Still a Mystery

      

      If Pluto is not causing the perturbations in 
      the orbits of Uranus and Neptune, then what is? 
      
 For ten years after he discovered Pluto, Clyde Tombaugh continued his 
        search, which included observing nearly 45 million stars. If there was 
        another planet out there, even much dimmer than Pluto, Tombaugh should 
        have seen it. Still, planets have been observed before and have gone unrecognized 
        for what they were.
      
 Some astronomers believe that the deviations in Uranus and Neptune are 
        really observational errors. Before about 1910, sightings of the planets 
        were not precise enough -- by today's standards -- to predict planetary 
        motion. Discarding early data leaves only minute discrepancies in the 
        orbits, which can still be explained away as observational errors. Based 
        on this, there is little need to postulate a tenth planet as the cause 
        of these orbital discrepancies. 
      
 Other astronomers feel differently. One theory has the outer solar system 
        populated by hundreds of rock-ice bodies about 1000 to 2000 kilometers 
        in diameter, orbiting at 40 astronomical units from the Sun. If more of 
        these are found we might learn that, rather than having nine planets, 
        the solar system has hundreds. Pluto would finally be in good company. 
        By sheer number, these small bodies would be considered the "normal" planets, 
        and the other eight planets would be the oddities.
      
 
      

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