We might be edging closer to a full deck of planets, again.
Our solar system has had just eight known planets since Pluto was demoted, but evidence pointing to another contender for Planet Nine status has furrowed astronomers' brows for years, suspecting that it's out there, lurking in the edges of our solar system. But we may have finally pinned down the precise trajectory of the missing cosmic body, according to a recent study shared on a preprint server.
Planet Nine can run, but it can't hide. Unless, of course, it doesn't exist.
Planet Nine researchers respond to criticism
All evidence for Planet Nine stems from the gravitational pull it appears to have on other bodies in the outer solar system. If there's a planet, a gravitational anomaly makes sense, and all astronomers need to perform a bit of math on the affected wobbles of other nearby planets, to interpolate the new one. This is how astronomer John Couch Adams and Urbain Le Varrier discovered Neptune, when they observed Uranus exhibiting a "tugging" motion from an unseen planet. But for Planet Nine, nobody has seen unusual motion or "tugging" in the trajectory of other planets. The only gravitational evidence available consists of an atypical clustering of small icy bodies in the outer solar system, within the totality of Kuiper belt objects (KBOs). If no planets existed beyond the Kuiper belt, the orbits of KBOs would be spontaneously arranged within the solar system's orbital plane. But that's not what's happening.
Instead of the standard random motion, astronomers are observing clustered KBO orbits, and, while this could be a fluke, it's also a highly unlikely one. In 2016, researchers analyzed the statistical distribution of KBOs, and announced that the unusual clustering behavior was due to an undetected outer planet. They even calculated its mass to be that of five Earths, and roughly 10 times Neptune's distance from the sun. The older study also specified which region of the sky the planet was likely hiding, but widespread searches found no Planet Nine, which led some to suspect it doesn't exist. The new study looks at the initial 2016 work and considers some of the criticism on the posited ninth body.
The jury is still out on Planet Nine being real or not
One of the issues in locating a planetary body in the outer solar system lies in the inherent difficulty of locating anything out there. This forces astronomers to look wherever it's convenient, which means the clustered KBOs might be the result of biased data. The recent study's authors account for this observational bias, and concluded that the clustering bodies still constitute an unusual phenomenon, with only a 0.4% chance of happening without a nearby body with significant mass, like a planet. But, most crucially, the study authors further localized the mysterious object when they repeated their calculations of Planet Nine's likely orbit, which places it closer to the sun than we thought.
If Planet Nine is real, astronomers should detect it very soon. But, with many astronomers still more skeptical than not about it (some of whom even suspected it of being a primordial black hole), the coming years will likely see it either ruled out as a possible explanation for the KBO clustering, or reveal world-historical data about a new ninth planet. Time will tell what the final verdict on Planet Nine will be.
Editor's note: An earlier version of this article had a title that was ambiguous about how certain scientists are about Planet Nine. This has been corrected to reflect that its existence remains unknown.