For years, astronomers have toyed with the idea that the Sun hides an extra planet so distant and dim that even the best telescopes have missed it. Now, a new study digs into decades-old infrared maps of the sky and turns up one faint object whose slow drift could fit the bill—renewing hopes (and doubts) about the long-sought “Planet Nine.”

Astronomers have nudged the Planet Nine debate back into the spotlight in their study accepted for publication in the Publications of the Astronomical Society of Australia after exhuming a faint, slow-moving speck of light from two infrared sky maps taken a generation apart.

The object, noticed first in the 1983 survey of NASA’s Infrared Astronomical Satellite (IRAS) and again in the 2006–07 all-sky pass of Japan’s AKARI observatory, sits exactly where a distant planet could have drifted during the twenty-three years that separate the two missions.

Why a ninth planet has been suspected all along

Far beyond Neptune lies the Kuiper Belt, a ring of icy leftovers from the Solar System’s early days. Several of those objects, including the dwarf planet Sedna, follow orbits that cluster in one sector of space instead of being spread evenly around the Sun. Computer simulations in 2016 showed that a hidden planet five-to-10 times Earth’s mass could shepherd those orbits into the observed pattern. Other explanations exist, but none fit the data as neatly.

The combined catalogues from the AKARI/IRAS sky maps held roughly two million point sources. Lead author Terry Long Phan, an astronomer at National Tsing Hua University in Taiwan, and his colleagues first calculated how bright Planet Nine should look at the five-to-10 times Earth’s mass ratio and sits 500–700 astronomical units (AU) from the Sun—a distance where one orbit would take about 10,000 years.

They rejected anything too bright, too faint, or too close to the dusty plane of the Milky Way, then paired every surviving IRAS dot with AKARI dots 42–70 arc-minutes away—the amount a slow-moving planet should drift in 23 years. The cull left 13 possible pairs.

Twelve of the 13 pairs turned out to be image noise or background dust clouds. The remaining pair behaves just as theory predicts: the IRAS source appears only in 1983, the AKARI source only in 2006, and the two lie 47 arc-minutes apart—right in the middle of the expected range. If the two detections are from the same body, its infrared brightness hints at a size similar to Neptune and a present distance near 700 AU.

Not everyone is convinced

Despite the findings, there are naysayers who don’t think the astronomical world should be jumping to conclusions.

“It is pretty amazing to think that something as big as Neptune could be sitting out there and no one would have ever noticed it,” University of Pennsylvania astronomer Gary Bernstein, who was not involved in the work, told Science.

Some experts don’t think the signal, which is just a single pair of faint dots, will hold up under close inspection and further observations. But if it does, the object is on a path that is very different from what was predicted for Planet Nine—rendering it an entirely different planet.

This mismatch “doesn’t mean it’s not there, but it means it’s not Planet Nine,” says Mike Brown, an astronomer at the California Institute of Technology who, along with his colleague Konstantin Batygin, came up with the Planet Nine proposal nearly a decade ago. “I don’t think this planet would have any of the effects on the Solar System that we think we’re seeing.”

What happens next

Two dots separated by two decades do not make an orbit. Phan’s team hopes to point the four-meter Blanco telescope in Chile at the spot where their candidate should be today. Its Dark Energy Camera can detect objects as faint as 26th magnitude—roughly what a Neptune-size planet at 700 AU would look like—in about an hour. A handful of detections spread over months would trace a curved path, proving the object orbits the Sun and revealing how massive and distant it really is.

Even if this candidate fades on closer inspection, the search is poised to speed up. The Vera C. Rubin Observatory, due to begin full operations in Chile later this year, will photograph the whole southern sky every few nights and is expected to discover tens of thousands of new Kuiper Belt objects. If Planet Nine lurks out there, Rubin’s nightly movies of the heavens should either pin it down or finally rule it out.

Finding—or definitively losing—Planet Nine would reshape our picture of the Solar System. A confirmed discovery would force textbooks to add a big, cold world whose year lasts longer than recorded human history, while a null result would send theorists back to their keyboards to explain those odd Kuiper Belt orbits some other way. Either outcome will sharpen scientists’ understanding of how planets take shape and migrate—a process now known to sculpt hundreds of planetary systems across the galaxy.

For now, the mysterious IRAS-AKARI object remains encouraging: old data is still relevant and is a fresh reminder that even dusty archival images can still change the map of our cosmic neighborhood.