"Planet Nine" has yet to be truly discovered. No one has seen it with a telescope. Instead, its existence is implied by the orbital behavior of a cluster of small, icy dwarf worlds far, far away.
But while some scientists are asking where Planet Nine is, others are asking how it got there — and running models to check the probability of their theories.
Researchers from the Harvard-Smithsonian Center for Astrophysics (CfA) may have a few clues. The institution has a paper on the subject set for publication in the Astrophysical Journal Letters and another two submitted to the Astrophysical Journal.
The first paper (the one already accepted for publication) focuses on three versions of a common theory: Back when our sun was young and still surrounded by a cluster of nearby stars, the gravity of one of its neighbors messed with Planet Nine's orbit. Or, alternatively, our sun messed with the orbit of another star's planet — so much so that we lassoed it for ourselves and made it part of the family.
The latter is quite unlikely — a group of researchers previously estimated a 50/50 chance for an exoplanet with a far-flung orbit to be stolen by our solar system in this fashion. But when they factored in the likelihood that a nearby star would have the right kind of planet — one of the right size, and in an orbit far out enough that it could be pulled into a far-out orbit around our sun — to begin with, that probability dropped to around 1 percent or less.
CfA astronomer Gongjie Li, lead author on the new star cluster study, agreed that this scenario is quite improbable. But even less statistically likely was one of the other scenarios she tested, where she modeled Planet Nine as a free-floating, rogue exoplanet pulled in by stellar wonkiness.
Some scientists estimate that millions of the planets float aimless without tethering to a star's gravitational pull, and the closest one ever found is just seven light-years away from Earth. But Li and her co-author Fred Adams of the University of Michigan are fairly confident that Planet Nine was never a lone wolf, setting the probability of such an event at less than 1 percent.
Even Li's most likely origin story — which is that a passing star's gravity pulled one of our planets into a distant orbit without managing to tug it free of the sun's influence — hovers around just 5 percent probability because it would be way easier for the star to tug it completely out of place. But despite the murkiness of Planet Nine's origins, Li said, "I think it has a high probability to exist."
And she hopes that her research can help in the hunt for Planet Nine, which will have scientists scanning the skies starting in a few months. She points out that her most likely scenario was found to be even more probable when Planet Nine was modeled with a close, circular orbit before the stellar mishap, and factoring that in could help astronomers puzzle out its current orbit — even though that orbit is now very oval and very far-flung, taking the hypothetical planet an average of 20 times farther away than the eighth planet, Neptune.
CfA's Scott Kenyon, working with Benjamin Bromley of the University of Utah, proposes alternatives in his two papers.
"The simplest solution is for the solar system to make an extra gas giant," Kenyon said in a statement. Perhaps a gas giant formed far out and was pushed even farther by a passing star, or perhaps it formed close to home and was ejected by its own sibling planets.
That's not as crazy as it sounds: Based on the way our solar system is laid out, many scientists believe that a ninth planet — a gas giant — had a run-in with Jupiter that flung it out into space. In fact, some scientists believe that Jupiter bullied several planets out of place with its incredibly strong gravity, clearing room for our own world to form.
Either way, it's possible that Planet Nine was a victim of Jupiter's massive pull. But this theory runs into the same problem posed by the most likely stellar scenario: If Jupiter tugged a neighbor out of place, that planet would have to catch a pretty lucky break for the sun to keep it in orbit. It would be more plausible for the displaced world to be destroyed or go hurtling into interstellar space.
So until we find a more likely explanation, we just have to assume that Planet Nine — should it exist — is a marvelously unlikely planet.
The leaders of the study that put Planet Nine on everyone's minds are hedging their bets by splitting down the middle. Mike Brown favors the dragged gas giant idea but says that his co-author Konstantin Batygin is in the star cluster camp.
"We argue about it daily. Which is easy to do given that we don't have any current way to figure out which one really happened," Brown told The Washington Post. With any luck, he added, they'll find more clues if and when they track down the would-be world.