Astronomers from the California Institute of Technology have discovered the signs of a planet lurking far beyond the orbit of Pluto, signalling that the Solar System’s catalogue of planets might still be incomplete. The supposed body, temporarily dubbed Planet Nine, could exist because, if it did, it would explain away many other observations made in the last few years in its neighbourhood that’ve defied understanding.
If it exists, Planet Nine is floating in the Kuiper Belt, a disc of icy rocks extending beyond the orbit of Pluto, clumps of materials leftover from the stuff that formed the system’s eight planets. Nine’s distance from the Sun is thought to be about 90 billion km on average (since it follows an elliptic path), and it takes between 10,000 and 20,000 years to orbit the Sun once. It hasn’t been directly observed yet.
It has been conjectured into existence at all by Mike Brown and Konstantin Batygin because of a bunch of Kuiper Belt objects – at least six – with strange very-elliptical orbits; Brown had co-discovered the first of them in 2003. In 2014, two other astronomers had found that the points of closest approach between all of these objects to the Sun lay along the plane of the Solar System – the plane on which the system’s planets sit. They also found that the six moved in a common direction when crossing the plane. Both observations were far too orderly to have been random; the odds were almost 1 in 14,300. And they drew the attention of Brown and Batygin.
The duo’s planet, should it exist, will be able to account for both peculiarities. In fact, because of its predicted mass – ranging from 5-20 times as much as Earth (based on notes on Batygin’s blog) – it will also be messing with the orbits of some Kuiper Belt objects, causing them to run at an angle of more than 60º to the plane of the Solar System.
Four such objects have already been found but spotting more would help make a stronger case for this super-Earth beyond Pluto. Such a case could also be reassuring because super-Earths are the most common type of exoplanets that astronomers have found, especially with the Kepler telescope, yet a type that’s been conspicuously absent in our planetary neighbourhood.
In the meantime, Brown and Batygin have published a paper – in The Astronomical Journal on January 20 – in order to draw the attention of their peers and hopefully enlist their help in looking for Nine. Nature reported Batygin as saying it could be “smaller than Neptune and icy with a gassy outer layer”. But to settle the question of its features conclusively, astronomers will have to spot the planet first, possibly with the powerful twin Keck Telescopes on Mauna Kea in Hawaii. And until then, we’ll have to be open to the possibility that there might not even be a Planet Nine, no matter that the evidence seems strong.
Incidentally, Brown was indirectly responsible for Pluto’s demotion from planet to dwarf planet after he discovered a Kuiper Belt object in 2005, called Eris, whose size rivalled Pluto’s but more importantly indicated that Pluto wasn’t unique after all. In a press statement announcing the possible existence of Nine, Brown (whose handle on Twitter is @plutokiller) said, “All those people who are mad that Pluto is no longer a planet can be thrilled to know that there is a real planet out there still to be found.”