Scientists have now excluded 78 percent of the search space for the hypothetical Planet Nine, significantly narrowing down the possible location of the planet that could explain some of the strange phenomena of our solar system—if it even exists.
The Planet Nine hypothesis proposes that there’s an undiscovered planet lurking at the far reaches of our solar system, and that this could explain the strange orbits of around half a dozen objects at its outer edges. These Extreme trans-Neptunian objects (ETNOs) mostly tend to cluster together as they get closer to the sun and their orbits are all oddly tilted. The gravitational pull of another, massive planet could explain these observations.
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No one’s actually seen this mysterious ninth planet, but a preprint published on arXiv has significantly narrowed the search field and explains that Planet Nine existing is still the researchers’ preferred hypothesis.
Astrophysicists have been slowly ruling out sections of space where they might find Planet Nine. Previous analyses using data from the Zwicky Transient Facilities archive and The Dark Energy Survey excluded 61 percent of that space. This latest work—by Planet Nine-searchers Michael Brown, Konstantin Batygin from CalTech and Matthew Holman from Harvard University—brings that up to 78 percent.
While it may seem small, that trimming of space represents a trimming of potential objects that could be Planet Nine from 1.2 billion to 244 million, with researchers hesitant to trim the fat further. “While searching for Planet Nine through this large data set remains a formidable task,
we could find no additional filters that appeared to safely further reduce the data set,” the preprint authors write. “Any one of these cuts in the data has the possibility of removing real detections of Planet Nine. Our calibration method must by necessity take this possibility into account.”
Study co-author Michael Brown explained the grueling work that went into narrowing the search. “It’s basically a brute force method of computer simulations,” he told Motherboard. The team simulates objects in the outer solar system, plug in varying specific parameters for a potential Planet Nine then watch gravity at work over four billion years to see how the orbits of those objects change. “You put all that together to get a probabilistic distribution of where the planet might be.”
But the existence of a mysterious planet isn’t the only way to explain the bizarre phenomenon in our neighborhood of space. One is that a set of distant objects—including other dwarf planets, comets, and moons—might collectively be messing with the orbits of ETNOs. Others say that it’s a far-off disk of ice and rock, or a small black hole.
Some researchers, including theoretical physicist Harsh Mathur, say that an adjusted law of gravity—called Modified Newtonian dynamics, or simply MOND—could rule out both Planet Nine and dark matter.
Mathur and his collaborator hadn’t sought to discount Planet Nine when they published their 2023 study, but were interested in what effects this theory would have on a solar-system level.
“We’re taking this theory of gravity that already exists and explaining something else which is galaxy dynamics. It’s an alternative to dark matter that works very well in the galactic realm,” Mathur, from Case Western Reserve University, told Motherboard.
In fact, Mathur describes his view towards the Planet Nine hunt as “agnostic.” “If we find Planet Nine I think that would be the coolest thing because how often do you discover a new planet in the solar system? But for us it’s interesting either way. If Planet Nine is found that means the MOND explanation isn’t working and that tells us something about gravity, and if it’s not found then the evidence for MOND begins to pile up, which is very exciting too because it’s a new law of gravity.”
Brown favors some alternate explanations over others but says that, ultimately, Planet Nine makes the most sense. “Having a planet out there is the most mundane explanation there could be. We see planets like that around other stars on these distant, eccentric orbits. There are good reasons to think our solar system made more planets and may have ejected them. There’s nothing profound about the idea that we have something else out there.”
An alternative “flavor” of the Planet Nine hypothesis that Brown concedes is that it’s actually a much bigger, much more distant planet sitting somewhere in the Oort Cloud some 3.2 lightyears away.
The other big—and potentially disappointing—explanation for ETNOs that comes up often is that the orientation and clustering astronomers are seeing is simply observational bias. Mathus explains that for objects that are this far away, we can only really see them when they venture close to the sun and therefore by nature cluster together. Or that it’s from using data from only one survey or telescope, which only scans a certain portion of the sky.
It’s a critique one that Brown says they took into account from the moment they proposed Planet Nine some eight years ago. However, through extensive statistical analysis—which Brown and Batygin have detailed in on X and blog posts—they’ve concluded it’s unlikely to explain what they’re seeing. “It is the most important question to ask when we see something like this. Therefore we asked it the very first time.”
And they aren’t blinded by their quest for the mysterious planet. “So far, we have not found a reason to discard the Planet Nine hypothesis. Will one some day? Perhaps. If the evidence for the existence of Planet Nine were to unravel we would be sad, but we would have to give up the idea. We have been prepared for that moment since the day we first proposed Planet Nine,” Brown and Batygin wrote in one post.
Finally observing Planet Nine might come down to some of the latest generation telescopes and observatories, says planetary astrophysicist Malena Rice. “We actually have to stack together all of the data for about a month with the TESS [Transiting Exoplanet Survey Satellite] dataset to try and find something that is both moving at right speed to be in the outer solar system and that’s bright enough that it would have to be a planet,” she told Motherboard. “There shouldn’t be anything else besides Planet Nine that would fall within that intersection.”
“There’s one game in town that’s really going to make a huge difference that’s the LSST [Legacy Survey of Space and Time],” echoes Brown, referring to the sky survey about to get underway at the Vera C. Rubin Observatory in Chile. “It’s a big telescope with the biggest camera on a telescope in the world and they take pictures of the sky night after night after night.” This telescope images the entire sky every three nights, compared to the telescope data Brown and colleagues were using, which only did so 10 to 15 times over a five year period.
And even though Brown has been saying it for the past ten years, he’s confident that this time around someone will spot Planet Nine in the next couple of years. “But this time it’s true!” he jokes.
Rice says that, in the search for Planet Nine, we may end up finding other planets in the far reaches of our solar system, similar to how Pluto was discovered. “I’d be quite surprised if we found exactly the planet that was predicted but that doesn’t mean we won’t find something.”
She adds that, although she hasn’t yet reviewed the latest preprint, that she’s excited to read more. “It looks like a really nice way to use an existing dataset and just do innovative science with it.”
