This week, science news was flush with the confirmation of the discovery of 2015 TG387, unofficially nicknamed “The Goblin.” It’s a cold world of ice, an estimated three hundred kilometres across–in an extremely elliptical orbit. At its closest, the Goblin is little more than twice as distant as Pluto–at its furthest, the Goblin is a staggering sixty times as distant. It takes forty millennia to complete a full orbit. Its discovery was largely pure luck; had the Goblin been any further out, it would have sailed on through the dark.
“The Goblin” is merely the latest in an ever-growing rogues gallery of minor planets and other objects that orbit the Sun in the shadowy regions outside the orbit of Neptune. It’s quite remarkable; not thirty years ago, there was only Pluto and its lonely moon Charon.
Most schoolchildren will have heard the story of Pluto’s discovery: it was pinpointed in 1930 after a decades-long search for an unknown “Planet X” that would explain irregularities in Neptune’s orbit. At first, it seemed that Pluto was the answer; astronomers in the 1930s thought Pluto was the mass of Earth. But as decades passed, and as observations improved, Pluto’s mass shrunk: first a tenth of Earth’s mass, then a fortieth, then a hundredth, then–after Charon was discovered in 1978–a measly five hundredth.
Not only was Pluto far too small to be Planet X, its highly slanted and irregular orbit made it an oddity amongst then-planets, and so the search proceeded. Doubts mounted that Planet X existed at all. Meanwhile, the far outer solar system continued to be populated by other speculation. Much of this was fuelled by the mystery surrounding the origin of comets–in particular, long-period comets that had extremely elongated orbits apparently originating in the solar system’s edge.
One theory was that there might lie a second asteroid belt beyond Neptune–this one made instead of ice and comets. This would come to be known as the Kuiper belt, after a Dutch astronomer named Gerard Kuiper. Another theory stated that there additionally existed an entire cloud of billions of comets, stretching light-years out from the Sun–this became known as the Oort cloud, after another Dutchman, Jan Oort.
These theories peeked into reality in 1992, when 1992 QB1 (now called 15670 Albion) was discovered, joining Pluto to become the second know object orbiting outside Neptune. What followed in the late 1990s and early 2000s was a rapid flood of new finds: cold, shadowy planetoids orbiting further than Neptune. Some lay in the Kuiper belt, others in erratic paths much like Pluto’s that lift and scatter out of the Solar System’s plane. They have been given names such as Chaos, Varuna, Ixion, Quaoar, Sedna, Haumea, Orcus, Salacia and Makemake. Thy are more than just comets; some are hundreds, even thousands, of kilometres wide, and a few even have little satellites of their own.
Pluto’s status as a planet was increasingly threatened; why should it be considered a planet if it isn’t dominant over its own rather crowded orbital area? The final straw was the 2003 discovery of Eris, the first object more distant than Neptune that was also more massive than Pluto; for comparison, Eris’s aphelion, the most distant point in its orbit, is twice as distant as Pluto’s. Eris proved that Pluto’s discovery really had been little more than an extremely lucky accident, and in 2006, the International Astronomical Union declared that Pluto wasn’t a planet.
With a whole wealth of minor planets now known to exist in the Kuiper belt, and the Planet X question still hanging over the head of them all, many still wondered if, perhaps, there were even bigger fish?
This was where Sedna’s orbit comes in. It takes more than eleven thousand years to complete and stretches out to where the Oort cloud is believed to lie. Many similar objects are believed to have been scattered into odd orbits by Neptune’s gravity, but Sedna is much too distant for even that. Another, larger object might explain it. The Goblin, even more than Sedna, has an unexplained, highly elongated orbit.
These orbits also appear to form numerical ratios with each other–known as resonance–suggesting they are “controlled” by something else. Since neither Sedna nor the Goblin is remotely big enough, that object must be far larger.
What, then, is it?
Perhaps the most intriguing suggestion of all was that, somewhere in eternity, the Sun has an extremely dim companion star: Nemesis. It was speculated that Nemesis orbited in a highly elongated ellipse at the very outer edge of the Oort cloud, more than a whole light-year away from the Sun. Nemesis was proposed to be a brown dwarf–an extremely cool, dim star that is barely a star at all. Nemesis might explain why Sedna’s orbit is so odd.
Nemesis was first proposed in the early 1980s to explain what appeared to be a repeating pattern in the history of Earth’s mass extinctions–violent events such as that which is believed to have killed the dinosaurs. Every twenty-five to thirty million years, it was said, Nemesis would interact with the Oort cloud and send a hailstorm of comets careening into the inner solar system, some of which would collide with the planets.
Unfortunately for Nemesis, infrared sky surveys that would surely detect such a brown dwarf have turned up with nothing, and the initial “pattern” that precipitated the Nemesis hypothesis is now commonly believed to be a statistical artefact. The prospects of the Sun having a companion are now, sadly, rather dim.
What about something slightly more modest? After all, thousands of Jupiter-sized planets have been discovered in other star systems, at all sorts of distances from their parent star; there is nothing theoretically stopping one from existing in the Oort cloud. In the late 1990s, a planet named Tyche was proposed: a gas giant drifting several hundred times more distant than Neptune. It would be the culprit for the manipulated orbits. Unfortunately, sky surveys seem to have denied Tyche’s existence, too.
While the possibility of stars and gas giants seem to have been ruled out, what about Earth-sized planets? An Earth-sized object this far away would still be a major find, and would definitely explain the orbits of Sedna and the Goblin.
This is where “Planet Nine” enters the picture–a hypothetical world, perhaps ten times Earth’s mass, orbiting deep past the Kuiper belt.
Searching for Planet Nine has begun; so far, there has been little luck, but the Goblin marks a significant step in trying to trace it down.
Discovering Planet Nine would be staggering. Although it is far, far more distant than the Planet X once pictured, in spirit it is quite the same: an unknown planet wielding gravitational influence. It would be proof that we have barely scratched the surface of what exists in the outer solar system.
Thirty years ago, it seemed that there were nine planets, full stop–end of story. The comfort of this established picture is why so many non-scientists were so willing to throw themselves behind Pluto’s cause in 2006, after we had known how different the reality actually was. At the edges of the solar system, our picture is ever changing, and far complete.