The Hunt for Super Pluto Comes Up Empty—For Now

Back in 2011, a couple of astronomers presented circumstantial evidence of a giant, undiscovered planet they named Tyche, for the Greek goddess of good fortune, lurking at the outer edges of the Solar System. This faraway world would be at least as big as Jupiter, but hundreds of times more distant than Pluto, so it could easily have escaped the notice of nearly all of the world’s most powerful telescopes.

NASA’s orbiting Wide-field Infrared Survey Explorer (WISE) should have been able to spot it, though—and if Tyche really existed, the proof would already be stored somewhere in the telescope’s vast database of observations. If so, said WISE principal investigator and UCLA astrophysicist Ned Wright at the time, “we might be able to tell you something in a year or two.”

That year or two has now come and gone, and the answer is…better luck next time. An exhaustive look through the database, chronicled in two separate papers in the Astrophysical Journal, has turned up all sorts of unexpected objects, including thousands of previously unknown dim stars and hundreds of the bigger-than-a-planet-smaller-than-a-star objects known as brown dwarfs. But the WISE data also ruled out anything as big as Saturn out to 250 times Pluto’s distance, and anything as big as Jupiter to about one light-year away—a quarter of the distance to the nearest star.

That doesn’t mean it was crazy to guess there might be a big planet out there. The evidence offered by Daniel Whitmire and John Matese, of the University of Louisiana, was based on a suspiciously large number of long-period comets coming from a particular part of the sky. In theory, such comets originate in a vast comet reservoir known as the Oort Cloud, which forms a kind of spherical halo around the Solar System.

If that’s true, then comets from the cloud should fall in toward us from all directions in an equal distribution. If they were coming in from one direction more than others, Matese and Whitmire argued, then the gravity of some large object must be diverting them. Wright called the evidence “kind of flimsy” at the time, but didn’t dismiss it entirely.

Still, few people thought the odds of finding the mysterious body were terribly high, mostly because the kind of gravitational reasoning on which the hunt was based has produced more duds than discoveries. One example of the latter: astronomers in the 1800’s noticed that Uranus was moving in ways that didn’t make sense unless some other object was pulling on it. They calculated where this unknown planet should be—and there, it turned out, was Neptune. In the 1920’s, it was Neptune that seemed to be moving strangely. A young Clyde Tombaugh looked where calculations said something should be—and there was Pluto.

Except Pluto was far too small to affect Neptune’s orbit, and Neptune, it turned out on closer examination, hadn’t been orbiting strangely in the first place. The fact that Pluto was where “Planet X” was supposed to be turned out to be a concidence. In the 1980’s, Berkeley astrophysicist Richard Muller and several colleagues proposed the existence not of a planet, but a dim companion star to the Sun they called Nemesis. Their reasoning: catastrophic extinctions, like the one that killed the dinosaurs, seemed to come with a regular rhythm. Maybe Nemesis was knocking big comets out of their orbits whenever it came around, sending them to smash into Earth. But again, extensive searches turned up nothing (and WISE has just re-confirmed the non-existence of Nemesis).

Nonetheless, the new results from WISE have revealed plenty of other cosmic bodies, including a pair of brown dwarfs orbiting each other just six or so light-years from Earth and several dim stars not all that much further away. “We think there are even more stars out there left to find with WISE,” said Wright in a statement. “We don’t know our own Sun’s backyard as well as you might think.” That’s good news for planet-hunters, who now think dim red M-dwarf stars, which are far more plentiful than Sun-like stars in the Milky Way, are prime places to look for habitable worlds.

And there could be even better news still to come. WISE was put into hibernation in 2011, when its primary mission ended. But last fall, it was dusted off for a new assignment, to look for the sort of near-earth asteroids that could someday threaten our planet. In doing so, it will be covering the same enormous swath of space it already scanned twice during its first incarnation, adding six more all-sky surveys to the two it has in the can. Anything that changes position from one survey to the next—or from the 2010-2011 batch to the current round—is something in motion.

Most of what WISE will see moving will indeed be asteroids (as was the case did in its first two rounds), but it will also see comets, brown dwarfs, M-dwarfs—and it just might see Tyche after all. “We didn’t actually rule it out with 100 percent certainty. If there’s a Jupiter-mass object within a light-year,” says project scientist Peter Eisenhardt, “there’s still a very slim chance we could have missed it.” When it comes to space, never say never.