The Flap Over Pluto
Sources: NASA and the International Astronomical Union.
News Flash (August 24, 2006)—
Misleading reports that Pluto was about to lose its status as a planet in January 1999 caused an unexpected public uproar and a subsequent stir over the planet's standing in the astronomical community. The flap began when some of the national media announced that Pluto was going to be reclassified as a minor planet or, even worse, a lowly asteroid. Although the proposal turned out to be false, the publicity generated by the media hype left many people confused about Pluto's official classification.
Since 1992, a substantial number of smaller objects have been discovered in the outer solar system, beyond the orbit of Neptune, with orbits and possibly other properties similar to those of Pluto. These characteristically “Pluto-like” bodies are known as Trans-Neptunian Objects (or TNOs).
The controversy over Pluto's designation began when an astronomer suggested that Pluto be assigned a number in a technical catalog or list of such TNOs so that observations and computations concerning these objects could be conveniently collated. This proposal was not in any way intended to change Pluto's standing as a planet. Unfortunately, the purpose of including it in a specialized listing became misinterpreted and was erroneously reported.
The hullabaloo that followed over Pluto's supposed demotion was soon denounced by the Paris-based International Astronomical Union (IAU), the organization that decides the classification of objects in the solar system, which promptly issued a statement confirming that Pluto will remain our ninth planet and that there was no such initiative to reclassify it as some lesser type of heavenly body.
Surprisingly, there is no set scientific law as to what constitutes a planet, but as a rule of thumb, a planet: 1) must directly orbit a star; 2) must be small enough that it has not undergone internal nuclear fusion (i.e., it is not a star or starlike object); and 3) must be large enough that its self-gravity gives it the general shape of a sphere.
Pluto, the smallest planet in the solar system (it's smaller than Earth's moon), has remained enigmatic since its discovery by astronomer Clyde Tombaugh in 1931. It is so unique in comparison to its eight siblings that it almost defies classification. Though it orbits the Sun, Pluto neither qualifies as a terrestrial nor as a gas giant planet. All the other planets in the outer solar system are gaseous giants, whereas Pluto is a small, solid object. Although it behaves like a comet by periodically warming and losing its atmosphere into space, Pluto is far too large for that category. Astronomers speculate that Pluto may be the last survivor of a lost population of objects called “ice dwarfs” that inhabited the primeval solar system.
Pluto's satellite, Charon, discovered in 1978, is larger in proportion to its planet than any other satellite in the solar system. Pluto and its moon are also considered a “double planet system,” which occurs when two bodies are reasonably close in mass and so orbit around a common center of gravity (or barycenter), analogous to two children balancing on a teeter-totter. It is thought that Charon may have been born through a head-on collision between Pluto and another large ice body, in much the same way as the Earth–Moon system is believed to have formed.
Pluto is the only planet whose orbit crosses that of another planet (Neptune, normally the eighth planet). Pint-sized Pluto's elliptical orbit takes 248 years, and carries it as close as 2.8 billion mi from the Sun and as far as 4.6 billion mi from the Sun. In September 1979, Pluto crossed within Neptune's orbit again, making Neptune the farthest planet. Pluto remained closer to the Sun than Neptune for most of the 1980s, reaching its closest point (perihelion) to the Sun by late 1989. According to NASA, the last time Pluto was this close to the Sun, George Washington was a boy! In February 1999, Pluto crossed Neptune's orbit again as it headed away from the Sun and regained its status as the most distant planet. NASA's New Horizons Pluto-Kuiper Belt mission was launched in Jan. 2006 to study Pluto and Charon as well as the Kuiper Belt beyond Neptune's orbit.