Anna Mallach follows “the Goblin” into the outer reaches of the solar system in the hunt for the mysterious ninth planet.
When Pluto was downgraded as a planet to a dwarf planet, a real sense of loss was felt. A planet had been forfeited and the other eight never quite seemed enough. But what if there was another one, a planet hiding in the outer realms of the solar system? And if there were, how would one go about finding it in the darkness of space?
In October 2015, during a normal survey of the skies, researchers in Hawaii found a small dwarf planet in the outer solar system, which looked like it could be part of the answer. Nicknamed the Goblin, at a glance this dwarf planet itself is not particularly interesting: only 300 kilometers in diameter, circling far away from the known planets. The interesting thing about this newly found object is its orbit. At its closest to the sun, it is already 65 times further away from it than we are on earth, but then it leaves the known realms of the solar system and reaches the furthest point away from the sun 20.000 years later and 35 times even further away. In our solar systems, there have only been two other confirmed objects, whose closest point to the sun is further away than the Goblin, and both of them have rounder orbits, meaning they never reach the final distance to the sun the Goblin does.
Not only are the orbits of these far away worlds very elongated, but surprisingly, they have similarities. They all come closest to the sun at similar points in the sky and have synchronous orbits. The orbits can further be analysed in simulations and it was found, that they are stable throughout the life of the solar system. One of the researchers involved in the find of the Goblin, said: “These simulations do not prove that there’s another massive planet in our solar system, but they are further evidence that something big could be out there”.
The dwarf planets observed so far are too far away from Jupiter and Neptune to be influenced by their gravitational pull, but they seem to be moving as if they are influenced by a big gravitational force. Something, some other source of gravity, in the outer solar system is shepherding them into stable, similar orbits. The hunt for this ominous ninth planet started years ago, when orbital perturbations of Neptune suggested something could be out there. Whilst the hunt eventually uncovered Pluto, it soon became obvious that Pluto is too small to create a gravitational pull to influence Neptune in this way. And so the search for the so called Planet X, as this mystery was eventually called, continued.
Now, the orbits of these dwarf planets again suggest that Planet X could be out there, influencing the objects in the outer reaches of our solar system. Experts believe it to be a massive planet, ten times heavier than the earth, and of similar size to Neptune. This rocky planet could have a 3.600 year-long orbit around the sun and be 90 billion kilometers away from the sun. However, no one has been able to observe and track this object yet.
So what is next? Finding other dwarf planets from the outer solar system could further support the hypothesis that Planet X influences their orbits. The problem is that these worlds are so far away from the sun, their dim reflections are difficult to observe with our telescopes. The Goblin was only discovered when it was at the closest point to the sun and 99% of the time, it would have been too dim to be picked up. More data and more orbits could lead to more sophisticated models of the orbit and size of this supposed Planet X and with enough knowledge, a targeted hunt may be successful. The discovery of the Goblin is only the start of the process, but it could be a process ending in the discovery of a new planet!