AUDIE CORNISH, HOST:
Now a step back - a look at the universe. At some point, most American kids have to memorize the planets of the solar system. Want to give it a go, Robert?
ROBERT SIEGEL, HOST:
OK. Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
CORNISH: Yes, and nice work leaving Pluto out. It's now off the list, considered a dwarf planet.
SIEGEL: But in the future there may be a whole new yet-to-be-named planet in its place. Scientists suspect the outer reaches of our solar system may have an undiscovered planet the size of Earth or even larger.
CORNISH: NPR's Nell Greenfieldboyce has the latest on the search for Planet X.
NELL GREENFIELDBOYCE, BYLINE: You may have thought that by now astronomers would surely have found all the major planets in our solar system. Well, think again.
KEVIN LUHMAN: There is a huge volume of space in the outer solar system. We know almost nothing about what might be out there.
GREENFIELDBOYCE: Kevin Luhman is an astronomer at Penn State University. He says for over a century, scientists have observed various things that they thought could be explained by the presence of an unknown planet - Planet X.
LUHMAN: Planet X is kind of a catchall name given to any speculation about an unseen companion orbiting the sun.
GREENFIELDBOYCE: Some conspiracy-minded folks even think that Planet X has already been discovered.
LUHMAN: There are a lot of these people on the Internet who think that, for instance, NASA knows about an unseen planet, but it's on a collision course with Earth and it's going to destroy us, but they don't tell us about it.
GREENFIELDBOYCE: Nobody's told him anyway. So to search for Planet X, he recently scanned the sky using a NASA space telescope that detects infrared light. It would have found anything the size of Jupiter or Saturn because gas giants like these are big enough and warm enough that they produce a lot of infrared light. He didn't see any planet like that - not that everyone believes him.
LUHMAN: I got a few emails after that, just from random people in the public. I shouldn't be admitting that on a recording.
GREENFIELDBOYCE: Still, there may be smaller, cooler planets out there. Astronomer Scott Sheppard says for a long time, there was just no way to go look.
SCOTT SHEPPARD: Up until a year or two ago, we just didn't have the technology to do this because we didn't have large cameras and large telescopes.
GREENFIELDBOYCE: He says any planet out there would be very faint because light would have to travel billions of miles from the sun to the planet, bounce off and then travel all the way back to our telescopes.
SHEPPARD: And because of that, it's actually - if you put an object twice as far away, it becomes 16 times fainter, so things get very faint very fast.
GREENFIELDBOYCE: Sheppard and his colleagues have been searching for very faint objects using a massive camera on a powerful telescope in Chile. Last year, they announced that they had found one - a dwarf planet, a little, pink ice ball that's far beyond Pluto. There's a framed photo of it hanging on the wall of Sheppard's office at the Carnegie Institution for Science in Washington, D.C. And that wall could soon have other photos.
SHEPPARD: We believe there's probably a lot of objects bigger than Pluto still out there. And there could easily be objects as big as Mars or even Earth, out beyond in the very far distant solar system.
GREENFIELDBOYCE: He's already found hints of something big. When he looks at the orbits of his dwarf planet and some other small, icy bodies, he sees a pattern.
SHEPPARD: And you wouldn't expect that. You expect the orbits to be completely random.
GREENFIELDBOYCE: One possible explanation is that they are all being influenced by the force of a large, unknown planet - or maybe more than one. Scientists will have a good shot at finding out once they have something called the Large Synoptic Survey Telescope. It will scan huge swaths of sky for faint objects. The building site for it is already being prepared on top of a mountain in Chile, and construction will begin in earnest this year.
Scott Kenyon is an astrophysicist at the Smithsonian Astrophysical Observatory in Cambridge, Mass.
SCOTT KENYON: With the next generation of telescopes, or if we're lucky with the current generation of telescopes, it will be possible to detect the light from this planet. If we can see it and pinpoint its position, then everybody will get excited.
GREENFIELDBOYCE: He says finding a new planet would be like meeting a new neighbor.
KENYON: You like to know people on your street or in your apartment building. I think that part of this search for these planets in the outer part of the solar system is trying to find out our neighborhood. I think to find out more about our neighborhood is just really a cool thing.
GREENFIELDBOYCE: Nell Greenfieldboyce, NPR News.
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