Lurking Giant

Well this is quite interesting. There’s some latest news about the possibility of a giant planet lurking on the outskirts of our solar system. The near mythological “planet X” which astronomers have been hunting for for decades. This latest study come from Konstantin Batygin and Mike Brown, and I must say, their actual work seems a lot more cautious than many of the news headlines being thrown about. But that’s no shock. So let’s break this down, because it’s really quite interesting.

People have been hunting for planets beyond Neptune for a long time. Neptune was officially discovered in 1821, after it was noticed that Uranus had some perturbations in its orbit. The theory was that a previously unknown large planet was tugging on it gravitationally. Neptune was found to be the culprit, validating the hypothesis. But Neptune also has perturbations in its orbit, and so planet hunters continued their toil, eventually confirming the existence of Pluto in 1930. Pluto, however, is too small to have much effect on something the size of Neptune. The hunt continued awhile for the so-called Planet X♠︎ until the idea eventually fell from favour. Turns out, after more closely examining Neptune, it behaves exactly as it’s supposed to. But there are still a few things unaccounted for in the outer solar system.

Sedna‘s ludicrously eccentric orbit, for instance, can only be explained feasibly in 3 ways. It was captured from somewhere outside the solar system, it was yanked into a bizarre orbit by a passing star, or it was flung out that way by a large planet. Even if it’s simply that Sedna’s adopted, there are other unexplained phenomena in the mix.

All alone in the dark

The idea is not new. The wraithworld which was discussed in 2011 met with some staunch opposition, for instance. But Batygin and Brown are actually rather cautious in their paper (which is freely downloadable if you’d like to take a look yourself). They aren’t claiming the existence of any such planet, but simply presenting a hypothesis. Their ideas all seem perfectly sound to me.

I'm quite fond of eccentric things. Can't imagine why...One of the things which their hypothesis aims to explain is the orbits of several highly eccentric solar system objects. Notably, these objects all have a perihelion (the point at which they’re closest to the Sun) at a very similar distance. This clustering of perihelia is quite unlikely, with only a 0.007% likelihood of having happened by chance. No good scientist likes to explain things away with coincidences. Even if Sedna is adopted, it wouldn’t explain this.

Some detailed modelling suggests that this, together with a few other features of Kuiper belt objects, can be explained by the presence of a large planet in a highly eccentric distant orbit. “Large” in this case, means about 10 Earth masses, though a planet as small as 5 Earth masses may suffice. Perhaps our solar system does have a super-Earth planet in it after all. So that’s nice. Overall the hypothesis is consistent with observations, explains a number of different phenomena by only postulating a single entity, and fits with existing knowledge. A rather promising hypothesis then. There’s more info directly available from Mike Brown himself, who will no doubt do a much better job of explaining it than I would.

So the only thing remaining to prove this hypothesis is to find the smoking gun – if this planet exists, no one knows where it is, though we know roughly where to look. I have my suspicions that if it’s out there, we’ve probably already seen it in one or more of the many sky surveys that astronomers are constantly doing. Unfortunately, given how slowly it would appear to move at such a huge orbital radius, we may not have realised what it was and not known what we were seeing♣︎. An obvious thought is to search for it by looking for stars which move between the different survey dates. But all stars and other objects move. Some of them quite fast. It’s going to be a long search, probably with many more computer simulations to work out all of the details.

Actually, it’s quite likely that Neptune was first spotted by Galileo in 1612, who didn’t realise that he was looking at a planet. It was seemingly in the background of some of the sketches he made while his attention was focussed on Jupiter.

♠︎ That was originally a roman numeral, by the way. Planet X = Planet number 10.

♣︎ Just like Galileo.

Image credits:
Upper – Artists impression created by myself. Starfield background freely downloaded under a CC0 license.
Lower – ©2016, The American Astronomical Society. Taken from figure 2 of Batygin & Brown (2016) and reproduced here in accordance with fair use policies for the purposes of research, criticism, review, and news reporting.


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