The story of the new ninth planet in the Solar System is quite possibly the most exciting stories to come out of solar system astronomy in quite a long time. Largely because it suggests that the solar system is still a kind of wild west, full of the strange and the unknown, and we could get another ninth planet to replace the void Pluto left in many people’s hearts. The story is really much longer and stranger than that, though, and it’s over a hundred years old…
The saga of the Ninth Planet really starts with the Eighth planet. In the years following the discovery of Uranus, astronomers made serious advances in accounting for the tiny pulls of the planets on each other allowing them to produce accurate tables of planetary positions. The French astronomer Urbain Le Verrier discovered that Uranus did not follow the calculated path, which he interpreted to mean something else had to be gravitationally affecting Uranus. In 1846 he made calculations and determined what kind of orbit this additional planet would have, and where it would be in the sky; Johann Galle found the Eighth Planet almost exactly where Le Verrier predicted.
The first attempt at finding a ninth planet actually went inward. Emboldened by the success with Neptune, Le Verrier went on to predict a planet even closer to the Sun than Mercury, based on the ways the motion of Mercury didn’t match its predicted orbit. This planet, dubbed Vulcan, could have fairly easily been missed because it would always appear very close to the Sun. It was even “seen” a few times (probably sunspots or sungrazing comets) but unfortunately, Vulcan does not exist. The deviation of Mercury’s orbit was explained completely by general relativity. There could still be an asteroid belt there, but they’d all be too close to the Sun to easily detect, and the IAU’s Minor Planets Center doesn’t list any asteroids there at all. Yet. (You’d be looking for something with “a (AU)” less than 0.38 in that table).
The other search for the ninth planet is beyond Neptune. There were inconsistencies with Neptune’s orbit as well, leading to (among other things) Percival Lowell setting up telescopes at Lowell Observatory just to look for it. This led to the almost accidental discovery of Pluto in 1930. I’ve covered Pluto elsewhere, but suffice it to say it was quickly recognized as being too small (particularly after the 1978 discovery that what we thought was Pluto, was Pluto and a gigantic moon Charon) and not alone (after the 1992 discovery of 1992 QB1) and not even the largest object in its region of space (the 2005 announcement of Eris). In any case, more recent calculations suggest there’s actually nothing wrong with the orbit of Neptune after all.
Coming at Planet Nine from another angle, a theory proposed during the 20th century was that the Sun had a stellar companion, called Nemesis. This star was supposed to have a highly elliptical orbit that brought it close to the Solar System every 26 million years, stirring up comets that would eventually cause mass extinctions. It’s an intriguing theory (many stars have companions), but every infrared all-sky survey from the 1983 IRAS survey to the 2010 WISE survey has turned up nothing, when even a tiny star or cold brown dwarf should have shown up as a bright object moving fairly quickly across the sky. Every so often, someone re-evaluates the evidence and the maximum size of an undetected object in the solar system drops. The largest things that fit now are smaller-than-Jupiter-sized objects.
This Nemesis theory seems to have been chopped up and repurposed as “Nibiru”, a planet that was supposed to destroy the Earth in 2003. Or 2012. The allegations are, of course, that NASA has known about this planet for decades and is covering it up… Of course, the conspiracy is ridiculous. Even if NASA had every single space agency on the Earth under its thumb, amateur astronomers would be able to see its effect on the positions of the other planets, and calculate, as Le Verrier did, the location of the planet. Heck, as it’s usually described, Nibiru would be something you could go outside and see just by looking up.
There have been other attempts over the years, (many listed here) mostly whittling down the regions where an additional distant planet could hide without producing detectable effects. Very recently, ALMA studies found apparent planets, but they seem more likely to be spurious detector effects or two distant galaxies rather than one moving object.
On the other hand, there ARE some detectable effects, and that’s what Kontantin Batygin and Mike Brown went after with this potentially real Planet Nine.
