In the 1800s observations of the planet Uranus had some interesting irregularities. When compared against the predicted motion of Uranus due to the Sun and other planets, the actual motion didn’t quite match. This led some astronomers to suspect that there was another planet beyond Uranus. In 1846 Neptune was discovered close to its predicted position. Now a new paper in the Astronomical Journal, points to similar evidence that a large planet may lurk in our outer solar system.
The evidence comes from the behavior of the outermost known solar system bodies (the six with an average distance greater than 250 AU). These bodies are so distant from the Sun that we can’t measure their gravitational deviation directly. We just haven’t observed their orbits well enough. So instead the authors of this new work looked at the statistical characteristics of the bodies.
If one assumes the orbits of outer solar system bodies are Keplerian (which is a reasonable assumption) then you can plot them in terms of their orientation. Most of the outer solar system bodies are distributed fairly randomly, but the most distant ones are clustered. Their orbits seemed to be clumped together in a way that you wouldn’t otherwise expect. The authors calculate the likelihood of this happening purely by chance is about 0.007%. In scientific terms that’s not quite unusual enough to be conclusive, but it does strongly hint at either a bias in the way these outer bodies are discovered or a mechanism that has caused them to cluster.
The authors contend that a good explanation for the clustering is a gravitational perturbation by a Neptune-mass planet further away and on the other side of the solar system. While it’s certainly a good explanation for the clustering of these outer bodies, that doesn’t guarantee there’s a large world out there. There are other possible explanations that could account for the effect. But it is worth looking into, and that’s exactly what’s planned. If there is a planet out there, then infrared sky surveys such as NEOWISE have a chance of finding it.
Paper: Konstantin Batygin and Michael E. Brown. Evidence for a Distant Giant Planet in the Solar System. The Astronomical Journal, Volume 151, Number 2 (2016)