The orbits of the most distant Kuiper Belt objects bunch together, as if herded by a planet five to ten times Earth's mass
In 2014 Chad Trujillo and Scott Sheppard noticed that Sedna and 2012 VP113, the two most distant known Kuiper Belt objects, had similar orbital orientations. In 2016 Konstantin Batygin and Mike Brown of Caltech showed that the long, eccentric orbits of six such objects point the same way and are tilted the same way, which Neptune cannot explain. Their simulations reproduced the pattern with an unseen planet on an orbit pointing the opposite way. A 2021 update put the clustering at 99.6% confidence and the planet at about 6.2 Earth masses, an average distance of 380 AU (300 to 520 AU), 16° tilt and closest approach about 300 AU, taking about 7,000 years per orbit. The same planet would explain Kuiper Belt objects orbiting nearly perpendicular to the planets and the 6° tilt of the Sun's spin axis.
| Object | Average distance AU | Closest AU | Farthest AU | Tilt |
|---|---|---|---|---|
| Sedna | 499 | 75 | 923 | 12° |
| 2012 VP113 | 261 | 81 | 441 | 24° |
| 2010 GB174 | 370 | 48 | 692 | 22° |
| 2014 SR349 | 288 | 46 | 530 | 18° |
| 2004 VN112 | 318 | 48 | 588 | 26° |
| Leleākūhonua | 1,085 | 65 | 2,105 | 12° |
Skeptics argue the clustering is an artifact of where surveys have looked
Distant objects are found near their closest approach, and only in the parts of the sky surveys cover, away from the Milky Way and in good seasons. The Outer Solar System Origins Survey and a 2021 analysis of Dark Energy Survey objects found their samples consistent with no clustering once those biases are modeled. Other explanations include the combined gravity of a massive disk of small icy bodies, or a star that passed close to the young Sun. Ten years of searching visible-light surveys has also ruled out most of the brighter part of the predicted range: Planet Nine, if it exists, is fainter than about magnitude 21.
The Rubin Observatory should find Planet Nine or rule it out within a few years
The Vera C. Rubin Observatory in Chile, now starting its ten-year Legacy Survey of Space and Time, maps the southern sky every few nights down to about magnitude 24, deep enough for most of the predicted range. It will also find thousands of new distant Kuiper Belt objects, enough to settle whether their orbits really cluster. Mike Brown has said Rubin will either find the planet or show convincingly that it is not there. A planet at 300 to 500 AU would move about 3 arcminutes a year against the stars, so it shows up as a slow, faint mover between visits.
A few infrared candidates have been proposed; none has been confirmed
A planet that far out reflects little sunlight but still glows faintly in far-infrared light from its own internal heat. In 2025 a team led by Terry Long Phan in Taiwan matched sources in the IRAS survey of 1983 and the AKARI survey of 2006 and found one pair that moved by the amount expected for a planet 500 to 700 AU away. It needs to be found again in deep new images before it can be called a planet; earlier infrared candidates have not held up.
Planet Y: a smaller planet closer in may be tilting the Kuiper Belt
In 2025 Amir Siraj, Christopher Chyba and Scott Tremaine at Princeton found that beyond about 80 AU the average plane of Kuiper Belt orbits is tilted by about 15° from the plane of the planets. Their best explanation is a planet between Mercury and Earth in mass, 100 to 200 AU from the Sun, on an orbit tilted at least 10°. They call it Planet Y, separate from Planet Nine; both could exist. The signal is tentative, and Rubin's new Kuiper Belt objects should confirm or erase it within two or three years.
A primordial black hole the size of a grapefruit could do the same job; most astronomers think a planet is far likelier
In 2019 Jakub Scholtz and James Unwin pointed out that an object of five to ten Earth masses could be a primordial black hole, formed in the first moments after the Big Bang and captured by the Sun, and would be only about 5 to 10 cm across. It would explain why no telescope sees it. Primordial black holes have never been detected, and capturing one is about as likely as capturing a free-floating planet. Proposed tests include gamma rays from dark matter gathered around it and a fleet of tiny laser-driven probes whose paths it would bend. Mike Brown has said there is no reason to think Planet Nine is a black hole. The idea becomes interesting only if Rubin finds no planet but the clustering survives.
Planet X led to Pluto by accident, then turned out not to exist
Percival Lowell searched for a Planet X to explain wobbles in the orbits of Uranus and Neptune, and Clyde Tombaugh found Pluto in 1930 searching for it at Lowell Observatory. Pluto proved far too small to cause the wobbles, and when Voyager 2 measured Neptune's mass in 1989 the wobbles vanished. Planet Nine rests on different evidence: the orbits of distant small bodies, not of the giant planets. Pluto · Dwarf planets · Sedna