It’s not every day we find a new planet in our solar system. The last discovery was Neptune in 1846 since Pluto is no longer regarded as a major planet. Could there be more planets out there? New data is leading some astronomers to think we’re closing in on a giant planet far beyond the orbit of Neptune. Mike Brown and Konstantin Batygin, both astronomers at Caltech, have published a paper excitingly titled “EVIDENCE FOR A DISTANT GIANT PLANET IN THE SOLAR SYSTEM”. You had my curiosity, but now you have my attention.
Nobody has discovered a genuine planet since 1846 but Brown is no stranger to smaller objects and previously led the team that discovered dwarf planets Sedna and Eres. Pluto is bigger than Sedna but smaller than Eres, which is why we can no longer think of Pluto as a major planet. Minor planets are discovered more often than you might think. Two years ago, 2012 VP113 (also known as Biden) was discovered by Scott Shepard of the Carnegie Institution for Science. Both Sedna and Biden have interesting orbits that suggests something strange is happening out there beyond the orbit of Neptune. They are among several small objects with strange orbits that appear to be influenced by an unseen, giant object.
Brown and Batygin created a model to test the hypothesis that a huge planet is causing Sedna and other small objects to orbit on their unexpected planes. The model worked and could explain the orbits of the objects in the image above (purple orbits). In my opinion, the strength of a theory lies in its ability to make predictions. Excitingly, the model suggested that the orbits of some other known bodies would be influenced in predictable ways by the mystery planet if it exists. Brown and Batygin used historical data to look at the orbits of five of these small objects and found that their objects were exactly as predicted by the model. The blue orbits in the next image show the objects predicted by the model and confirmed with historical data. A theory that makes accurate predictions is a good sign that scientists are going in the right direction, so there might be something to this idea of a unseen planet.
The orbits of these small objects can be used to calculate the size of the potential ninth planet. It would be smaller than Neptune but 10 times bigger than Earth. It might seem unlikely that such a giant planet could go undiscovered for so long but it’s definitely possible given its distance from the sun. The outer planets are far fainter than our nearest neighbours and this potential giant could be around 200 times further from the sun than we are. It might too faint for people to have found it by chance already. By taking into account the interesting orbits of the small objects like Sedna and Biden, astronomers might be able to determine where the planet should exist in the night sky. With more data from other small objects, astronomers will get closer to pinpointing its location and it’s likely that within a few years we’ll be able to find it using telescopes. That’s if it actually exists.
As exciting as a genuine ninth planet would be, perhaps it would be more interesting if it doesn’t exist. What else could explain the unexpected orbits of these objects so far beyond Neptune? What influences them if not a mini-Neptune or super-Earth? Not everyone is convinced that the ninth planet is out there. It will take more data and eventually an actual observation of the planet before we’ll know for sure. However, the current data leaves Mike Brown feeling confident:
OK, OK, I am now willing to admit: I DO believe that the solar system has nine planets.
— Mike Brown (@plutokiller) January 20, 2016
Main image © Caltch/R. Hurt