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."
ROBERT SIEGEL, HOST:
An unseen planet about 10 times more massive than Earth is lurking in the outer reaches of our solar system. That is the bold claim made today by two astronomers at Caltech. The idea sounds crazy, but as NPR's Nell Greenfieldboyce reports, it's worth taking seriously. One of these guys has a solid track record of finding things in this frigid, distant part of our cosmic neighborhood.
NELL GREENFIELDBOYCE, BYLINE: Mike Brown once called "How I Killed Pluto And Why It Had It Coming." It recounts his discovery of Eris, one of thousands of icy bodies beyond Neptune. Eris was a big deal because it's more massive than Pluto, proving that our old friend wasn't special enough to be considered the ninth planet. This did not exactly make Brown popular.
MIKE BROWN: I get hate mail. I get obscene phone calls. I get drawings from kids where Pluto is crying and saying, why can't I be a planet anymore? I don't get as much of that as I used to. I think the kids have mostly gotten over it.
GREENFIELDBOYCE: The adults - not so much. Brown spotted other icy worlds, too. He discovered a dwarf planet called Sedna in a region of space beyond Pluto that was thought to be a no man's land. For a decade, it seemed like a freakish loner. Then a few years ago, a couple of astronomers spotted another dwarf planet, a pink ice ball they nicknamed Biden after the vice president. And Brown says this team noticed something weird about the orbits of Sedna, Biden and some of the other most distant known objects.
BROWN: The weirdness that got people's attention is sufficiently obscure that it's nearly impossible to describe without resorting to big, three-dimensional diagrams.
GREENFIELDBOYCE: The orbits were clustered in something known as the argument of perihelion. Never heard of it? You aren't the only one.
BROWN: If you were to ask 20 people who study the outer solar system what it is, probably 18 of them would first go to Wikipedia to look it up, including, probably, me two years ago.
GREENFIELDBOYCE: The researchers said the clustering might be caused by the gravitational pull of a planet bigger than Earth. That got Brown's attention, so he walked down the hall to see a colleague at Caltech named Konstantin Batygin. The pair decided to do their own analysis and soon found other oddities in the orbits that could also be explained by a giant planet. Still, Brown tried to be skeptical.
BROWN: Belief is a dangerous thing. As a scientist, you try really hard not to believe your own theories too much.
GREENFIELDBOYCE: Here's what finally convinced him. 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 orbits like that, and he found some.
BROWN: My jaw hit the floor. That was - that just came out of the blue, so being able to make a prediction and having it come true in five minutes is about as fun as it gets in science.
GREENFIELDBOYCE: Today in The Astronomical Journal, the researchers lay out their evidence so that telescopes can go hunting for this giant planet. Brown's already looking.
BROWN: I want to see it. I want to know what it's like. I want to see that it's really there.
GREENFIELDBOYCE: Now, it may be hard to believe that an object 10 times more massive than Earth could be out there and no one has seen it yet. But astronomer Scott Sheppard says keep in mind this would be very, very, very far away.
SCOTT SHEPPARD: So objects get very faint very fast. We really don't know the distance of this object, and if it's at the further ends of where we think it might be, then it would be too faint to see.
GREENFIELDBOYCE: Sheppard works at the Carnegie Institution for Science. He's one of the researchers who discovered the strange cluster of orbits that first suggested the presence of a big, hidden planet.
SHEPPARD: When we announced our thing two years back, we thought either it'd be debunked really fast, or someone would take it further. And someone has now taken it further and shown that what we said is possibly real.
GREENFIELDBOYCE: Still, even he is skeptical.
SHEPPARD: We really need to find more of these objects - more of these smaller objects that can lead us to the bigger object. I think it's still a tossup if it's really out there or not. I think we just need more data.
GREENFIELDBOYCE: With luck, they should be able to nail it down in a few years. And if this ninth planet is out there, but too faint to be seen with existing technology, he says there is a telescope already under construction in Chile that should be able to spot it. Nell Greenfieldboyce, NPR News. Transcript provided by NPR, Copyright NPR.