spaceSpace and Physics

Evidence For Planet Nine May Rely On Selection Bias


Stephen Luntz

Stephen has a science degree with a major in physics, an arts degree with majors in English Literature and History and Philosophy of Science and a Graduate Diploma in Science Communication.

Freelance Writer

TNO orbits

The orbits of Trans-Neptunian Objects we have found are not randomly distributed, but is this because a planet's gravity is herding them, or we are looking in specific places that mean we mostly find the objects with particular orbits. Image Credit: Tomruen CC-By-SA- 4.0

The basis of claims for a large “Planet Nine” has been challenged, with a team of scientists claiming it's been made using unrepresentative data. However, those who argue for a large object lurking in the outer reaches of the Solar System are holding their line.

The idea of an additional object orbiting the Sun but barely warmed by it is an old one, and many scientists have tried to make the case for it. The astronomical world only really started to take it seriously, however, in 2016. Caltech's Professor Mike Brown, whose Twitter handle is @plutokiller for his part in reducing the planetary count to eight, and Professor Konstantin Batygin presented evidence that six Trans-Neptunian Objects (TNOs) have orbits bunched as if something was herding them. That something, the pair proposed, was an object with a mass 5-10 times that of the Earth so far out we have yet to notice it.


The evidence since then has been mixed. On the one hand, efforts to observe Planet Nine directly have come up empty-handed so far, on the other hand, further TNOs have been found with orbits consistent with being pulled by Planet Nine.

However, skeptics have argued we're only searching for TNOs in small areas of the sky, so it is not surprising those we find seem to have similar orbits. We can only see most TNOs when they are near their closest point to the Sun, which biases the sample from the start, even without the limited areas searched. Brown and Batygin attempted to account for these biases in surveys of TNOs, but not everyone is convinced they succeeded.

So University of Michigan PhD student Kevin Napier and a large multi-institution team examined which parts of the sky TNO searches had studied. In particular, they considered three search projects that between them have found 14 TNOs.
In a paper to be published in Planetary Science Journal (preprint on Napier and co-authors report the TNO detections to have more to do with where the telescopes were looking (and could look deeply without interference) than any distant gravitational influence.

The findings don't conclusively rule out Planet Nine. We will need a much larger sample of TNOs to confirm or reject it. Instead, we have something that can be interpreted in line with people's prior thinking. Napier thinks it makes Planet Nine unnecessary, but Batygin doesn't see it as changing anything.

Napier told Science Magazine the clustering “is a consequence of where we look and when we look,” he says. “There’s no need for another model to fit the data.” Batygin disagrees, saying Napier's work can't distinguish between clustering and a random TNO distribution.


Planet Nine isn't the only theory that would be headed for the scrapheap if Napier is right. A black hole of planetary mass has also been proposed. Astonishing as this sounds, it would explain why we haven't spotted it, but this idea is also unnecessary if there is no clustering to explain.

Finding enough TNOs to settle the question one way or another is one of the reasons the Vera Rubin Observatory is being built, with an opening set for 2023, Starlink permitting. We may just have to be patient.


spaceSpace and Physics