New Discovery Strengthens the Case for Elusive Planet 9

Years after its proposed detection, Planet 9 remains obstinately absent from our astronomical charts. Evidence for its existence is found only indirectly, in the orbits of a number of trans-Neptunian objects, or TNOs, which follow trajectories around the sun that collectively imply the presence of another, larger planet in the distant boundaries of the solar system.

The biggest problem with Planet 9 is that we haven’t found it. The biggest problem for those who want to argue it doesn’t exist is that we keep finding evidence it does. The most recent piece — rock? — of evidence is 2015 TG387, colloquially known as “The Goblin.” What makes the Goblin so interesting is what it doesn’t do: namely, interact with other planets in the solar system. It never comes close enough to Jupiter, Saturn, Uranus, or Neptune to be gravitationally influenced by them. Yet its orbit around the solar system shows that it’s clearly being influenced by something.

All of the “Giant planets” are located in the far right-hand side of the image. That’s how far away from us 2015 TG387 is.

“It never interacts with anything that we know of in the solar system,” says Scott Sheppard, an astronomer at the Carnegie Institution for Science and a co-discoverer of 2015 TG387. “Somehow, it had to get on this elongated orbit in the past, and that’s the big question: What did it interact with to get [there]?”

Mathematical simulations show only one real possibility — 2015 TG387 was shifted into its highly elongated orbit by interactions with a larger body, one that fits the assumed characteristics of our hypothetical Planet 9. Part of the problem with finding the actual planet is that bodies out this far from the Sun are exceedingly faint. The Goblin spends most of its time too far from Earth to be detected by telescopes and can only be seen when it’s on its closest approach to the Sun, something that only happens every 40,000 years. In other words, the only reason we found it is because it’s in the right place in its orbit to be found (at roughly 300km in diameter, Goblin is substantially smaller than Ceres).

There are critics of the Planet 9 theory, including those who believe that the collective gravity of these small objects may have nudged them into strange elliptical orbits, or that the entire issue is a sampling artifact from only examining a small portion of the sky. If we see these unusual elliptical orbits all over the solar system, it would mean they’re being caused by something else (nobody expects space to be full of invisible planets whizzing about).

It’s early in all cases. Planet 9 could simply be too far away from Earth to be observed at the moment, thanks to a combination of dim surface albedo, distance, and us not knowing where to look. Previous sky surveys have clarified a lot of objects that aren’t out there; we don’t think, for instance, that there’s any way a Jupiter-size planet could still be hiding anywhere nearby. But there are still gaps in our knowledge that could be cloaking a distant ice ball.

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