A team of astronomers says it has identified a single, slow-moving infrared object that checks all the right boxes for it being the long-theorized ninth planet lurking beyond Neptune.
The hunt for Planet Nine—the solar system’s ghostly gravitational troll—is longstanding, and is based on the peculiar clustering of rocky bodies in the Kuiper Belt beyond the orbit of Neptune. This clustering suggests the presence of some faint, massive object. But the team behind the new study took a new approach to the search: investigating infrared data that’s been collecting dust for decades.
The research, posted this month to the preprint server arXiv and set to publish in the Proceedings of the Astronomical Society of Australia, comes from a group of scientists in Taiwan, Japan, and Australia. The group pored over archival data from two infrared space telescopes, NASA’s IRAS mission from 1983, and Japan’s AKARI satellite from 2006 to 2007. Their goal: find any object that’s cold, faint, and slow enough to be Planet Nine.
The search builds on a theory that made headlines in 2016, when Caltech astronomers Mike Brown and Konstantin Batygin proposed a massive, hidden planet based on the Kuiper Belt objects’ clustering patterns. In a 2021 update, the duo refined the theory, estimating Planet Nine to be just over six times Earth’s mass, with an orbit of 7,400 years—close enough to detect, possibly, with the upcoming Vera Rubin Observatory.
Enter the infrared search. Astronomer Terry Long Phan from National Tsing Hua University in Taiwan and his colleagues used the 23-year gap between IRAS and AKARI to look for extremely distant objects that moved just a little bit between snapshots. After applying filters for stars, galaxies, and other noise, they were left with one candidate. It doesn’t match any known object and appears to have moved just enough to plausibly be a missing planet.
“The paper is a nicely done analysis, but it is missing one thing that I would have hoped to have seen: a discussion of the probability that the detections are simply noise or some astrophysical transient that happen to be close together over a 23-year period,” Brown told Gizmodo in an email.
The object’s faint signature and extremely slow motion suggest a planet far beyond the orbit posited by Brown and Batygin, as well as other Planet Nine estimates. It’s a new lead on the phantom object, albeit one leading in an entirely different direction.
“The other interesting point is that the orbit of this object is not consistent with our predictions for Planet Nine,” Brown added. “That doesn’t mean it is not real, just that if it IS real it is not the predicted Planet Nine, it is something unpredicted.”
“My suspicion is that it is simply noise, but it will be fun to find out,” he said.
So astronomers remain cautious. Two data points decades apart aren’t enough to pin down an orbit, and the source could still be an unrelated background object or a data artifact. Still, the authors suggest the object is bright enough to be re-detected using ground-based telescopes, such as the Dark Energy Camera (DECam) in Chile.
The new research also undercuts some of the more fringe Planet Nine theories—like the idea that it’s a primordial black hole—since the apparent object is emitting light in the infrared. If follow-up observations confirm a planetary object, that would be a major win for the team’s approach—and validate the longstanding notion of a hidden planet in our system, albeit a different one than previously postulated.
For now, Planet Nine remains theoretical. But this could be the first real glimpse of something real out there—a faint point of light drifting through the dark, waiting for our technology and ingenuity to catch it in the act of being.
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