A long quest has ended: there is no tenth planet. But there may be a hundred or a thousand mini-planets in the dark beyond Pluto.
For more than a century, a few dedicated astronomers have braved the skepticism of their colleagues to champion an alluring idea: the idea that the known planets are not enough. In the dark and frigid boondocks of the outer solar system, these researchers have argued, there lurks a large, undiscovered planet, Planet X, whose gravitational pull is needed to explain why Uranus and Neptune have sometimes seemed to deviate slightly from the orbits astronomers (and Newton’s laws) have prescribed for them. When Pluto was discovered in 1930, some astronomers thought Planet X had been found, but Pluto turned out to be far too puny to pull Uranus or Neptune out of line. The search continued, long and fruitless.
Now at last it may be coming to an end: Planet X, says astronomer Myles Standish of the Jet Propulsion Laboratory, is dead. Standish recently recomputed the orbits of Uranus and Neptune with unprecedented accuracy. He also reexamined the historical data that had supposedly shown small discrepancies between the observed positions of the two planets and their calculated orbits. Under this close scrutiny, Standish found, the discrepancies all but vanished--and with them the evidence for Planet X.
In the case of Uranus, it was the orbital computations that turned out to be wrong. The source of the error was the mass of Uranus’s neighbor, Neptune. After Voyager 2 flew by Neptune in 1989, the planet was found to be about .5 percent less massive than had previously been thought. That meant Neptune’s tug on Uranus was slightly weaker. Standish modified Uranus’s orbit accordingly. He found that the historical observations of Uranus’s positions fell right on the modified orbit--with no discrepancy, and no need for Planet X’s pull.
In the case of Neptune’s orbit, though, the problem lay primarily with the historical observations. Standish found that a decade-long series of observations made at the U.S. Naval Observatory starting in 1895-- observations that were central to the case for Planet X--were plagued by procedural errors and were inconsistent with Neptune positions recorded at other observatories. Some of the other supposed discrepancies in Neptune’s orbit were based on observations made even before it was identified as a planet in 1846. Galileo, for instance, observed Neptune with his primitive telescope in 1612 and 1613 (he mistook it for a star) and recorded its location in relation to Jupiter. But those records, says Standish, are ambiguous; they allow several possible interpretations of Neptune’s position. They’re certainly not enough to hang a Planet X on.
Standish’s work does not prove that no other planets exist in the solar system, only that no planet fitting Planet X’s description--a large planet with a mass between Earth’s and Neptune’s--exists close enough to Uranus and Neptune to distort their orbits. Says Standish: There isn’t much hope for a major planet out there unless it’s way out there. And that’s not terribly likely.
Minor planets, though, are another story. Some astronomers, notably Alan Stern of the Southwest Research Institute, have predicted that a multitude of mini-planets will be discovered on the outskirts of the solar system. In fact, they’re apparently being discovered already. In the past year David Jewitt, of the University of Hawaii, and Jane Luu, of the University of California at Berkeley, have found the first two bodies orbiting the sun beyond Pluto: chunks of dirty ice 150 miles or so across-- a tenth the size of Pluto itself--that probably belong to the belt of comets first postulated by astronomer Gerard Kuiper in 1951.
So far Jewitt and Luu have searched an area of sky only four times the size of the full moon for Kuiper belt objects. If we’ve found two in such a small area, there must be a lot of them, says Luu. There are also likely to be even bigger minor planets in the Kuiper belt--in which case, far from adding a Planet X, astronomers may one day have to consider demoting Pluto out of the ranks of the major planets. There is a revolution going on in planetary science, says Stern. What we have come to recognize is that instead of a tenth planet, there’s probably a collection of hundreds or maybe a thousand small planetary bodies in the outer solar system.