A Cold Clue in the Sky: Using Infrared Light to Hunt for Planet Nine
The idea of Planet Nine came from an odd observation: some distant objects in the Kuiper Belt—icy bodies far beyond Neptune—seem to cluster together in a way that gravity alone doesn’t explain. In 2016, Batygin and Brown suggested that a massive, unseen planet might be shaping these orbits. Some researchers have challenged this idea, arguing that observational bias could explain the effect. But newer studies—including one from 2024—have strengthened the case for Planet Nine. Simulations show that a planet with a mass between 6 and 12 Earth masses and an orbit hundreds of times farther from the Sun could create the patterns we see in distant space rocks.
A Different Way to Look: Heat Instead of Light
Most previous searches for Planet Nine used visible-light telescopes, hoping to catch sunlight bouncing off the planet. However, light from the Sun fades quickly with distance—at a rate of d⁴, where d is distance from the Sun. Thermal radiation (heat), on the other hand, fades more slowly (at a rate of d²). That makes it possible to search for the planet’s own glow in the infrared. Chen and his team used data from AKARI, a Japanese satellite that scanned the entire sky in infrared wavelengths between 50 and 180 micrometers, perfect for spotting objects around 30–50 Kelvin in temperature—exactly the expected range for Planet Nine.
Narrowing Down the Field: Data Selection and Filtering
The team used a special AKARI dataset called the Single Scan Detection List (FISSSDL), which includes even one-time detections that may have been missed in more conservative catalogs. From over 5 million sources, they focused on a promising sky region identified in past simulations. To ensure they weren’t looking at known stars or galaxies, they compared the AKARI list with nine major star catalogs. Only sources without known counterparts were kept. They also filtered out sources in areas with strong infrared background noise (like cirrus clouds), and chose only those with a strong, well-measured signal at 90 micrometers—the most sensitive AKARI filter.
Looking for Movement: How Planet Nine Would Appear
Because of its great distance, Planet Nine would not appear to move in images taken only hours apart. However, over six months, Earth’s motion around the Sun should cause the planet to appear to shift in the sky—a phenomenon called parallax. The team searched for sources that stayed still over hours but disappeared (or appeared) when viewed again months later. Out of hundreds of filtered candidates, they rejected any that appeared in multiple months (indicating they were fixed background sources), or that were only seen once (suggesting noise or cosmic rays).
Separating the Real from the Fake: Image Inspection
After applying all these filters, the researchers visually examined detection maps to eliminate false positives. Cosmic ray hits, scan edges, and artifacts caused many of the remaining “detections” to be discarded. Only 13 objects survived this inspection, and three of those were moving too quickly to be Planet Nine—possibly unknown asteroids. After further cuts, just two sources remained. These two objects were each detected twice within a day and then not seen again six months later, a motion profile consistent with a distant planet.
Could These Be Transients or Just Noise?
The authors considered whether the candidates might be short-lived astronomical events—such as supernovae or stellar outbursts—rather than a planet. But calculations showed that the chances of such bright, short-lived events occurring in their search area were extremely low (roughly one in a hundred). They also estimated the odds of two random, fake detections coinciding to mimic a moving object and found the probability was similarly tiny. This makes the two remaining candidates particularly intriguing.
The Next Step: Confirmation
Although these two candidates are compelling, the team emphasizes that further observations are needed to confirm whether either is truly Planet Nine. Each object was seen only twice, not enough to determine its orbit. However, follow-up studies using powerful telescopes like Subaru could check these sky regions for motion since the original AKARI scans. Depending on how far away the object is, it may have moved up to 2 degrees since its last detection—within reach of wide-field imaging.
Conclusion: A Promising Lead in the Planet Nine Mystery
By creatively using infrared data and a method designed to find slow-moving objects, Chen and colleagues have identified two potential Planet Nine candidates. These detections don’t prove the planet exists, but they provide new targets for astronomers to investigate. If confirmed, discovering Planet Nine would reshape our understanding of the Solar System and help explain the puzzling behavior of distant objects orbiting the Sun. For now, the mystery continues—but the hunt has just gotten a lot more interesting.
Source: Chen
