Take, for example, a 19th-century cautionary tale that has remained on the minds of Brown and Batygin since they started scouring the skies with telescopes, hoping to image Planet Nine directly. The French mathematician Urbain Le Verrier followed up his discovery of Neptune in 1846 by proposing the existence of Vulcan, a planet orbiting between the sun and Mercury. He ended up being wrong about that.
The discovery of more objects like 2015 BP519 would help, and Becker and her team will be looking for more. Each new object would, theoretically, bring astronomers closer to pinpointing the location of Planet Nine, and then aiming their telescopes to try to snap a picture.
At the same time, future discoveries could actually weaken the Planet Nine hypothesis, says Ann-Marie Madigan, an astrophysicist at the University of Colorado at Boulder.
Madigan says the discovery of 2015 BP519 and its weird orbit bolsters her theory for the clustering objects, and it has nothing to do with a hidden, massive planet. The clustering could be explained by the interactions between the small objects—as many as 10 Earth masses’ worth—littering the outer solar system. Small bodies alone don’t have much gravitational influence, but their collective movements can jostle the orbits of their neighbors. In a 2015 paper, Madigan predicted this mechanism would lead to a population of objects like 2015 BP519 off the plane of the solar system.
“The reason I like the ‘self-gravity,’ as I call it, of small bodies collectively doing this by themselves is that you don’t need anything new,” Madigan says. If “we’re saying, okay, Planet Nine did it, Planet Nine is responsible for why the outer solar system is behaving so oddly, then we have to say, okay, how did Planet Nine get there?”
Batygin, who is a friend of Madigan’s, says his computer simulations are unable to simulate the self-gravity mechanism in the solar system as it is, and if they could, they would produce a picture of the Kuiper Belt that wouldn’t line up with existing observations. “But could you still cook up a scenario where you don’t need [Planet Nine]? I mean, yes,” he says.
Brown and Batygin welcome the next piece of evidence, the next clue, in their search. But they’re raring to find Planet Nine once and for all. They even sound a little impatient, as if the planet should have revealed itself months ago. “Of course, the obvious [next step] is just to go out and find the damn thing,” Batygin says.
Even if Planet Nine never turns up, a search party at the edges of the solar system is a worthwhile endeavor, says Laughlin. The icy and rocky inhabitants of this region are the leftovers from the beginnings of our solar system. The closer we get to them, the richer our understanding of the way our home in the universe came to be. Small objects out there “provide a good way of understanding why our solar system is so much different than the average solar system in the galaxy, and that’s kind of the real story,” Laughlin says.
“We’re weird, for sure,” he says. “And the best clues as to why we’re weird are imprinted in the most primitive bodies in the solar system, the most distant, untouched fringes.”