Mystery Planet: Is a Rogue Giant Orbiting Our Sun?

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Tim Kiusalaas / Corbis

The quest for Planet X always starts out with celestial objects behaving badly. Astronomers notice that a known planet, or a bunch of comets, begin moving in ways Newton's laws of motion can't explain. They propose that it's caused by the gravity of something massive and still undiscovered lurking out in the Solar System, and they head to their telescopes to search for it.

Most often it's all a big mistake; the unexplained motion turns out to be just an incorrect measurement. (The great exception concerned Neptune, spotted in 1846 after observers noticed Uranus wandering from its predicted path). So when a pair of University of Louisiana astronomers, writing in the journal Icarus, advanced their recent theory about a new mystery planet in our solar system — and a very, very big one — their colleagues mostly just listened politely, then went back to work.

They reckoned without the Web, though. A few days ago, the British Independent ran an article about the possible planet, and suddenly the idea went viral. The likely reason the story caught fire: a key sentence that read "But scientists now believe the proof of its existence has already been gathered by a NASA space telescope, WISE, and is just waiting to be analysed."

That's just close enough to the truth to be dangerous, something that John Matese, co-author of the Icarus paper, admits — sort of. "What we're really saying," he explains, "is that there's suggestive evidence there might be something out there." And if a new planet exists — something Matese is emphatically not claiming at this point — then the Wide-field Infrared Survey Explorer (WISE) satellite should already have an image of it stored somewhere in its enormous database.

How suggestive the evidence actually is, though, depends on whom you ask. If you ask Ned Wright, a UCLA astrophysicist and WISE principal investigator, he'll tell you, "It's really kind of flimsy. It's there, but they don't have super data."

The argument Matese and his colleague Dan Whitmire have been making since the late 1990s is that some comets seem to be moving in toward the Sun from a skewed direction. They start out in the Oort Cloud, a vast collection of perhaps trillions of small, icy chunks that hover at the very outer edges of the Solar System. Every so often, a passing star or the tidal effect of the Milky Way itself jostles the cloud, sending some of the chunks sunward to light up the night sky as comets.

When Matese and Whitmire analyzed the orbits of these Oort Cloud comets, about 20% of them seemed to come not from the random directions you'd expect, but from a narrower section of sky. This might suggest a giant planet, at least the size of Jupiter and maybe up to four times as big. Its size would not be its only remarkable feature; it's remote orbit would be another — a tidy trillion miles from the Sun, or more than a thousand times more distant than Pluto. "This is not a crazy idea" says Bad Astronomy blogger Phil Plait. And indeed, WISE project scientist Davy Kirkpatrick went so far as to propose a name for the possible new world: Tyche, for the Greek goddess of good fortune.

That might have been a subtle dig, though. In Greek mythology, Tyche was usually invoked along with Nemesis, the goddess of bad luck. Nemesis was also the name given to a mystery object that was in vogue in the 1980s, shortly after it was generally agreed that a comet might have killed the dinosaurs. A few astronomers thought they could see even more of a dino-cosmos link — a pattern of mass extinctions occurring like clockwork in the geologic record. Their explanation: a faint, far-off companion star to the Sun was sending down a rain of comets when it reached just the right point in its orbit. That sounds an awful lot like the current thinking about Tyche's possible influence — and thus the possible dig.

After a brief flurry of interest, most astronomers decided the evidence of Nemesis was pretty flimsy, and the idea went away. So did the long-ago speculation about another huge putative planet, one that was said to be messing with Neptune's orbit; astronomers who went looking for that version of Planet X in the 1920s could only scare up puny Pluto — whose influence on schoolchildren is huge, but which doesn't affect Neptune a whit (the original evidence for Neptune's orbital anomalies turned out to be wrong). And Pluto itself has recently lost its planetary distinction and been busted down to dwarf planet. Then too there was the mystery planet said to be orbiting Barnard's Star, detected via wobbles in the star's motion by Swarthmore astronomer Peter Van de Kamp in the 1960s.. Turns out that the wobbles were caused by the removal, cleaning and replacement of his telescope's mirror. No planet there either.

So while the latest version of Planet X could certainly exist in theory, it's way too early to start rewriting the textbooks. The evidence isn't even strong enough to have triggered an active search for Tyche; it's only because WISE happens to be surveying the heavens anyway that it could be found at all. If Tyche really is out there, says Wright, "we might be able to tell you something in a year or two."