Planet 9: Can We Get Excited?

I’m back! And very pleased to be so, I must say, I’ve missed writing about science. It’s not going to be easy coming back though, the reason being that it’s going to be nigh on impossible deciding which story to go with. In the week I’ve been away we’ve discovered the largest ever creature to have walked the earth, the brightest ever supernova explosion and that 2015 was by far the hottest year on record; no surprises there. For me, though, there is only one thing I could write about this week: Planet 9.

I mean, seriously, how cool would that be? Having a ninth planet back in the neighbourhood. What, though, is the evidence? And should our sceptical alarms be going off at such a bold claim? There have been claims about ‘missing’ planets made in the past and they have never come to anything. Let’s try to puzzle this out.

The paper (open access) has been published by serious astronomers from a serious institution in a serious journal, so that’s a good start. Konstantin Batygin and Michael Brown, both of Caltech, are theoretical and observational astronomers respectively and the story that culminates in their claim goes back a long way; all the way back to 2003 and the discovery of Sedna.

Sedna was discovered by Brown along with one of his post docs, Chad Trujillo. It is a world not too different from Pluto but a lot further away. Currently it is about twice as far from the sun as Pluto is, but its highly elliptical orbit will take it more than 10 times further out than that during its eleven and a half thousand year orbit. The problem with Sedna, Brown realised, was that there was likely to be lots of other objects out there waiting to be discovered just like Pluto and Sedna. If Pluto was a planet, then Sedna had to be a planet too, and all the yet to be discovered objects too. So we were left with a situation where we were either going to have to have dozens of planets in our solar system or we could exclude Pluto, go down to 8 planets and come up with a new category of dwarf planet for all the Kuiper Belt objects that were big enough.

We all know how that story ended.

Fast forward a decade and Trujillo was still working on finding and describing Kuiper Belt objects, a tough task given they’re quite small and long way from the sun. Last year, now working independently of Brown, Trujillo and his colleague, Scott Shepherd, published a paper noting that 13 Kuiper Belt objects had a quirky orbital feature in common with each other. All 13 of them had orbits at the same sticky out angle, about 30 degrees from the plane of the 8 known planets.

One of several possible solutions to this anomaly suggested by Trujillo and Shepherd was the presence of a ninth planet; though they suggested it tentatively. Brown didn’t think this at all likely but he did find it an interesting problem to solve and so he walked down the corridor to the office of Batygin and so began an 18 month collaboration that Brown described as, “…perhaps the most fun year of working on a problem in the solar system that I’ve ever had.”

The combination of Browns observations and Batygin’s theoretical modelling proved a powerful one and before long they were forced to the conclusion that a ninth planet was actually quite a good way of explaining the observations. They came up with a set of characteristics for the theoretical planet that produced models that most closely looked like the real solar system we see.

Planet 9, as they have dubbed it, would be about ten times the size of the earth; so a proper planet, then, none of this dwarf planet nonsense. It would have to have a highly elliptical orbit coming no closer than 200 AU (1 Astronomical Unit is about 150 million km) to the sun and reaching as far out as 1000 AU. Pluto averages about 40 AU from the sun so we are talking about something very, very far away here.

Interestingly, the position of Planet 9 at perihelion would have to be on the opposite side of the sun to the other Kuiper Belt objects when they are at perihelion. Initially they thought this would be a nail in the coffin of the theory as they guessed that the orbits would be unstable and they would ultimately collide with one another. It didn’t work out this way, however, as a mechanism known as mean-motion resonance meant that, far from being unstable, this was by far the most stable way for them to coexist.

Even at this point Batygin was sceptical about Planet 9 having never seen a mechanism like this before in celestial mechanics. As they pushed the model further, though, they were able to use it to make predictions about the orbits of other objects in the night sky which Brown was able to confirm. This is strong evidence that the hypothesis has some weight to it. It’s all well and good making up an idea that fits a phenomenon that you originally wanted to explain, but when it also starts making accurate predictions about unrelated items that you hadn’t at first considered then that kind of makes you sit up and take notice. Batygin says it’s like killing to birds with one stone but one of the birds was in a nearby tree that you hadn’t even realised was there.

So there you have it, that’s our evidence to date. No direct observations yet, obviously that’s what we need next and hopefully there are plenty of scopes out there looking in the likely areas. Personally, I think we can be optimistic, I don’t think it’s crazy to say we have a whole new planet in the sky as opposed to pie in the sky.

I’m sure it won’t have escaped your notice that it is the same man that got Pluto dropped as a planet that is now putting forward the argument for a new ninth planet. Michael Brown, Twitter handle @plutokiller, had a rough time of it from people who felt very strongly about Pluto’s demotion; I wonder, if he’s proved right, if they will be able to forgive him.

P9_KBO_orbits_labeled
Image courtesy of Batygin and Brown
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