For one thing: the Kuiper belt now clearly has an outer edge (the “Kuiper Cliff”). It doesn’t just trail off into nothing. In the solar system and in extrasolar planetary systems, edges of disks are a sign that something’s sitting just outside it, tossing out anything that strays.
For another thing: the entire solar system basically lies in a single plane, until you go thousands of astronomical units away and reach the Oort cloud. The only exceptions are comets (which come in from the Oort cloud), and objects that have been flung around by the planets (Pluto, for instance, interacts with Neptune). The dwarf planet Sedna is on an odd highly-elliptical and slightly inclined (tipped, relative to the plane of the solar system) orbit but it doesn’t come close enough to Neptune to have interacted with it. And actually, there are six other distant Kuiper Belt Objects (KBOs) like it, all of which have similar orbital orientations. Explaining that was the problem that’s occupied quite a few solar system astronomers’ time since 2003 (this is not the first Planet Nine prediction based on the distant Kuiper Belt orbits), most recently Batygin and Brown.
What they found was that these orbits were best explained by a planet. The tricky bit of their prediction was that a planet close enough to force these KBOs to align, would also be so close it SHOULD affect the orbit of Neptune and disrupt the Kuiper Belt. The SIMULATION they did, however, shows that these six strange KBOs could be in semi-stable orbits where they should be kicked out, ANTI-aligned with a more distant planet. The preferred planet they came up with is on average 700 AU from the Sun with an eccentricity of 0.6, coming as close as 280 AU and as far as 1120 AU, inclined (tipped) by 30 degrees to the plane of the Solar System. It is, on average, ten earth masses (for comparison, Uranus is 14 and Neptune is 17) and its year (time to orbit the Sun) would be over 18,000 years long. Ironically, we may have to infer the other properties of this solar system object from extrasolar planet studies, rather than the other way ’round as is usual.
This theory has one unexpected bonus: It explains where a few outer-solar-system objects on really highly inclined orbits (almost at right angles to the rest of the Solar System) came from. Also, this planet probably formed much closer to the Sun but was flung out by repeated interactions with other objects… perhaps this is the object that explains why Uranus is tipped on its side.
Right now, not much is known about Planet Nine. It could be more distant and more massive, it could be closer and lower-mass… and not knowing the exact orbital shape means there’s a lot of sky to examine if you want to find this thing. It should be extremely cold such that it wouldn’t have already shown up in infrared surveys like IRAS or WISE. (XKCD has a good breakdown of the limits on what kind of planet this would have to be.) It will be moving across the sky very fast – at its closest, I calculate that its proper motion, or apparent angular movement across the sky, will be about 276 arcseconds per year; at its most distant, it’ll move 34.5 arcseconds per year… That’s an awkward range, because it’s a lot slower than Kuiper Belt Objects that planetary scientists are used to – Eris is currently moving at 1379 arcseconds per year – and a lot faster than stellar astronomers like me are used to. The fastest moving star, Barnard’s Star, is moving 10 arcseconds per year; usually nobody is looking for anything faster than half an arcsecond per year.
And of course, there’s a possibility that Batygin and Brown and friends are wrong about all of this. For one, the alignment of the KBOs could be a coincidence; the next six could be completely scattered and we were just enormously (un)lucky with the first six. Mike Brown has entire page of reasons why you could be skeptical here (it seems to be down now, but you may have more luck). It’s decidedly not my field of expertise, but right now I’m tending toward the idea of this Planet Nine being real. It’s going to be something to watch.
Update April 17, 2016: A seventh distant KBO has been found, and guess what? Its perihelion angle is aligned with the other six. That’s further evidence in favor of the existence of Planet Nine.
Other posts about the edge of the Solar System: Pluto was Never a Planet, on the Exploration of Pluto, in which Pluto is Revealed, on the Most Distant Object in the Solar System, on the Search for Planet Nine (Part 2), on the Secrets of Pluto