very eccentric, long, narrow ellipses, and way more consistent with each other than you would expect. In the absence of anything strange going on, you’d expect these distant objects to be scattered randomly though the outer solar system – and so far, that’s not what they’ve found.
So, to explain this clustering of the orbits of very distant objects, both sets of authors suggest that a large object might be responsible. If there was something else out there, it could be rattling the orbits of the smaller objects, in the same way that Neptune rattles the orbits of more nearby objects. Hence: Planet Nine.
This Planet Nine has been theorized to be about ten times the mass of the Earth, which puts it at about 60% as big as Neptune, which a bit heftier, at 17 times the mass of the Earth. Neptune is probably a decent point of comparison here. It seems likely that if Planet Nine is out there, it’s likely to be a smaller version of the ice giant planets we’re already familiar with, such as Neptune and Uranus. Uranus, for reference, is about 14.5 times as massive as the Earth, so our potential Planet Nine is about 2/3rds the size of Uranus.
So now the hunt is on to see if we can find it. An object ten times the mass of the Earth isn’t undetectable, and there’s a limited area of space in which it could be hiding. We can rule out some sections of that area already because it should have been spotted by surveys that have already looked at those patches of sky. There are also some new surveys starting observations soon, which should have the ability to spot Planet Nine, should it exist.
The other option, of course, is that the data will improve, and will instead fill out the distribution of distant objects so that they don’t look so clustered together. However, that would also require some weird biases to be present in the data we have now, and all four authors suggesting a distant planet have worked hard to understand the biases in their data. So we can’t say for sure if there really is a Planet Nine out there until we find it, but in the mean time we’ll be looking both for Planet Nine and for any discoveries that might remove our need for a distant planet to explain the data.
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