Update, 20 January: Mike Brown and聽Konstantin Batygin say that they have found evidence of 鈥淧lanet Nine鈥� from its effect on other bodies orbiting far from the sun
Take a look at a late Victorian orrery: its eight mechanical planets trace out neat, concentric orbits around the sun, from Mercury to Neptune. This depiction reflects the confidence we had in the contents 鈥� not to mention the shape 鈥� of our solar system. Recent discoveries are shaking that confidence. Is the outer solar system hiding something more? Bigger than a comet, bigger than a dwarf planet, bigger even than Earth?
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It wouldn鈥檛 be the first outing for 鈥淧lanet X鈥�. It was first dreamed up as an explanation for mysterious glitches in the orbits of Uranus and Neptune. Even Pluto鈥檚 discovery in 1930 couldn鈥檛 account for these strange perturbations. It now seems they were simply down to poor observations, but the suspicion that Planet X is lurking in the dark reaches of the solar system has never gone away.
What we do now know is that there is more out there than we imagined. Pluto is just one of many millions of icy bodies in the vast Kuiper belt. At least two of these are other dwarf planets about the size of Pluto.
Then there鈥檚 the Oort cloud, the spherical swarm of a trillion comets loosely bound to our solar system. Its outer edge stretches more than halfway to the next star. Objects in the Oort cloud are too small and far away for us to observe directly, but a 2009 survey by NASA鈥檚 exoplanet-hunting telescope Kepler was able to investigate. It found nothing substantial. 鈥淭he outer solar system probably does not contain a large gas giant planet, or a small companion star,鈥� says Kevin Luhman, an exoplanet scientist at Pennsylvania State University.
But what about something smaller? Scott Sheppard at the Carnegie Institution for Science in Washington DC thinks we might still find an undiscovered super-Earth within the solar system, a rocky world several times more massive than our own.
His evidence? Sedna. In 2003, Mike Brown and his team at the California Institute of Technology found this dwarf planet swinging in an elliptical orbit in our solar system, ranging from 75 astronomical units to almost 1000 AU away from the sun (1 AU being the distance from the sun to Earth). Its solar 鈥測ear鈥� lasts about 10,000 Earth years. A decade later, Sheppard found a second distant object, 2012VP113, on its own highly elliptical orbit (see diagram).
Echoing the mystery of Neptune鈥檚 wobble, these orbits can鈥檛 be explained by our current understanding of the solar system, Sheppard says. Elliptical orbits happen when one celestial object is pushed around by the gravity of another. But both Sedna and 2012VP113 are too far away from the solar system鈥檚 giants 鈥� Jupiter, Saturn, Uranus and Neptune 鈥� to be influenced. Something must be stirring the pot.
If there is something else out there, have we been looking in the wrong place? The orbits of both Sedna and 2012VP113 are highly inclined relative to the plane in which the eight major planets orbit. Planet X is likely to be too. In other words, our solar system may actually be a 3D structure, its furthest outer bodies orbiting at preposterous angles.
Last month, Sheppard鈥檚 idea got some traction when two separate teams of astronomers announced they had seen unexplained objects that might correspond to a Planet X 鈥� one , the other a . Both lie far from the traditional plane of the solar system.
Not everyone takes these findings seriously. According to Brown, finding something this big in a telescope鈥檚 minuscule field of view implies there should be 200,000 super-Earths, enough to destabilise the entire solar system. Still, as long as there are telescopes to do it with, people will hunt for Planet X. Pity the orrery makers if they find it.
(Image: Claus Lunau/Science Photo Library)
This article appeared in print under the headline 鈥淚s there a planet x?鈥�
