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29 Jan 2016

Where is Planet Nine?

According to astronomers, a planet the size of Neptune is lurking somewhere in the outer reaches of the solar system beyond Pluto. Currently known as "Planet Nine" or "Planet X", this unseen planet is said to orbit the sun every 15,000 years. While the presence of Planet Nine has been inferred from a peculiar clustering of six previously known objects orbiting beyond Neptune, if truth be told, we still don't know exactly where it is. 

Evidence of the mysterious planet comes from Konstantin Batygin and Mike Brown of the California Institute of Technology (Caltech) in Pasadena. Published in The Astronomical Journal on January 20 in a paper called “Evidence for a Distant Giant Planet in the Solar System”, Batygin and Brown said “We motivate the existence of a distant, eccentric perturber.” Brown has already made headlines in recent years, with his 2005 discovery of the ice-world Eris responsible for the reclassification of Pluto as a dwarf planet.

The quest to find another planet has been active for a long time now, with discoveries usually met with scepticism and outright disbelief. According to Brown, "If you say, ‘We have evidence for Planet X,’ almost any astronomer will say, 'This again? These guys are clearly crazy'... Why is this different? This is different because this time we’re right.” Despite not actually sighting the planet, according to Brown, there is "0.007% chance, or about one in 15,000, that the clustering could be a coincidence."

According to the paper, Planet Nine was knocked out of the planet-forming region near the sun roughly 4.5 billion years ago, eventually settling down into a stable elliptical orbit somewhere on the outer reaches of the solar system. Planet Nine is said to have the mass of 10 Earths, with this mass helping to shepherd the six objects into strange elliptical orbits that are tilted out of the plane of the solar system. The ninth planet was discovered by observing the orbits of six Kuiper belt objects, all of which are stretched out in roughly the same direction. Renu Malhotra, a planetary scientist at the University of Arizona in Tucson, offers a simple analogy: “Imagine having pencils scattered around a desktop... If all are pointing in the same quarter of a circle, that’s somewhat unusual.”

The planet is said to stir up the orbits of other Kuiper belt objects, making them roughly perpendicular to the rest of the solar system. The existence of a ninth planet could also explain other oddities in the outer solar system, including the unusual location of the dwarf planets Sedna and 2012 VP113 far from other known planets. Despite these peculiarities, however, not everyone is convinced. According to Scott Tremaine, an astrophysicist at the Institute for Advanced Study in Princeton, while "The orbital alignments are striking... That can be very misleading... The numbers that won the Powerball lottery are an unusual combo, but that [particular combo] doesn’t mean anything.”

The orbit of Planet Nine is both stretched and tilted, with its closest approach to the sun being seven times farther than Neptune at 200 astronomical units (AU). At its furthest reaches, Planet Nine could reach between 600 to 1200 AU, although with each AU being about 150 million kilometres, this is a pretty large variation. Despite his surety, Brown seems to understand that no-one will believe in the ninth planet until they see it with their own eyes: “Until there’s a direct detection, it’s a hypothesis - even a potentially very good hypothesis.”

Image source: Vadim Sadovski/