When Mike Brown first proposed that a hidden, massive planet lurks in the outer reaches of our solar system, he was confident someone would prove him wrong. “Planet Nine,” as the hypothetical world was nicknamed, was his explanation for the strange movements of half a dozen distant, icy planetoids that are farther away and smaller than Pluto: In theory, this huge, somehow-undiscovered planet could sway their orbits. But surely astronomers would be quick to find a more obvious explanation.
“Shockingly, in a year and a half, nobody has,” says Brown, an astronomer at the California Institute of Technology. “There have been so many claims of planets in the last 170 years, and they were always wrong. But I’m clearly a true believer at this point.”
Brown, the self-titled “Pluto Killer” who led the campaign that demoted the dwarf planet, and Konstantin Batygin, his coauthor at Caltech and a young star who plays in his own rock band, know how to spark debate. Since their proposal about Planet Nine, the lack of definitive evidence for or against its existence has divided the planetary community. Other astronomers have put forth alternative explanations, and some contend Brown and Batygin’s data are biased. Until someone clearly spots the new mystery planet in a telescope, they’ve come to an impasse.
The peculiar clustering of the six faraway objects that Brown and Batygin’s initial hypothesis highlighted is extremely unlikely to happen just by chance. According to the duo’s mathematical arguments, it would be naturally explained, though, by a planet about 10 times as big as Earth in the region known as the Kuiper Belt. Batygin has come up with more recent evidence, too: The orbits of other distant solar-system objects yoked to Neptune have gotten “detached” as well, and other objects’ orbits have gotten tilted to the side or reversed, so the solar system as a whole no longer resembles a thin record or CD with the sun at the center. Planet Nine, if it exists, could explain all of those phenomena.
“When all of these things come together, it’s becoming evident that without Planet Nine, the solar system has these weird puzzles and features that stand out,” Batygin says. His approach echoes that of astrophysicists when they inferred the existence of dark matter—which still can’t be seen—based on the rapid motions of stars in the outer realms of galaxies, and then clinched the idea with a more diverse array of evidence.
But other experts remain skeptical. Even the most basic facts are in dispute. Scientists with the Outer Solar System Origins Survey, or OSSOS, argue that Brown and Batygin’s data are actually biased by factors like bad weather and their telescope’s location, which could influence what’s seen and what’s missed, thereby introducing a spurious trend. If this is true, then there’s nothing weird about the little balls of rock and ice in the first place. The OSSOS researchers say these objects mostly seem randomly oriented and not aligned by some unseen force. If there’s no clustering, then Planet Nine’s linchpin disappears.
“While my research is skeptical of this planet, that’s not at all to say there isn’t a planet out there,” says Cory Shankman, an astronomer at the University of Victoria who led the research. He advocates for continuing the search for these hard-to-detect objects and understanding the biases while doing it. It’s slow, painstaking work.
Shankman’s survey only covered one-twentieth of the sky, however. Other astronomers, such as those affiliated with the Dark Energy Survey, question their findings, just like Shankman questions Brown and Batygin’s. “The more objects you can find that bear on the story, the easier it is to talk about them as a population rather than a small handful of curiosities” says David Gerdes, a University of Michigan astrophysicist. One way or another, he believes, the answer will be clear within the next year or two.
A theory’s more powerful if it doesn’t just explain what’s already known, but also makes successful predictions about things that haven’t been seen yet. If scientists find more objects throughout the Kuiper Belt and these objects are hardly clustered at all, it will deal a blow against Planet Nine. If the objects are similarly clustered as Brown and Batygin expect, it will strengthen their case.
And yet, another possibility remains, which is that the only solutions people find are the ones they have access to, like the proverbial story of the person who lost their keys at night and only looks for them under the streetlight. “Scientists are often good at doing contrastive assessments, like a Sherlock Holmes–style argument: Here’s my suspects, and here’s the one most likely to have done it,” says Christopher Smeenk, a philosopher of science at the University of Western Ontario. “But do you have the right list of suspects?”
There have been many planetary misses in history, Smeenk points out, such as 17th-century claims of a moon orbiting Venus, which better data demonstrated not to exist. Two centuries later, astronomers attributed Mercury’s slightly peculiar orbit to the gravitational forces from an unseen inner planet, dubbed Vulcan. But when Albert Einstein’s theory of general relativity came along, it explained the orbit, debunking Vulcan claims.
In the case of Planet Nine, Ann-Marie Madigan, an astrophysicist at the University of Colorado Boulder, believes everyone has missed a key suspect. The gravitational forces in the outer solar system could be more complicated, and the unlikely alignment of those icy bodies could all be a temporary coincidence.
She argues that there are millions, if not billions, of planetoids—more than previously thought—orbiting in that distant, dusty disk of material around our solar system. Most astronomers have assumed that the forces of these tiny objects are so small that they can be ignored in models, and it’s difficult to model their behavior. But Madigan includes them all in her models, and has found that, as they orbit over and over again, the nebulous pull of their gravity gently and gradually clumps some objects together over time. This “self-gravity” mechanism, as she calls it, could explain the other lines of evidence brought forth by Batygin as well.
“People think gravity’s dominated by Jupiter, Neptune, Uranus, and Saturn, and they’re not really thinking of the collective effects of all the smaller bodies,” Madigan says. “The main criticism I get from Planet Nine advocates is that there’s no evidence for such a mass of small bodies. But I don’t pay too much attention to that, because they haven’t seen Planet Nine yet either.”
Incidentally, both Batygin and Madigan invoke the principle of Occam’s razor, the notion that the simplest explanation is likely the correct one. But they come to completely different conclusions, highlighting that this seemingly straightforward principle is actually rather complicated, with no clear answer yet in sight.