Astronomers Close In On ‘Planet Nine' As Hints Found

Forbes

21-05-2025

An artist's illustration of a possible ninth planet in our solar system, hovering at the edge of our ... More solar system. Neptune's orbit is shown as a bright ring around the Sun.
Scientists in Taiwan looking for a ninth planet in the solar system claim to have found hints of it in archive images of the night sky taken by long-dead infrared telescopes. The evidence is thin — and the hypothetical Planet Nine or Planet X is yet to be directly observed — yet astronomers are confident that they're on the cusp of either either finding it or ruling it out.
There are currently eight planets in the solar system — Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune — and have been since 2006 when Pluto was demoted to a dwarf planet.
Astronomers think there may be a ninth planet 20 times further than the orbit of Neptune in the Kuiper Belt, a region of the solar system that's home to Pluto, other dwarf planets, and comets. They even know it must be midway between Earth and Neptune in mass.
Here's everything you need to know about the search for Planet Nine — and why, if it does exist, it will likely be found in the next 18 months.
There is an unusual clustering of minor bodies in the Kuiper Belt. Six objects — Sedna, 2012 VP113, 2004 VN112, 2010 GB174, 2013 RF98 and 2007 TG422 — all have highly elongated yet confusingly similarly oriented orbits. They appear to have been herded by the gravitational influence of a planet.
However, that hypothesized planet's predicted orbit is far beyond Neptune, so it reflects very little sunlight. So, even if it does exist, it would be challenging to see in observations. Its orbital motion would also be very slow, making it hard to ascertain its orbital path.
The six most distant known objects in the solar system with orbits exclusively beyond Neptune ... More (magenta) all mysteriously line up in a single direction. Moreover, when viewed in 3-D, the orbits of all these icy little objects are tilted in the same direction, away from the plane of the solar system.
A new paper, published as a pre-print and not yet peer-reviewed, claims to have found hints of a distant planet in archival images from two far-infrared all-sky surveys, NASA's IRAS (1983-84) and Japan's AKARI (1983-2011). Since they're infrared, they're able to detect optically faint objects. The images come from 1983 and X06, with a 23-year gap between them, enough for a possible Planet Nine to have moved significantly enough in its orbit to show up in a different position.
The researchers — Terry Long Phan and Tomotsugu Goto at National Tsing Hua University — found one object in the IRAS data set that was not in the AKARI data set and one in the AKARI data set that was not in the IRAS data. Crucially, they were close enough to plausibly be at the distance of Planet Nine.
The authors state, however, that the detection in AKARI and IRAS images is not enough to determine the full orbit of the Planet Nine candidate — and that follow-up observations are required.
Planet Nine-hunters are excited but skeptical of this candidate. 'It's a great paper. It is the right thing to do. But it's not the thing that's causing these effects in the sky,' Michael Brown, professor of planetary astronomy at the California Institute of Technology, told EarthSky this week. His work in 2016 with colleague Konstantin Batygin postulated the existence of a Planet Nine to explain why the orbits of six small bodies in the outer solar system are so similar. 'The obvious question is, 'is it even real?' It's a tough one to answer,' said Brown. 'I would not believe anything that just had two data points without seeing a third, a fourth, and maybe a fifth and a sixth.'
The Vera C. Rubin Observatory will begin science operations in July 2025.
Brown and colleagues have been looking for Planet Nine in images from the Zwicky Transient Facility and Pan-STARRS all-sky telescope surveys, but they have had no success so far. However, a new observatory now undergoing testing in Chile and soon operational, the Vera C. Rubin Observatory, will likely find Planet Nine — if it exists. The world's largest digital camera will shoot a 10-year time-lapse of the universe, taking a high-resolution image every 39 seconds. If a Planet Nine is lurking in the outer reaches of our solar system, Rubin ought to find it.
'If you were to hand me a billion dollars to build a telescope to find Planet Nine, I would give it back because the Vera Rubin observatory is absolutely perfect,' said Brown. 'It's going to just take pictures in the sky night after night, and we just need to find that one faint object slowly moving across the sky.' If it's on the moderate to brighter end of predictions, Rubin will find Planet Nine — probably in the next 18 months as it begins science operations. Until it does, Planet Nine will remain one of astronomy's greatest mysteries.
Wishing you clear skies and wide eyes.