The Hunt for Planet Nine

Since Pluto was demoted to a “dwarf planet” in 2006, a Caltech team has been searching for a hypothesized planet on the edge of the Solar System. Could its gravity be tugging on distant objects?

By Liz Boatman | July 13, 2023

Credit: Nagualdesign/Tom Ruen. Background from ESO/S. Brunier.

Artist's impression of a hypothetical Planet Nine. Neptune's orbit rings the Sun.

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In August 2006, hundreds of astronomers gathered at the International Astronomical Union (IAU) meeting in Prague to debate a planet-sized problem. A Caltech astronomer had recently discovered a string of objects in the Kuiper Belt, an icy band on the outskirts of the solar system. The objects — Haumea, Makemake, Quaoar, Sedna, and Eris — were as large as or larger than the planet Pluto. Were they planets, too?

If so, Pluto and Charon, Pluto’s largest moon, would be the only binary planet in the solar system. It would also make the Caltech astronomer, Mike Brown, “the greatest planetary discoverer of all time,” he says.

It was a title he didn’t want. “I just found it very depressing,” Brown says. “It was clear [to me] by the late 1990s that Pluto was part of this Kuiper Belt and that it should not have been classified as a planet to begin with.” If Pluto didn’t deserve planetary status, neither did the other objects Brown had discovered.

Brown doubted that the IAU astronomers would agree, he says. He didn’t attend the meeting, opting instead to “hide out” on a family trip to an island off Washington state.

But to Brown’s surprise, the IAU didn’t decide that the Kuiper Belt objects were planets. Instead, the astronomers created a new classification scheme. Pluto and the other objects were now dwarf planets.

Pluto had actually never been Brown’s focus — his team was working to catalog new Kuiper Belt objects. But they began to notice an odd pattern: A subset of those objects had orbital inclinations and geometries that defied explanation. Stranger still, these oddballs were clustered on one side of the solar system, not randomly distributed throughout the Kuiper Belt.

Perhaps it was observational bias, and similar objects did exist in other locations but hadn’t been seen yet. Maybe Brown’s data was wrong. Or maybe there was an undiscovered planet, much larger than Pluto. But Brown, the infamous “planet killer,” thought another planet seemed unlikely.

When Pluto was glimpsed in 1930, “it was the only thing out there beyond Neptune that had ever been seen before,” says Brown. Astronomers leapt at the chance to declare it the ninth planet. But in the end, Pluto “was an accidental discovery of something that they thought was going to be a planet, and it wasn't,” says Brown.

Now, Brown’s observations of these Kuiper Belt objects, oddly clustered and tilted, were best explained by a ninth planet, he says. But Brown didn’t want to be the next astronomer searching for a planet to solve a set of mysteries. He didn’t want a Pluto repeat. To explain their observations, “we tried so hard to come up with anything else,” says Brown. “Nothing else worked.”

Finally, something clicked. “It’s real — there’s actually a giant planet out there,” Brown remembers realizing. “[My] heart just kind of dropped.” In 2016, Brown and his colleague Konstantin Batygin published a paper in The Astronomical Journal, presenting their evidence for “Planet Nine.”

Around then, a doctoral student at the University of Michigan named Juliette Becker was studying a strange Kuiper Belt object herself. A decade earlier, Becker had been a ninth grader when her teacher told the class that Pluto was no longer a planet. Many students were confused. Becker remembers thinking, “What happened to Pluto? Where did it go?”

Now an aspiring astronomer, Becker’s object, BP519, was odd, with an orbit tilted by 54 degrees from the plane of the solar system. “The big question I was trying to answer was how this distant object could have such a high inclination,” she says.

That’s when Becker stumbled across Brown and Batygin’s paper on Planet Nine. Seeing a possible connection, she ran new simulations of the solar system with both BP519 and Planet Nine. “The amazing thing was, if we started BP519 out in a disc with all other Kuiper Belt objects and put Planet Nine in the simulation,” she says, it “recreated the current geometry of BP519’s orbit.” Becker’s “weird” object turned out to be a key piece of evidence for Planet Nine.

After Becker completed her doctorate in 2019, Batygin invited her to Caltech. As a postdoc in his research group, she worked to help craft the theory for Planet Nine and constrain its orbital parameters.

Today, the theoretical portion is largely wrapped up — at least, based on known Kuiper Belt objects, says Becker. The team predicts that Planet Nine will be six or seven times the mass of Earth, residing so far from the Sun that one sweep around would take 10,000 Earth-years.

The team doesn’t know how the hypothesized Planet Nine might have come to be. It might have formed close to Uranus and Neptune, Brown says, but then passed “too close to one of those larger planets — to Saturn, more likely — and then was ejected to the outer part of the solar system, where it’s been lurking.”

Now, the team must find observational evidence, surveying the sky or existing images, watching for any “speck” that changes position between frames. That speck could be Planet Nine.

“A part of me thought that it was going to be bright enough that it would be found within a few years, and it was just a matter of who’s going to find it first,” says Becker. But it’s been six years since she saw the first evidence of Planet Nine’s existence. This fall, she’s starting her own faculty career at the University of Wisconsin, Madison.

And what if the Planet Nine theory is wrong? Many astronomers think so, and plenty of other explanations for the Kuiper Belt objects have been offered.

If Planet Nine isn’t out there, that’s okay, says Brown. He likes that science self-corrects. But “if we’re going to be wrong,” he says, “we would like to tell everybody we’re wrong before someone else tells us.”

Liz Boatman is a staff writer for APS News.