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Hunting Planet X: a nothing that counts.

Hunting Planet X: A nothing that counts

The search for "Planet X,' a possible10th planet, continues to produce nothing. But now one scientist has concluded that the latest round of nothing may in fact be a clue to the whereabouts of such a Planet X, whose gravitational influence some researchers believe is evident in measurements of the motions of Uranus and Neptune since at least the early 19th century.

The crux of the latest nondiscovery isthe failure to detect, after half a decade of radio-tracking, any changes in the paths of the distant Pioneer 10 and 11 spacecraft not attributable to the pull of the known planets. The two probes, controlled from NASA's Ames Research Center at Moffett Field, Calif., are now heading out of the solar system in nearly opposite directions, following their visits to Jupiter and Saturn in the 1970s.

Neptune was discovered in 1846 afterthe presence of a massive object was inferred from ananalysis of irregularities in the orbital motion of Uranus, the next planet in toward the sun. Pluto, the ninth and most recently discovered planet, was found in 1930 when Neptune (and Uranus as well) kept showing up slightly out of its expected position. But Pluto turned out to be a case of mere serendipity, since its mass was later shown to be far too low to account for the perturbations of the other planetary orbits that had prompted the successful search for it. Thus the possibility of a still-undiscovered Planet 10, or X, has been a tantalizing one ever since.

John Anderson, "celestial mechanics'experimenter for the two Pioneers, admits that he had originally expected the object responsible for deflecting the paths of Uranus and Neptune to reveal its location by its effects on the spacecraft trajectories. "Well,' says Anderson, who works at Jet Propulsion Laboratory in Pasadena, Calif., "we've ruled that out.'

Yet those early observations byground-based astronomers have strongly persuaded some researchers at the U.S. Naval Observatory (USNO) in Washington, D.C., that something is out there. The need, then, is to reconcile past evidence that says "yes' with findings from two spacecraft that say "no.' The problem, says Anderson, is that the present model of the solar system is not complete enough to predict the motions of Uranus and Neptune to the level of accuracy with which they can in fact be measured.

The USNO group's conclusion is thatPlanet X may be in an orbit tilted 30| or more from the paths of the known planets, so that sometimes it is too far away to affect them. Anderson adds that the suggested orbit also needs to be extremely elliptical, since if it were round, and large enough for Planet X not to be measurably affecting the Pioneers in the present, it would probably not have affected Uranus or Neptune in the past, either.

In its proposed path, Planet X wouldtake about 700 to 1,000 years to go around the sun once, and would have about five times the mass of the earth. (With more than about 10 "earth masses,' its effects would probably be far more conspicuous than they are, elliptical orbit notwithstanding.) Another possibility, though Anderson finds it unlikely, is that it was a one-time visitor, not circling the sun at all.
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Author:Eberhart, Jonathan
Publication:Science News
Date:Jul 11, 1987
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