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.