A new initiative by NASA and the University of California at Berkeley, called Backyard Worlds: Planet 9, is crowdsourcing the hunt for Planet Nine. It will use archived observations from NASA's Wide-field Infrared Survey Explorer (WISE) mission, which scanned the skies for asteroids and other faint objects. It's possible that Planet Nine — or perhaps a “brown dwarf” star or two — is lurking in its speckled images of space.
This planet could be 500 times as far from the sun as Earth is, but it would still be part of our solar system, with a highly elliptical orbit that never takes it anywhere close to the sun. Planet Nine should not be confused with the many “exoplanets” discovered orbiting distant stars. (Nor is it the planet known as Nibiru, which exists only in the imagination of people peddling pseudoscience and apocalyptic narratives about worlds in collision.)
The mystery planet's existence is inferred from the orbits of many smaller bodies in the outer solar system. They orbit the sun and cluster in a manner that suggests the possible gravitational influence of an unseen, large planet. The evidence for its existence has been getting stronger, Caltech astronomer Mike Brown told The Washington Post.
“I’m just going to tell you: It's there,” Brown said.
Not everyone is persuaded.
“The evidence is very intriguing, but I don’t think I can put a high likelihood on it yet,” said Renu Malhotra, a University of Arizona professor of planetary science. “I see the evidence as being quite soft, still.”
Even the name is iffy. Planet Nine? Planet X? Planet 10? It's confusing. There used to be nine officially recognized planets in our solar system — the ninth being little Pluto, discovered in 1930. But after Brown and other astronomers began discovering small planetlike objects in the outer solar system, a debate erupted over the definition of a “planet.” In 2006, the International Astronomical Union decided that Pluto was a member of a class of objects to be known as “dwarf planets.”
Astronomers kept discovering these distant objects in the remote Zip codes of the solar system, in what is known as the Kuiper belt. And then a pattern seemed to reveal itself.
In 2014, astronomers Scott Sheppard of the Carnegie Institution and Chad Trujillo of the Gemini Observatory in Hawaii published a paper suggesting that there may be a massive “perturber” in the outer solar system that affects the orbits of smaller objects. Sheppard and Trujillo said those smaller objects orbit the sun at an angle as if avoiding the hidden, larger planet. “It would resemble a giant frozen snowball,” Sheppard told The Washington Post in 2015.
Brown set out to debunk Sheppard and Trujillo's conjecture, but he wound up providing supporting evidence. Last year, Brown and Caltech colleague Konstantin Batygin published a paper in the Astronomical Journal that offered a possible orbit for the hidden planet, which a news release from Caltech referred to as “Planet Nine.”
Brown said last week that additional observations of distant, small objects and ensuing calculations and modeling suggest that Planet Nine is roughly eight times as massive as Earth and slightly closer to the sun than previously thought. He said it is probably the core of a giant planet that was ejected from the inner solar system long ago. He said it's likely to have an atmosphere, which would make it broader, warmer and easier to detect. If it's a dense, rocky planet, it'll be smaller, colder, darker and harder to find.
Sheppard said he has also become more optimistic about the existence of the planet.
“We still see a clustering trend. That brings me to about 90 percent sure that this planet exists beyond Pluto,” he told The Post this week. “I don’t think there’s a credible alternative hypothesis right now.”
He doesn't call it Planet Nine, though. Nor Planet X — which, historically, is what astronomers called a hypothetical planet beyond Neptune before the discovery of Pluto.
“I like to call this Planet Y, because it's beyond Planet X,” Sheppard said.
Malhotra points out that this hypothesis is based on relatively few observations — that this is still in the “regime of small numbers.” But she says there's another possibility that emerges from the way the orbital plane of the solar system is warped beyond Pluto. That could be a sign that there's yet another planet, maybe the size of Earth, that's not as far away as the hypothesized Planet Nine, Malhotra said.
So why wouldn't we have seen it?
“It could be hiding in the galaxy,” Malhotra said. She means the Milky Way. Finding a dim, icy planet against the thick river of bright stars is a challenge. “There are parts of the sky that our surveys haven't covered well,” she said.
So apparently the universe isn't about to run out of surprises.