In a gigantic "screw you" to Pluto, the ex-planet now demoted to minor planet, we could be on the cusp of locating Planet Nine – if it actually exists, that is.
Why haven't we found Planet Nine?
Planet discovery isn't as simple as looking up at the sky and noticing a big blob. Well, it used to be when Mars was discovered all the way back in Ancient Egypt and China. But since then, planetary discovery has become more complicated by the fact that space is giant, and in comparison, planets are teeny tiny (and also quite far away).
For planets around other stars, we have the transit method. Essentially, when a planet goes past a star we are observing, we see a dip in light: boom, planet discovered. Planets in our own star system are not as helpful, other than Mercury and Venus. Unless Planet Nine suddenly takes a trip to the center of the Solar System, we are not going to discover it in this way.
However, by observing the effects of a planet on other bodies, it's possible to pinpoint where a planet or other body should be (if it exists).
How did we discover the planets?
This isn't hypothetical but was how we discovered Neptune. Uranus (grow up) had been moving in unexpected ways, as predicted by the Newtonian theory of gravity. Though the discrepancies were small, there was a difference between the observed orbit of Uranus and the way Newtonian physics predicted its orbit to be.
In 1846, astronomer and mathematician Urbain Le Verrier sat down and attempted to locate the object that was causing this discrepancy, and made precise predictions about where the planet should be. Happy with himself for completing the entirely mathematical exercise, he left it to someone else to check he was correct. He was. On September 23, 1846, Gottfried Galle looked at the spot Le Verrier had predicted the planet would be and found Neptune to within one degree of the spot.
In a similar way, for the last few decades astronomers have theorized about the existence of a Planet Nine, way out beyond Neptune, by the way that objects cluster in the Kuiper belt, almost as if a large body was pulling at them and messing with their orbits.
Yet all our attempts to locate and observe the supposed planet have (thus far) failed, with a recent study going as far as to suggest that evidence for the planet may be down to selection bias. The research, carried out by Kevin Napier of the University of Michigan, suggested that the initial clustering of objects seen by Professor Mike Brown and Professor Konstantin Batygin only appeared clumped because they observed a small portion of the sky for a short time during a specific time of the year.
In short, to claim that Planet Nine is causing objects to cluster, would mean that there is a usual orbit that these objects go through, from which they have been pulled by Planet Nine. In their own surveys of the sky, they didn't find enough evidence to support this idea, making the Planet Nine theory not entirely dead (more observations are needed) but at least wounded.
Where is Planet Nine?
Enter the new study by Brown and Batygin, which has updated for calculations of observational biases and claims to have found "that the clustering remains significant at the 99.6\% confidence level".
"All telescopic surveys contain observational biases," the team wrote on the pre-print server Archiv, addressing the study. "Correctly understanding and implementing these biases into our modeling is critical to correctly using the observations to extract orbital parameters of Planet Nine."
The team puts the planet much closer and brighter than previous predictions. Excitingly, many of the potential distances that they predict the planet could be are within range of "sky surveys being performed with modest telescope[s]", and could potentially be picked up by the Vera Rubin telescope when it goes into operation in 2023.
"Despite recent discussions, statistical evidence for clustering in the outer solar system remains strong, and a massive planet on a distant inclined eccentric orbit remains the simplest hypothesis," the team conclude.
"Detection of Planet Nine will usher in a new understanding of the outermost part of our solar system and allow detailed study of a fifth giant planet with mass common throughout the galaxy."
Here's hoping it doesn't turn out to be another Planet Vulcan, the 19th Century planet that turned out not to exist.