Far beyond Neptune, in the frigid darkness at the edge of our solar system, something massive may be lurking. For nearly a decade, astronomers have been hunting for this elusive giant—a hypothetical ninth planet that could revolutionize our understanding of how our cosmic neighborhood formed and evolved.
The story begins not with a telescope pointed at the stars, but with a computer screen displaying the strange mathematical dance of distant objects in the Kuiper Belt.
The Mathematical Detective Story
In January 2016, two Caltech professors made headlines with an extraordinary claim. Mike Brown, the Richard and Barbara Rosenberg Professor of Planetary Astronomy, and Konstantin Batygin, an assistant professor of planetary science, discovered evidence of a giant planet tracing a bizarre, highly elongated orbit in the outer solar system through mathematical modeling and computer simulations, though they had not yet observed the object directly.
What caught their attention wasn’t a glint of light in a telescope image, but a pattern in data. The six most distant known objects in the solar system with orbits exclusively beyond Neptune all mysteriously line up in a single direction, and when viewed in three dimensions, they tilt nearly identically away from the plane of the solar system.
The mathematical odds of this happening by chance? Astronomically small. Brown and Batygin calculated the probability that the clustering is spurious at around one in 500. Something massive had to be shepherding these distant objects into their peculiar alignment.
Characteristics of the Hidden Giant
The object, which the researchers nicknamed Planet Nine, has a mass about 10 times that of Earth and orbits about 20 times farther from the sun on average than does Neptune. It would take this new planet between 10,000 and 20,000 years to make just one full orbit around the sun.
To put this in perspective: Planet Nine is at 5,000 times the mass of Pluto and is sufficiently large that there should be no debate about whether it is a true planet. Unlike the class of smaller objects now known as dwarf planets, Planet Nine gravitationally dominates its neighborhood of the solar system.
Brown, who famously led the effort that resulted in Pluto’s demotion to a dwarf planet, noted with characteristic wit that Planet Nine “dominates a region larger than any of the other known planets—a fact that makes it ‘the most planet-y of the planets in the whole solar system.'”
A Breakthrough in Ancient Data
While the search for Planet Nine has continued for years without success, a recent development has renewed excitement in the astronomical community. Researchers from Taiwan, Japan, and Australia have found a candidate for Planet Nine by analyzing data from two far-infrared all-sky surveys: IRAS (launched in 1983) and AKARI (launched in 2006). The 23-year separation between these surveys provided enough time to detect the expected slow orbital motion of such a distant planet.
After a careful search, the team found a single object that appeared in one position in IRAS’s 1983 data but was located 47.4 arcminutes away in AKARI’s imagery—within the range that Planet Nine could have traveled in the intervening time.
However, this discovery comes with a significant caveat. Mike Brown, who originally proposed Planet Nine’s existence, says the candidate’s motion suggests it’s on a highly inclined and mildly retrograde orbit, nearly perpendicular to the solar system’s plane. With such an orbit, this object could not explain the unusual clustering of Kuiper Belt objects that led to the prediction of Planet Nine.
“Whether it is real or not, it is 100 percent NOT Planet Nine,” Brown states. Still, finding any new planet in our solar system would be a monumental discovery.
The Technology That Could End the Hunt
The mystery of Planet Nine may not remain unsolved much longer. A revolutionary observatory is poised to transform the search: the Vera C. Rubin Observatory, currently conducting final preparations on a mountaintop in Chile.
Mike Brown couldn’t have asked for a better tool for the hunt. “If you were to hand me a big wad of cash and say, ‘Go build a telescope to go either find this Planet 9 or find the best evidence possible for Planet 9,’ I would probably go and build the Vera Rubin Observatory,” he says. “It really is a telescope that is perfectly suited for making the next step.”
Starting in 2025, the Vera C. Rubin Observatory will increase the number of known objects circling the sun by roughly tenfold, spotting new comets, exotic asteroids from other stars, and perhaps even the elusive Planet Nine. The observatory will survey almost the entire southern sky every night, and its images will feed into computer systems that will constantly compare new images to previous ones, allowing it to detect anything that changes—like Planet Nine moving across the sky.
If Planet Nine is real, researchers estimate this observatory has around a 70 to 80 percent chance of finding it, though it’s not a sure thing because of the many uncertainties about the planet’s size, reflectivity, and distance.
A Solar System Transformed
The discovery of a ninth planet would fundamentally change our understanding of how our solar system formed. Current models don’t easily explain how such a massive world could end up in such a distant, tilted orbit. Some theories suggest it could be a “rogue planet” captured from another star system, while others propose it was scattered outward by gravitational interactions with Jupiter and Saturn in the early solar system.
As Konstantin Batygin notes, there’s “a real thrill to the fact that this theoretical thing has a relatively near-term resolution that’s observational. Planet Nine’s either there or not there. And we’re going to know, not on a timescale of a century, but on a timescale of a decade from 2016, so any moment now.”
The Bigger Picture
The hunt for Planet Nine represents more than just the search for another world in our solar system. As researcher Terry Long Phan puts it: “Personally, I feel it is unacceptable that we do not even fully understand our solar system, while 100 billion planetary systems are in our Milky Way galaxy alone, and 100 billion such galaxies in the universe. It is my belief that before we try to understand stars, galaxies, or the universe, we must first come to understand our own home: the solar system.”
Brown remains confident in their calculations. “This is the 5th largest planet in our solar system, lurking out there, waiting to be found,” he says. Whether Planet Nine is real or not, the search itself has already advanced our knowledge of the outer solar system and pushed the boundaries of observational astronomy.
With the Rubin Observatory beginning operations and more sophisticated surveys planned, we may soon know definitively whether our solar system harbors a ninth planet. Until then, the hunt continues—a testament to humanity’s insatiable curiosity about the cosmos and our place within it.
The scientific papers and observations mentioned in this article represent ongoing research, and the existence of Planet Nine remains a hypothesis awaiting observational confirmation.