Astronomers May Have Found the First Direct Hint of Planet Nine

A faint infrared object may be the long-sought planet hiding beyond Neptune.

06 Oct 2026, 21:37 by Tibi Puiu · ZME Science

Illustration of hypothetical Planet Nine. Credit: ZME Science.

For years, astronomers have wondered whether another huge planet is lurking unseen in the cold darkness beyond Neptune. It sounds silly seeing how astronomers have found thousands of alien planets across the night sky, orbiting planets even thousands of light-years away. But the Solar System is a vast place, and its edges are so devoid of light that it’s actually harder to detect a planetary body at the Solar System’s frontier than in far-away star systems.

Nevertheless, the signals are hard to ignore. Several extremely distant objects appear to have clustered and unusually tilted orbits, as though an unseen planet has been slowly shepherding them with its gravity. That hypothetical world became known as Planet Nine.

Now astronomers may have found their first direct clue that something is actually there.

Researchers searching decades-old infrared observations have identified a faint object that appears to have moved across the sky between two surveys taken 23 years apart. Its brightness and slow motion fit what the team expected from a large planet hundreds of times farther from the Sun than Earth.

The researchers stress that they have not discovered Planet Nine, or at least not in any definitive way. But the object was picked from a search through millions of infrared sources and emerged as their single best candidate.

The result, described in a study published in the Publications of the Astronomical Society of Australia, offers an intriguing new target in one of astronomy’s longest-running planetary hunts.

Hunting for Planet Nine

Finding Planet Nine would not be like spotting Jupiter in a telescope.

If the planet exists hundreds of astronomical units away — one astronomical unit is the Earth-Sun distance — very little sunlight would reach it and bounce back toward Earth. That makes it extraordinarily faint in visible light.

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That’s where infrared comes in. A planet also gives off heat, and its thermal infrared radiation fades with distance more slowly than reflected sunlight does. Previous searches have therefore turned to spacecraft that mapped the sky at wavelengths invisible to human eyes.

Terry Long Phan of National Tsing Hua University in Taiwan and his colleagues compared observations from two such missions.

IRAS, the Infrared Astronomical Satellite, surveyed the sky in 1983. Japan’s AKARI spacecraft repeated an infrared survey in 2006 and 2007. The 23-year gap gave researchers a particularly long baseline: a very distant planet might barely seem to move from night to night, yet drift far enough across the sky over decades to stand out.

The team searched for objects appearing in both surveys at positions consistent with such slow motion. They focused on a possible planet between 500 and 700 astronomical units from the Sun and roughly 7 to 17 times Earth’s mass.

After filtering more than a million sources from each survey for brightness, location and infrared characteristics, the researchers produced 22 possible pairings. Additional checks reduced that number to 13. Visual inspection left just one.

The apparent source had shifted by 47.46 arcminutes between the IRAS and AKARI observations — about one and a half times the apparent width of the full Moon.

So have we found Planet Nine?

Not yet.

The researchers found a pair of infrared detections consistent with a slow-moving object. AKARI observations also suggest the source changed position over six months in the way a distant Solar System body should. But two positions separated by decades cannot reveal a reliable orbit.

TPlanet Nine was proposed not simply as another distant planet, but as an explanation for the unusual trajectories of objects in the outer Solar System. A candidate must therefore move on the right kind of path to exert the gravitational influence astronomers are trying to explain, which is why determining the orbit is crucial.

There are other uncertainties. One of the candidate’s AKARI infrared measurements appears unusually faint and may be unreliable. And the search itself depended on assumptions about Planet Nine’s mass, temperature, density and distance. A substantially smaller, colder or more distant planet could have escaped these surveys entirely.

The next step is straightforward in principle: look again.

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Phan and his colleagues suggest using the Dark Energy Camera on the 4-meter Blanco Telescope in Chile. More observations could reveal whether the source continues moving along a planetary orbit or disappears as another false lead.