Astronomers believe they have found evidence for a ninth planet in our solar system — not Pluto, but an object that is giant, mysterious, and entirely new.
In fact, they believe so-called “Planet Nine” exists for the same reason that dwarfy Pluto was demoted in 2006. Something appears to be gravitationally bullying a handful of objects in the Kuiper Belt, the icy region beyond Neptune. That bully, the researchers reported in the Astronomical Journal on Wednesday, could be a planet 10 times more massive than Earth, on an orbit so distant a single trip around the sun would take between 10,000 and 20,000 years.
The astronomers, Mike Brown and Konstantin Batygin of the California Institute of Technology, have not observed the new planet directly. But they say computer simulations convinced them the planet is out there.
“This is basically the start of the treasure hunt,” Brown told the Star. “We’ve published the treasure map, and now the hunt is on to go find Planet Nine.”
Other astronomers say Brown and Batygin’s conclusions are plausible, but more research needs to be done.
“There’s a reasonable chance, although it’s by no means 100 per cent, that they’ve found something significant here,” said Martin Duncan, a professor of astronomy at Queen’s University who specializes in solar system dynamics and the Kuiper Belt. Duncan noted that Brown and Batygin both have stellar reputations, and “have apparently explored the possibilities quite carefully. I think they’re probably not leaping to wild conclusions.”
Evidence for the existence of Planet Nine comes in the form of half a dozen distant objects behaving very strangely.
In 2014, two other astronomers, Scott Sheppard and Chad Trujillo, published a paper showing that several objects in the Kuiper Belt were orbitally “clustering,” and the phenomenon could be explained by the existence of a new planet.
Intrigued but skeptical, Brown and Batygin teamed up to investigate.
“We’ve been working on this for two years now, and for the first year and a half, we sort of treated this as an anything-but-a-planet project. Our goal was to prove (the idea) was wrong.”
First, the duo noticed that six of Sheppard and Trujillo’s objects had even stranger similarities: their orbits were aligned in the same direction and tilted at the same degree. The probability of that happening by chance, they calculated, was 0.007 per cent.
Experimenting with computer simulations, Brown and Batygin modelled a giant planet rotating around the sun on a strange “anti-aligned” orbit. That model explained the clustering.
But that wasn’t all: The simulations also predicted even stranger objects with orbits perpendicular to the usual plane of the Kuiper Belt. And, the researchers were startled to realize others had already reported finding exactly such objects.
“As a scientist, you try really hard not to believe yourself too much, because that’s where you go wrong,” says Brown. But this second class of strangely behaving objects jarred him into believing that the giant, distant planet might be real. “That’s the moment that it went from interesting pet theory to holy cow — this might be true.”
Interestingly, the last discovery of a true planet in our solar system occurred after 19th century astronomers noticed “perturbations” in Uranus’ orbit. Guided by that data, Neptune was found in 1846.
In the years since, other claims for new planets have crumbled and spacecraft surveying the sky failed to find evidence to support the existence of a long-heralded “Planet X.” Indeed, Paul Delaney, an astronomer at York University, notes that the effects attributed to Planet Nine could be explained by other phenomena or by simple chance — however remote.
“It’s interesting, but it’s not convincing,” Delaney said. “We need more data.”
Brown and Batygin would agree. The Large Synoptic Survey Telescope, when it is completed in 2023, will be able to map the entire sky in just a few nights, providing more data on distant Kuiper Belt objects that could both bolster evidence for Planet Nine and help pinpoint its current location. Telescopes like Subaru and Keck on the summit of Mauna Kea in Hawaii, currently the most powerful in existence, might be able to observe Planet Nine directly. Brown has already spent several nights at Subaru searching the skies.
Ironically, Brown was the astronomer whose observations led to Pluto’s demotion to dwarf planet — his Twitter handle is @plutokiller. His ten-year-old daughter has never forgiven him.
“She did tell me five years ago that I could make it up to her if I found a new planet,” Brown says. “She’s pretty excited.”