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I’ve gotten texted twice today about the story that Caltech astronomers have found more evidence there could be a massive planet in the outer solar system, perhaps ten times the mass of Earth. My initial reaction was to dismiss it as a crackpot theory, because there have been a couple of poorly-supported claims about this kind of thing in the past five-odd years. But this one seems better.
For one thing, the astronomers are well known and have reputations for reliable work and solid accomplishments. Mike Brown and Chad Trujillo have found other ice planets in the outer solar system, at least through direct observation. And whereas the earlier claims were based on pretty thin evidence, this one has stood up to a direct challenge from Brown already.
What’s happening is that the most distant ice planets in our solar system, like Sedna, turn out to have elongated orbits — ellipses — with the Sun near one end. Ordinarily, you’d expect that the point of closest approach, the perihelion, would be randomly distributed for each of these. Some would swing by on one side of the Sun, others on another side. But Trujillo found that they’re mostly oriented to swing past just one side of the Sun.
One possibility is that a massive planet is out there, regularly running through their orbits in such a way that it scattered them away, making them cluster together on the far side.
Mike Brown initially dismissed this idea, saying (pretty much correctly) that planetary astronomers like to invoke Planet X whenever they see something strange in a planet’s orbit. Go read anonymous’s review of The Hunt for Vulcan for a classic example. But as he tried to debunk the idea, he wound up convincing himself it was the best explanation.
The discussion I’m seeing among professional astronomers is a mix of the eager and skeptical, with the skeptics pointing out that there have been some infrared searches of the whole sky that were able to rule out any more undiscovered massive planets … but only for certain limits in mass and distance. There is debate about how brightly such a planet would glow in the infrared, and whether the existing searches should have ruled it out or not.
On a final note, I’ll say that Mike Brown’s scientific work is good enough for me to overlook for the moment his obnoxious behavior regarding Pluto. He was probably the big mover behind the International Astronomical Union’s move to unilaterally redefine the meaning of the word “planet” in every language on Earth just to keep Pluto out of the definition. I’ll write some other time about the dirty tricks that were used to pull that vote off, but I don’t want to go too far out on a tangent now.
Anyway, I’ve had the impression for years that he’s got a big ego, and he has some kind of weird personal grudge against the planet that goes beyond rational explanation. I don’t mean this as a flippant joke. His Twitter handle is @plutokiller. And with that in mind, I am not going to call this thing “Planet Nine.” That’s just feeding his ego about trying to erase Pluto from the books, and it’s really blatant. Notice in the chart above, which comes from Brown’s press release, they show the orbit of Neptune, but not the larger and more relevant orbit of Pluto, which is generally used for these kinds of comparisons. This is just to stroke Mike Brown’s ego and pretend that he has somehow made Pluto physically disappear.
But he does solid work, and while I hold out the possibility that he’s wrong, I’m not dismissing this as a crackpot idea. The next step is to look for the thing, and I read that a couple of observatories are already looking along the proposed orbit for to see if anything’s there. It’s an exciting possibility, and I’m hoping they find it.
For those interested in the details, here’s the paper.Published in