Among the solar system’s more intriguing mysteries is whether a large, icy planet lives in the outer regions of our cosmic neighbourhood, well beyond the orbit of Neptune. This hypothetical world, nicknamed “Planet Nine” by some of the scientists searching for it, has stirred up controversy since it was first proposed.
The unseen planet is predicted to exist based on its apparent gravitational influence on a group of small objects with odd, clustered orbits. But so far, searches for it have come up empty, and critics contend that the hints of its presence are just ghosts in the data.
Now, a new analysis predicts that if it’s out there, that skulking planet could be closer, brighter, and easier to spot than previously estimated.
Instead of orbiting our home star once every 18,500 years, astronomers calculate that it loops around the sun in about 7,400 years. That tighter orbit brings it much closer to the sun than previously expected, which means that Planet Nine may appear brighter to Earth-based telescopes.
“I think it’s within a year or two from being found,” says Mike Brown, an astronomer at the California Institute of Technology and an author of the new study, which has been accepted for publication in the Astronomical Journal. But, he adds, “I’ve made that statement every year for the past five years. I am super-optimistic.”
Brown’s latest analyses of Planet Nine’s gravitational shenanigans, calculated with his Caltech colleague Konstantin Batygin, suggest that the world is roughly six times as massive as Earth—which would likely make it either a rocky super-Earth or a gaseous mini-Neptune. If discovered, the planet would be the first large world to join the solar system’s cast of characters since 1846, when astronomers announced the discovery of Neptune—an ice giant whose presence was forecast by its gravitational influence on Uranus.
But over the years, skeptics have suggested that the gravitational signatures betraying Planet Nine’s presence are nothing more than observational artefacts. The apparent clustering of distant objects’ orbits doesn’t reflect the influence of an unseen world, critics argue, and is instead the result of natural biases in sky surveys.
“Most of these objects are discovered with large telescopes that have limited time for surveys of the outer solar system, and they look in the places they can look, which depends on where they are located,” says Renu Malhotra of the University of Arizona, who is agnostic about the planet’s existence and is working on her own estimates of its position. Astronomers have so far discovered only a handful of these distant objects, and without a more complete census of the outer solar system, it’s tough to tell whether these small, icy objects are truly behaving strangely, or are randomly distributed.
To aid searchers in the meantime, Brown and Batygin used their revised calculations to make a “treasure map” that points to a swath of sky where Planet Nine is most likely to be found. That area crosses the densely populated, sparkling plane of the Milky Way, which could have helped the planet hide during past searches.
“Now we really know where to look, and where not to look,” Brown says. “This should do it—unless we did something wrong.”
Ghost planets in the far solar system
Brown and Batygin originally announced their prediction of Planet Nine in 2016, but the pair is hardly the first to suggest that an undiscovered world is hiding in the solar system’s backcountry. For more than a century, astronomers have mused about such a planet, erroneously believing that something hefty was perturbing Neptune’s orbit. Astronomer Percival Lowell called the world Planet X and was so intent on finding it that he left a million dollars to fund the continuing search after his death in 1916. (In 1930, the Lowell Observatory’s Clyde Tombaugh found little Pluto instead.)
The Caltech team based their prediction of Planet Nine’s existence on how it apparently perturbs a group of Kuiper Belt Objects, or KBOs. These small, icy worlds beyond Neptune include a population of objects with extreme orbits that take them at least 150 times farther from the sun than Earth’s orbit.
In 2016, Batygin and Brown scrutinised six of those objects, whose oblong, tilted orbital paths have confounded scientists for years. The team concluded that an unseen planet about 10 times as massive as Earth must be gravitationally shepherding the objects onto their catawampus trajectories. The planet’s estimated mass sits between Earth and Neptune, making it a type of world that appears to be common throughout the galaxy, based on surveys of planets orbiting other stars, yet is conspicuously absent in our own solar system.
Soon after the announcement, though, astronomers began casting doubts on the Planet Nine hypothesis. Chief among their concerns was that the peculiar clustering of orbits might not be clustering at all. Instead, over the past five years, multiple teams using a variety of data sets have repeatedly concluded that the evidence pointing to Planet Nine is nothing more than an observational artefact.
Perhaps Planet Nine is an apparition, its supposed gravitational handiwork a false signature created by a small number of misleading data points. Astronomers are still working on resolving the controversy, and this latest analysis from Brown and Batygin is one attempt to do that.
“Good on them for making a detailed prediction and putting it out there,” says the University of Canterbury’s Michele Bannister, whose work challenged the Planet Nine hypothesis in 2017. “I will be very delighted if this thing turns out to exist—it’ll be a fun solar system to live in.”
Refining the search
Brown and Batygin based their newest predictions of Planet Nine’s size and orbit on a slightly different set of objects. Some of the original KBOs remain in their data set, but the team added new ones and threw out any objects whose orbits appeared to be influenced by Neptune’s gravity. In the end, they worked with 11 KBOs.
“If you include the Neptune ones you’ll fuzz out your signal and won’t know what’s going on,” Brown says.
The new study finds that there’s a 99.6 percent chance that the peculiar orbital alignments of these objects are the work of an unseen planet and not random chance. That sounds pretty good, Malhotra says, but it means that there’s a 1-in-250 chance the alignments are a fluke—which is much greater than the 1-in-10,000 chance Brown and Batygin published in 2016.
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