Pluto, once the ninth planet from the sun, is downgraded to “dwarf planet.” The solar system now has only eight planets, officially speaking.
You’d think that discovering a planet would grant you immortality, but astronomer Clyde Tombaugh may have been de-immortalized when Pluto was deplanetized. He discovered Pluto in 1930 by identifying a moving object against a background of stationary stars in sky photographs taken over several nights. The planet was officially named Pluto, after the Roman god of the underworld, on May 1, 1930.
Astronomers had trouble accepting Pluto as a full-fledged planet. Size (smaller than Earth’s moon) is one issue, and its orbit around the sun doesn’t match those of the eight “classical” planets. Astronomers also began discovering similar bodies near Pluto, diluting its cachet even more.
So, in 2006 the International Astronomical Union redefined planet thus:
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 Neptune’s orbit, it has not cleared the neighborhood, making it (along with Ceres, Eris, Haumea, and Makemake) a dwarf planet—which the IAU defined as (a) and (b) above, but “(c) has not cleared the neighborhood around its orbit, and (d) is not a satellite.” (Our solar system has seven moons bigger than Pluto: Io, Europa, Ganymede, Callisto, Titan, Triton, and Earth’s own moon.)
A lot of people reacted to the demotion like our solar system was losing a favorite kid brother. Pluto proponents still rail about its relegation to an inferior category.—DC, TL