In early 2016, astronomers made a stunning claim: A giant planet was patrolling the farthest reaches of our solar system. Planet Nine, as they called it, was too far away to see directly. So its existence was inferred from the way its gravity had herded six distant icy worlds into clustered orbits.
Since then, the case for Planet Nine has been bolstered by other evidence, such as a peculiar tilt to the sun’s spin axis, along with a few more of these strange objects, which have elongated orbits of more than 4000 years and never come closer to the sun than Neptune. Now, a survey has found four more of these extreme bodies. The problem: They don’t display the tell-tale clustering. That’s a substantial blow for Planet Nine enthusiasts.
“We find no evidence of the orbit clustering needed for the Planet Nine hypothesis in our fully independent survey,” says Cory Shankman, an astronomer at the University of Victoria in Canada and a member of the Outer Solar System Origins Survey (OSSOS), which since 2013 has found more than 800 objects out near Neptune using the Canada-France-Hawaii Telescope in Hawaii. In a paper posted to arXiv on 16 June and soon to be published in The Astronomical Journal, the OSSOS team describes eight of its most distant discoveries, including four of the type used to make the initial case for Planet Nine.
“I think it’s great work, and it’s exciting to keep finding these,” says Scott Sheppard, an astronomer at the Carnegie Institution for Science in Washington, D.C., who was among the first to suspect a large planet in the distant solar system. But he says three of the four new objects do have clustered orbits consistent with a Planet Nine. The fourth, an object called 2015 GT50, seems to skew the entire set of OSSOS worlds toward a random distribution. But that is not necessarily a knockout blow, he says. “We always expected that there would be some that don’t fit in.”
The OSSOS team says any apparent clustering in their new objects is likely to be the result of bias in their survey. Weather patterns and a telescope’s location, for instance, determine what areas of the sky it can look at and when. It is also harder to see faint solar system objects in bright areas on the sky like the galactic center.
Such biases make OSSOS more likely to find objects in regions that support the Planet Nine hypothesis, says OSSOS team member Michele Bannister, an astronomer at Queen’s University Belfast in the United Kingdom. When the team corrects for that effect, the apparent clustering vanishes. By contrast, the OSSOS team says, many details of the surveys behind the original six objects are unpublished, making it impossible to understand their biases.
That argument does not impress Mike Brown, an astronomer at the California Institute of Technology (Caltech) in Pasadena, who along with Caltech colleague Konstantin Batygin catapulted Planet Nine into the mainstream with their bold claim. “Their main conclusion is that their observations are hopelessly biased, and it’s true,” he says. “But they then kind of make the leap of faith that everybody else’s must be biased, too.” For Brown, any biases in the hodgepodge of surveys that found the earlier objects should average out. That would make the clustering real—whether caused by Planet Nine or not.
So far, astronomers have found only a dozen of the most distant probes of Planet Nine’s supposed sphere of influence. Finding more objects could help settle the question. So could the most direct kind of evidence: an actual image of Planet Nine, which other surveys hope to capture.
“Perhaps the most attractive thing about the Planet Nine hypothesis is that it has a well-defined observational resolution,” Batygin says. “It’s either there or not.”