Scientists examined infrared space surveys to narrow down candidates for Planet 9.
Planet 9 would be so far from the Sun that it is just not illuminated by much sunlight.
The two space surveys, conducted 23 years apart, could show how a secret planet orbits.
Since the demotion of Pluto to dwarf planet, there’s been a ninth-planet-shaped hole in the hearts of many Earthlings. How will we remember what My Very Excellent Mother Just Served Us now?
Well, if we’re lucky, we may soon be able to fill that gap. For some time now, scientists have wondered if a regular planet—not a dwarf planet—could explain some of the clustering of objects in the Kuiper Belt that starts at Neptune and extends outward from our Solar System. (Pluto is considered a Kuiper Belt object.) This hypothetical Planet 9 has never been directly observed and remains theoretical, but in new research uploaded to the preprint site arXiV, scientists share a very educated guess about the location of the long-theorized celestial body. (This study is not yet peer reviewed, but it follows a presentation from a 2024 conference and is listed as “Accepted for publication in Publications of the Astronomical Society of Australia (PASA).”)
To search for Planet 9, the scientists “search[ed] for objects that moved slowly between IRAS and AKARI detections.” IRAS and AKARI are far-infrared, all-sky surveys completed 23 years apart.
Though Planet 9 has never been confirmed, we know a lot about what a planet in its suspected area could be like. The team used qualities like this—such as a very long orbit around the Sun, feasible temperatures, and probable masses—to narrow down the IRAS dataset and the AKARI dataset separately. They then highlighted 13 total bodies that fit the criteria and showed up in both datasets, accounting for changes in epochs and positions in orbit.
From there, they inspected the surveys’ images and found one eligible bachelor… er, planet. The scientists are careful to explain that the criteria they chose would not necessarily identify the only possible candidates for Planet 9. For instance, if Planet 9 is over the size constraints they placed on their study, it’s still likely to be picked up at some point by infrared surveys like this. It could also be a gas giant-type planet on par with or larger than Neptune. But if it’s smaller, it’s likely below the threshold of observability at such a long range.
And long range is, honestly, underselling it—top theories for the positioning of Planet 9 place it around 400 astronomical units (AU). The furthest known planet, Neptune, is only about 31 AU. That’s already 31 times further than Earth is from the Sun, creating an orbit that lasts about 165 years. Planet 9 would also have an irregular orbit, scientists believe, so it would spent part of its cycle even further away.
This is why two surveys conducted 23 years apart may just be the perfect way to observe Planet 9. In that amount of time, the planet itself will have completed just a tiny fraction of its overall orbit—enough to travel from one spot into the adjacent spot on the subsequent survey. But a lot of other factors would need to line up exactly in order for this candidate to be the real Planet 9, and the scientists are transparent about that.
“The finalist of our Planet Nine candidate pair strongly depends on how the characteristics of Planet Nine are defined. [I]f the actual mass of Planet Nine is not sufficient to make its flux above the detection limits of two surveys, there is no chance of finding Planet Nine in this work.”
This group took a stab at observing Planet 9 using the IRAS and AKARI infrared surveys, which can “see” further than telescopes using the visible spectrum. And indeed, while the odds of actually finding the elusive planet here may be more like the odds of buying a winning lottery ticket, the work opens several next steps for other teams investigating Planet 9. The data may also help narrow down parameters for those using other tools.
“If the existence of Planet Nine can be confirmed by observations in the near future, it will improve our understanding of the history and structure of the entire Solar System in early stages,” the team concludes.
And it would get Pluto yet another big sibling.
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