About the image. Drawn as a cold, pale world lit only faintly by a distant Sun. Planet Nine has never been observed; even whether it is icy or rocky is unknown.
In early 2016 Konstantin Batygin and Michael Brown of Caltech noticed something odd about the most distant known objects in the Solar System. Six small bodies, far beyond Neptune, had orbits that pointed in roughly the same direction and were tilted the same way. They estimated only about a 0.007% chance that this was a coincidence. Their explanation: a large, unseen planet out in the dark, shepherding them.
What it would be
The estimates have moved over the years. The 2016 proposal called for a planet of 5 to 10 Earth masses on an orbit hundreds of AU from the Sun, taking 10 000 to 20 000 years to go around once. A 2025 study by Amir Siraj and colleagues puts it at about 4.4 Earth masses on an orbit around 290 AU, with a year of about 4 900 years, and suggests it might be more rock than ice.
At that distance it would be faint, cold and slow, which is why the searches have come up empty so far.
The doubts
Not everyone accepts the evidence. Large surveys such as OSSOS and the Dark Energy Survey found in 2020 that the clustering largely disappears once you correct for where telescopes happened to look. Newly found distant objects such as 2017 OF201 and 2023 KQ14 have orbits that might not survive if Planet Nine were there.
Until someone actually sees it, Planet Nine remains an idea: a planet inferred from the shadows it might cast.
How far out it would be
Planet Nine goes around once every 4 940 years, at 290 AU from its star. Dashed circles show Neptune around the Sun, on the same scale.
Earth days passing: 0
To scale · orbit size in AUNeptune: 30.1 · Planet Nine (est.): 290Semi-major axes. Neptune: 30.07 AU.
Gallery
Artist's impressions rendered from the same model as the view at the top of this page.
Timeline
2016Proposed by Konstantin Batygin and Michael Brown.
2020Large surveys find the clustering weakens after correcting for observation bias.
2021Estimate refined to about 6 Earth masses.
2025A new study suggests about 4.4 Earth masses on an orbit near 290 AU.
Questions people ask
Does Planet Nine exist?
Nobody knows. It has never been observed, and some astronomers think the evidence for it is an effect of where telescopes happened to look.
How far away would Planet Nine be?
Recent estimates put its average distance from the Sun around 290 AU, nearly ten times farther than Neptune.
Why haven't we found Planet Nine?
At that distance it would be cold, faint and slow-moving, and the sky area to search is enormous.