By Tony Long and Doug Cornelius
2006: Pluto, once the ninth planet from the sun, is downgraded to a mere “dwarf planet.” Our solar system loses a favorite kid brother and now has, officially, only eight planets.
Pluto was discovered by American astronomer Clyde Tombaugh on Feb. 18, 1930. Comparing photographs taken of the same section of sky over several nights in January, Tombaugh identified a moving object against a background of stationary stars.
Based on the predictions of two colleagues at the Lowell Observatory, he’d been searching for a planet outside the orbit of Neptune at the time. The photographs convinced him he had found it.
Tombaugh announced his discovery on March 13. The planet was officially named Pluto, after the Roman god of the underworld, on May 1, 1930.
But astronomers had trouble accepting Pluto as a full-fledged planet almost since its discovery. Size was always an issue — it’s smaller than Earth’s moon — and its orbit around the sun doesn’t conform to the path followed by the eight “classical” planets. Astronomers also began discovering similarly sized bodies in the vicinity of Pluto, diluting its cachet even more.
Finally, in 2006, the International Astronomical Union officially downgraded Pluto’s status to “dwarf planet.”
The IAU decided on this definition of a planet:
A celestial body that (a) is in orbit around the sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit.
Because Pluto crosses the orbital path of Neptune, it has not cleared the neighborhood of its orbit, throwing it (and similarly Ceres, Eris, Haumea and Makemake) into the category of dwarf planet.
The International Astronomical Union decided on this definition of a dwarf planet:
A celestial body that (a) is in orbit around the sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighborhood around its orbit, and (d) is not a satellite.
(It is important to keep the moons out of the definition because there are seven moons in our solar system bigger than Pluto: Io, Europa, Ganymede, Callisto, Titan, Triton and Earth’s own moon.)
So, Pluto is now merely a dwarf planet in the Kuiper belt — like it or not.
Image: Computer maps of Pluto made from 384 separate Hubble Space Telescope images/NASA, ESA, M. Buie (Southwest Research Institute)
Portions of this article appeared on Wired.com Feb. 18, 2007, and March 13, 2010.
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