For centuries, scientists have searched in vain for an elusive 10th planet that some believe is orbiting the sun and shaking up the orbits of two sister planets. They had counted on the two Pioneer spacecraft, now drifting through the outer regions of the solar system, to finally solve the mystery.
Last week, John Anderson of the Jet Propulsion Laboratory held a news conference to reveal that during a three-year study, the two spacecraft have failed to detect any such planet. Loath to give up the ghost of "Planet X," however, Anderson said this lack of evidence may not mean the planet isn't there.
Another explanation, he said, is a planet five times as massive as Earth traveling in an orbit at nearly right angles to the orbits of the known planets.
The orbit of "Planet X" would be 10 billion to 20 billion miles across and so elongated that it would near the sun and other planets only every 700 to 1,000 years, Anderson said. He is the principal scientific investigator in celestial mechanics for Pioneers 10 and 11, launched by the National Aeronautics and Space Administration in 1972 and 1973 to explore the solar system. The spacecraft, now 2 billion to 4 billion miles from the sun, are managed by NASA's Ames Research Center in California, where the news conference was held.
Between 1810 and 1910, scientists accumulated strong evidence that some unseen body was affecting the orbits of Neptune and Uranus, Anderson noted. He recently found to his surprise, however, that those effects seemed to have disappeared in the last 75 years.
Anderson acknowledged that, as other astronomers suggest, the most obvious hypothesis is that the old information was flawed and there never was a 10th planet. "It was the first thing that occurred to me as well," Anderson said. However, Kenneth Seidelmann of the U.S. Naval Observatory has studied the older data extensively and could find no errors, he said.
If the disturbances in the Uranus and Neptune orbits are believed to be real, Anderson said, then this strange planet is the best explanation for the data. "I certainly can't say it is the answer. . . but if you go beyond Planet X, the explanations get even more bizarre."
Leroy Doggett of the U.S. Naval Observatory confirmed that the team there "takes the old data seriously . . . . We've found no convenient error or other reason to throw it out."
But it leaves them somewhat baffled, he said. "If we compare our predictions, or mathematical models, for the orbits of Uranus and Neptune with all the observations, they just don't fit," he said. "We're left with something we don't understand here."