Planet Nine
UnsolvedSummary
A handful of small icy bodies in the far outer solar system have orbits that appear to be lined up. Their long axes point in similar directions. Their orbital planes are tilted the same way.
Something, the argument runs, is herding them. A planet, several times the mass of the Earth, very far out, on a long slow orbit, that nobody has ever seen.
The registry files this under Unsolved, and it wants to be precise about what is unsolved, because it is not the thing most people think.
The live question is not what is causing the clustering. It is whether there is any clustering at all.
What is documented
The objects. Extreme trans-Neptunian objects: small bodies far beyond Neptune, on very elongated orbits that keep them at enormous distances for most of their long years.
The proposal. In 2016, Konstantin Batygin and Michael Brown of Caltech published an argument in the Astronomical Journal. They noted that a set of these objects had orbits which appeared clustered: their closest approaches to the Sun occur in a similar region of physical space, and their orbital planes are tilted in a similar direction.
They calculated the probability of that happening by chance as very small, and proposed a shepherding planet.
The predicted planet. Something in the region of five to ten Earth masses, on a highly eccentric orbit with a semi-major axis of several hundred astronomical units, taking many thousands of years to go round once.
Nobody has found it. It has been looked for extensively, with large survey telescopes, and the searchable parameter space has been narrowed considerably. Nothing.
Leading explanations
Observational bias, and this is the objection that has to be answered first.
The extreme trans-Neptunian objects were not found by a survey that swept the entire sky uniformly. They were found by a number of different surveys, each of which pointed where it was convenient to point.
Telescopes avoid the plane of the Milky Way, because it is crowded with stars and it is hard to see a faint moving dot against it. They observe at particular times of year. They cover particular patches of sky, to particular depths.
If your surveys look preferentially at one part of the sky, then the objects you find will preferentially be in that part of the sky, and their orbits will preferentially have their closest approaches there.
You will find a cluster. And you will have made it.
The analyses. Kevin Napier, David Gerdes and colleagues examined the extreme trans-Neptunian objects from three surveys whose selection functions are properly characterised, meaning it is known exactly where those surveys looked and how deeply. They published in the Planetary Science Journal in 2021.
Correcting for where the telescopes had actually pointed, they found the clustering to be consistent with observational bias, and no statistically significant clustering to remain.
The OSSOS survey team had made a comparable argument earlier.
Batygin and Brown maintain the clustering is real and dispute the bias analyses. The argument continues, and it is a real scientific argument between serious people, and the registry does not adjudicate it.
If the clustering is real, a planet is a good explanation. That must be said. Batygin and Brown's dynamical work is careful, and a distant massive body would produce something like the observed pattern. The proposal is not fanciful and it has not been refuted.
It rests entirely on there being a pattern.
What the popular version gets wrong
"Astronomers have found evidence of a ninth planet." Astronomers have found a possible pattern in a small number of objects, and are arguing about whether the pattern is a property of the solar system or a property of where they pointed their telescopes.
"The orbits are clustered, so something must be causing it." This assumes the conclusion of the disputed step. Establish the clustering first.
"It is just a matter of time before we see it." It has been looked for hard, for a decade, and the region where it could hide is shrinking. That is not proof of absence, and it is not encouraging.
"The bias argument is a technicality." It is the entire case. This is exactly the structure of the Bermuda Triangle, where the question was never what caused the disappearances but whether the disappearance rate was elevated at all, and exactly the structure of the Indus script, where the question is not what the signs say but whether they say anything. (See AR-0014 and AR-0068.)
Before you explain a pattern, establish that there is one.
Current status
Unsolved. A proposed planet, based on a proposed pattern, in a set of objects found by surveys that did not cover the sky evenly, and searched for without success for a decade.
The registry keeps this record as an example of good science being difficult, rather than of anybody being foolish. Batygin and Brown may be right. Napier and Gerdes may be right. The way to find out is to survey the sky properly, and that is being done, and it will take time.
Sources
- Batygin, K. and Brown, M. E. (2016). "Evidence for a Distant Giant Planet in the Solar System." The Astronomical Journal 151, 22.
- Napier, K. J., Gerdes, D. W. et al. (2021). "No Evidence for Orbital Clustering in the Extreme Trans-Neptunian Objects." The Planetary Science Journal 2, 59.
- Shankman, C. et al. (OSSOS collaboration) (2017). On the observational biases in the detected extreme trans-Neptunian object population.
- Ongoing survey searches for the proposed body.
Last reviewed: July 2026. Records are provisional. Where the evidence changes, the entry changes. Found an error? Tell us.