Is there another planet in the solar system?

A small number of objects beyond Neptune have long, highly elongated orbits that never bring them inside Neptune’s. In 2016 two astronomers pointed out that the ones known then had their closest approaches clustered in roughly the same direction, and tilted the same way.

That is not what an undisturbed population looks like. Their proposal was a planet of ten Earth masses or more on a distant eccentric orbit, shepherding the others into alignment over long periods.

In 2021 the same authors reworked the analysis with eleven objects instead of six, and a bias model built from the ensemble of all known Kuiper belt detections rather than from the pointing history of any one survey. They reported that the clustering survives that correction at 99.6 per cent confidence, and used it to estimate a mass of about six Earth masses, an orbit and a brightness.

Predicting where an object should be is what makes this a scientific claim rather than a suggestion. It can fail.

Distant objects are found where telescopes look, and telescopes do not look uniformly. Each survey covers a limited area of sky, at particular times of year, to a limited depth.

In 2021 a separate group analysed fourteen such objects discovered by three surveys with published pointing histories, and calculated how likely the apparent alignment was if the underlying population were uniform. They found it consistent with uniform, and concluded that this sample provides no evidence of clustering.

The two analyses use different samples and different bias models. Brown and Batygin answer that the rival sample is dominated by surveys that looked in only one or two directions, which lack the sensitivity to detect the clustering even if it is there. This is what a genuinely open question looks like from inside.

If the alignment is real, a planet is not the only cause. A massive disc of small bodies could produce a similar effect through its combined gravity, and so, in some models, could a primordial black hole of a few Earth masses, which would be about the size of a grapefruit and effectively invisible.

The orbits alone rule out neither. The planet’s own proposers still call it the most likely explanation.

The object, if it exists, is faint, several hundred times farther from the Sun than Earth, and somewhere along a large arc of sky. By 2024 three surveys had together ruled out 78 per cent of the predicted range, and much of what remains is faint and near where the northern Milky Way crosses the plane of the planets. The Rubin Observatory survey should reach all but the faintest and most northern of the remaining positions.

A non-detection would not settle it on its own: the faintest predicted positions need telescopes of ten metres or more. And if the planet is not there, the orbits it was proposed to explain still need an explanation, unless the pattern was never there either.