ARI SHAPIRO, HOST:
Tonight on top of Mauna Kea in Hawaii, astronomers plan to point the giant Subaru telescope at a patch of sky near the constellation of Orion. They'll be searching for an undiscovered planet orbiting our sun - a planet that should be there, according to theories and models. If they find it, it will be the ninth planet in our solar system. Pluto used to be number nine, but it was demoted a decade ago. NPR's science correspondent Joe Palca has been following the hunt for this possible new planet as part of his project Joe's Big Idea, and he has this report on how the search is going.
JOE PALCA, BYLINE: At the Carnegie Institution for Science in Washington, D.C., you can frequently find the astronomer Scott Sheppard in his office staring at a large computer screen, displaying a recently taken picture of a patch of the night sky.
SCOTT SHEPPARD: Things that are circular are stars. Things that are more oval-shaped are galaxies.
PALCA: Sheppard isn't interested in the stars and galaxies on the screen. They're very far away and basically don't move with respect to Earth.
SHEPPARD: And we're looking for things that move, so we're looking for anything that's not a star or a galaxy, basically.
PALCA: Those things that move are most likely objects orbiting our sun - dim objects in the far reaches of the solar system, way past the orbit of Neptune. Sheppard's been using telescopes in Chile and Hawaii to search for these this distant objects, and he's found several of them.
SHEPPARD: All the objects we find are actually right on the edge of detection. They're very faint.
PALCA: But so far, no new planet. In 2014, Sheppard and a colleague published a paper suggesting a handful of these faint distant objects were behaving strangely, as if something even more distant with a strong pull of gravity was tugging on them. They suggested the behavior might be explained by an undiscovered planet. That speculation got a boost from a computer model made by two other astronomers at Caltech. The model made news headlines by confirming at least theoretically that the planet was really there. The model also predicted the planet would be 10 times more massive than the Earth, and it could even explain why the sun is slightly tilted with respect to the rest of the solar system. But David Jewitt thinks it's wrong to get all hot and bothered about a new planet at this point. Jewitt is an astronomer at the University of California, Los Angeles. He says the model predicting the new planet is interesting work.
DAVID JEWITT: But, you know, without meaning to sound too critical, it's relatively easy to explain almost anything that is observed in one way or another using a model.
PALCA: And he thinks the media attention to the model is overblown.
JEWITT: There's this natural tendency to jump on reports of, hey, we've got a planet. We might have a planet. That's something that people really want to think about a lot. It's a very cool thing to think about.
PALCA: Jewitt says theories and models might get the media fired up, and they do have value, but they only take you so far.
JEWITT: The answer as to whether there's another planet won't come from models, won't come from theories, can only come from observations.
PALCA: And so several astronomers, including Scott Sheppard, are out there observing.
SHEPPARD: To me, this is science at its finest. It's, like, this thing where science is leading us to say that something's definitely going on out there. Something weird is happening, and the best explanation for it is that it's a planet.
PALCA: Sheppard definitely hopes he'll be the one to find it. Joe Palca, NPR News.
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