Skip to main content

History Stories

Planet Nine is located in an icy, debris-ridden part of space known as the Kuiper Belt, some 20-100 billion miles away from the sun, which means it would take between 10,000 and 20,000 years to make one solar orbit. By comparison, Neptune, currently recognized as the most distant planet in the solar system, is a relatively paltry 2.8 billion miles from the sun at its furthest distance and takes approximately 165 years to circle the sun. It’s likely that Planet Nine’s distance, combined with its unique positioning (its orbit faces in the opposite direction of the eight other planets) allowed it to slide under the scientific radar for so long.

In a surprising twist, astronomer Mike Brown, who led the efforts to have Pluto downgraded to dwarf planet status in 2006, is part of the two-person Caltech team that may have identified its replacement. The irony was not lost on Brown, who cheekily tweeted (from his @plutokiller account). “OK, OK, I am now willing to admit: I DO believe that the solar system has nine planets.”

While some may find it somewhat convenient that Pluto’s “killer” was involved in finding its replacement,, Brown and his fellow Caltech planetary scientist Konstantin Batygin were actually working to disprove a 2014 study, written by Chad Trujillo, a former postdoctoral student of Brown’s, and his colleague Scott Sheppard. That early study focused on the unusual orbital behavior shared by some two dozen objects in the Kuiper Belt; behavior that Trujillo and Sheppard believed was dictated by the presence of a small planet.

space, planet nine

Caltech professor Mike Brown and assistant professor Konstantin Batygin. (Credit: Lance Hayashida/Caltech)

Scroll to Continue

Recommended for you

When the Caltech team got to work, they were quickly drawn to the elliptical orbit of six objects in particular, all of which point in the same direction (tilted at 30 degrees downward), despite the fact that travel around the solar system at different rates. As Brown explained in a press release, “It’s almost like having six hands on a clock all moving at different rates, and when you happen to look up, they’re all in exactly the same place…Basically it shouldn’t happen randomly.” Brown estimated the probability of the behavior being coincidental is at a 1 in 15,000 chance, or 0.007%, leading him and Batygin to believe that something in the distant Kuiper Belt was exerting gravitational power or, as Brown said, “shaping these orbits.”

Using mathematical and computer modeling, the Caltech team ruled out several possible scenarios before coming to the conclusion that the orbital abnormality could only be explained by the gravitational pull of a large planet. As Brown explained to NPR, “the only way to get these objects to line up in one direction is to have a massive planet lined up in the other direction.” According to the research team, it’s likely that Planet Nine first formed much closer to the other planets of the solar system, before being ejected to its present, far-flung location by a gaseous event.

The researchers also noted that Planet Nine’s size (5,000 times the mass of Pluto) should quickly end any debate about whether or not it’s a true planet, and that rather than upending the conventional wisdom, the existence of the planet actually makes perfect common sense, telling Mashable, “Throughout the galaxy, the most common type of planet is between the mass of the Earth and the mass of Neptune, and isn’t it weird that we don’t have this type of planet? And, it turns out, this is exactly what we have.”

Getting a glimpse of Planet Nine could prove difficult, due to its distance —and the fact that while astronomers have determined its orbital path, they’ve yet to pinpoint where in its orbital cycle it currently is. Brown, Batygin and the larger scientific community are turning to the public for help, rather than continuing the search in secrecy. As Brown said, “I would love to find it. But I’d also be perfectly happy if someone else found it. That is why we’re publishing this paper. We hope that other people are going to get inspired and start searching.”

FACT CHECK: We strive for accuracy and fairness. But if you see something that doesn't look right, click here to contact us! HISTORY reviews and updates its content regularly to ensure it is complete and accurate.