D+2: The Guide

In 2003, a Hispanic, an Anglo and a Jew walk into an observatory. This sounds like the beginning of a poor-taste joke, except that it actually happened. Because all three people involved (Chad Trujillo, Mike Brown and David Rabinowitz respectively) are badass astronomers, it resulted in the discovery of a dwarf planet called Sedna.

Sedna is very far away from the Sun, and is either on the outer edge of the Kuiper Belt or the inner edge of the Oort Cloud, depending upon whom you ask. Like most things that far out, she’s made mostly of ice and she doesn’t have a nice neat round orbit. Trujillo studied Sedna’s movement and concluded that when she was closest to the Sun, she would be 76 AU away (for reference, Neptune is 30 AU away) and at her furthest she would be an astonishing 937 AU, putting her on the near side of the Oort Cloud.

Sedna is a chunk of ice a thousand km across, which travels between two distances: “very far away” (we call this the perihelion) and “very, very, very far away” (we call this the aphelion.) It’s exciting to astronomers, but for the rest of us… well, are you interested? Be honest.

In 2012, Trujillo discovered a further lump of ice out in the void, which has not been properly named yet and so is called 2012VP113. He studied it and, along with Scott Sheppard, published a paper which told the world what its orbital measurements were. Again, astronomers were interested but the rest of us were busy with other things.

Trujillo and Sheppard then changed that: they pointed out that although 2012VP113 has a very different orbit from Sedna, they both have a similar perihelion. They not only both get closest to the Sun at around 80 AU, but both have a tilted orbit which tilts by the same amount at that perihelion point, and does it in the same direction. This might just have been coincidence, except that (as they pointed out in 2014) some Kuiper-belt objects have similar perihelions too. At this point it started to be a pattern, and the astrophysicists started to get interested.

Enter Konstantin Batygin, an astrophysicist. Astrophysicists study orbits, and we usually use computer simulations to do it. Batygin started studying this weird similar-perihelion pattern. What he noticed was amazing. See, most things in the inner solar system tend to have neat circular or nearly-circular orbits, and tend not to have tilted orbits. When you start looking at things that exist out past Neptune this stops being true. However, lots of the smaller Kuiper Belt objects have orbits which have a similar perihelion point to the one shared by Sedna and 2012VP113. Up until now nobody had thought to compare perihelions, but the more he looked the more he found a pattern. The crucial step was comparing a number called the “argument of perihelion.” Lots of the arguments of perihelion came out as around 300.

Space is big, and unlikely things can happen in it, but for arguments of perihelion to be the same is straight-up sorcery. By all rights they should change over the course of millions of years. Even if they were all the same once, the faint gravity from the gas giants in the inner solar system would nudge the outer objects into having different arguments of perihelion. Something had to be acting to make this happen.

Batygin then had a flash of genius: he started looking at the arguments of perihelion of other outer-system objects which are further out than 80AU. Most of them aren’t 300 (which is no surprise) but lots of them are closer to it than random chance would have it. It appears that some mysterious thing is pushing those objects’ perihelions towards that magic point.

The opposite of an argument of perihelion is an argument of aphelion. For objects closer to the sun than 80AU, the same mysterious thing would affect their aphelion instead of perihelion. When Batygin looked at their arguments of aphelion, he saw a similar pattern. One of the objects he looked at was the dwarf planet Pluto.

When Batygin saw Pluto acting funny, he knew that there was only one person he could talk to: Mike Brown. You may remember Brown from earlier in this essay, as one of the discoverers of Sedna. However, most people know Brown as the man who killed Pluto. (He is extremely proud of that, by the way; Brown’s twitter handle is @plutokiller.) Most importantly of all, the two of them are both based at Caltech and would pass each other in the hall every morning.

Batygin posed the question to Brown: there’s this mysterious thing that might be causing a lot of the weird orbits we see amongst the Kuiper Belt objects. It might even be the cause of Pluto’s weird orbit. We should probably check it out. Brown agreed. They managed to get a supercomputer, and started doing maths.

Supercomputer astrophysics is hard. You can’t just come up with the answer: you have to guess at what the answer might be, then check and see if you’re right. This takes time and often doesn’t give you any hints. They tried for a while. Eventually, however, they found a possible answer: it could be a planet. A distant planet with a tilted, eccentric orbit would give enough of a gravitational push to explain not only all the weird perihelion stuff, but also some other weird solar system things they hadn’t anticipated.

In January of 2016, Batygin & Brown published their paper, and it dropped like a dubstep bassline. Everyone was really excited.

The plan now is to refine Batygin’s simulations until we have a really good idea of where this mysterious guessed-at planet might be. Then, once we have an idea, we’ll give Brown the biggest telescope we have, point it at that place, and see whether we’re right or not. If we are, then there will be Nobel prizes to hand out.

For those who’ve been following my other column, this is very good news. For a long time we’ve assumed that the strange orbits of the outer objects meant that other systems would also have strange nonsensical orbits. However, if we find out that it’s all because of the influence of a single planet, then it means that space might be a tidier place than we think, which means that other systems might be tidier and thus your calculations will be easier.

Let’s wait and see.

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