The imagined view from "planet nine" back toward the sun. Astronomers think the huge, distant planet is likely gaseous, similar to Uranus and Neptune. (Caltech/R. Hurt/IPAC)
The astronomer whose work helped kick Pluto out of the pantheon of planets says he has good reason to believe there's an undiscovered planet bigger than Earth lurking in the distant reaches of our solar system.
That's quite a claim, because Mike Brown of Caltech is no stranger to this part of our cosmic neighborhood. After all, he discovered Eris, an icy world more massive than Pluto that proved our old friend wasn't special enough to be considered a full-fledged planet. He also introduced the world to Sedna, a first-of-its-kind dwarf planet that's so far out there, its region of space was long thought to be an empty no man's land.
Now Brown has teamed up with Caltech colleague Konstantin Batygin to do a new analysis of oddities in the orbits of small, icy bodies out beyond Neptune. In their report published Wednesday in The Astronomical Journal, the researchers say it looks like the orbits are all being affected by the presence of an unseen planet that's about 10 times more massive than Earth — the size astronomers refer to as a super-Earth.
"I'm willing to take bets on anyone who's not a believer," says Brown. He thinks existing telescopes have a shot at spotting this mystery planet in just a few years, since this new study points to a band of sky where astronomers should look.
The first suggestion that something big might be affecting the orbits of distant, icy bodies came in 2014. An international team of astronomers announced that they'd discovered a new dwarf planet, nicknamed Biden, that stays even farther out than Sedna. They also noted a strange clustering in the orbits of these objects, and in the orbits of about a dozen others. Perhaps, they hypothesized, the gravity of some unseen planet was acting as a shepherd.
"They were pointing out that there was something funny going on in the outer solar system, but nobody could really understand what it was," says Brown. "Ever since they pointed it out we've been scratching our heads."
The idea of a huge, hidden planet seemed kind of crazy. "No one really took it very seriously," says Brown. "It was ignored more than you might guess."
But he walked a few doors down to meet with Batygin and suggested they take this on. As they studied the freaky way that these objects lined up in space, Brown says, they realized that "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."
What's more, this planet naturally explains why the dwarf planets Sedna and Biden have weird orbits that never let them come in close to the solar system. "This wasn't something we were setting out to explain," says Brown. "This is something that just popped out of the theory."
But there was one moment that turned Brown into a believer. Their computer simulations predicted that if this hypothetical planet existed, it would twist the orbits of other small bodies in a certain way. So Brown looked through some old data to see if any icy bodies had been discovered with those kinds of orbits — and, lo and behold, he found five of them.
"They're objects that nobody has really explained or tried to explain before," says Brown. "My jaw hit the floor. That just came out of the blue. Being able to make a prediction and having it come true in five minutes is about as fun as it gets in science."
Their work suggests how big the planet must be, and more or less where it could be found. Brown has already started looking. He hopes other scientists will too.
"I want to know what it's like. I want to see that it's really there," says Brown. "It will hurt when somebody finds it and it's not me — but I assume it's going to happen, and I'm willing to feel that pain."
It may be hard to believe that something so big would not have been seen before now. But Scott Sheppard of the Carnegie Institution for Science explains that for us to see it, sunlight has to travel all the way out there, bounce off the object, then travel all the way back.
"Objects get very faint very fast," says Sheppard. "If you do the math, if you move something twice as far away from the sun, it gets 16 times fainter."
Sheppard is one of the researchers who, after discovering Biden and the strange orbits, suggested a large planet might be the culprit.
"What we published was a very basic analysis of this clustering of objects in the outer solar system," he says. "We just did some basic stuff."
The new analysis, he says, has gone much deeper and has more rigor. "It leaves me thinking that the possibility of there being this super-Earth or mini-Neptune out there is more and more real now," says Sheppard.
Still, he's not completely convinced. "We really need to find more of these objects — more of these small objects that can lead us to the bigger object," Sheppard says. "I think it's still a tossup if it's really out there or not. I think we just need more data. Hopefully within the next few years we'll really be able to nail this down."
Dwarf planets like Sedna and Biden are not exactly household names. But Sheppard says if the solar system indeed has an honest-to-goodness ninth planet — a distant, giant planet that's bigger than Earth — "that, I think, is something that would blow the mind of anyone here on Earth."
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