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The Changing Status of Pluto
| Article
# : |
15889 |
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Section : |
NATURAL SCIENCE
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| Issue
Date : |
1 / 1989 |
3,477 Words |
| Author
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Steve Mueller and William B. McKinnon
Steve Mueller is a research associate in planetary physics in
the Department of Geological Sciences at Southern Methodist
University in Dallas. William B. McKinnon is an associate
professor in the Department of Earth and Planetary Sciences
and the McDonnell Center for the Space Sciences at Washington
University in St. Louis. |
For such a faint and faraway world, Pluto has generated a surprising amount of controversy since its discovery by Clyde Tombaugh nearly sixty years ago. While most astronomers readily accepted the planetary status of the newly discovered object, others were reluctant. Pluto was simply not like the other planets known to exist in the outer solar system. Although its exact size would remain unknown for some time, it was clearly much smaller than either Uranus or Neptune, the smallest of the outermost planets. The orbit of Pluto was also peculiar in that it was both highly elliptical and inclined at a significant angle to most other planetary orbits. To some astronomers this seemed more reminiscent of a cometary orbit, and it was suggested that Pluto was nothing more than a large, inactive comet.
Other astronomers attributed particular significance to the fact that, as it makes its closest approach to the Sun, Pluto actually moves within the orbit of Neptune. They proposed that these two objects regularly cross paths because they share a common origin: namely, that Pluto is a lost Neptunian moon. This theory had the additional appeal of suggesting a possible solution to another unexplained curiosity of the outer solar system. Triton, the largest moon of Neptune, orbits in a direction nearly opposite to the rotation of the planet. This retrograde motion, it was argued, could have resulted from a close encounter between Pluto and Triton, both originally adjacent satellites of Neptune. Such an encounter might have ejected Pluto into a solar orbit and reversed the orbital motion of Triton.
Still, most astronomers felt that Pluto was indeed a true planet, despite the fact that virtually every new mass or size estimate involved downward revisions. Pluto, at least in the minds of astronomers, was becoming smaller and smaller. Each successive revision seemed to pose a greater threat to the planetary status of Pluto. In 1980, at an astronomical meeting commemorating the 50th anniversary of its discovery, it was proposed that Pluto be demoted to asteroidal status.
Is this controversial object, commonly referred to as the "planet" Pluto, truly a planet? Or is it merely an asteroid, an inactive comet, or perhaps even an errant moon? Recent improvements in astronomical technology, combined with the fact that Pluto is currently making its closest approach to the inner solar system in nearly 250 years, have resulted in our best look yet at this enigmatic little world. Pluto, it appears, may be in a class all its own.
A Closer
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