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How Did the Planet Pluto Get Its Name?

For a time, the distant, tiny world of Pluto was known as the solar system's ninth planet. That all changed in 2005, when the International Astronomical Union reclassified it as a dwarf planet. Discovered in 1930, Pluto got its name from an unusual source: It is the only world named by an 11-year-old girl. In honor of its important place in history, astronomers refer to all dwarf planets that orbit beyond Neptune as "Plutoids."

The Naming of Pluto
- Percival Lowell, an American astronomer, predicted the existence and location of Pluto in 1905. He based this prediction on irregularities in the orbits of Neptune and Uranus, which he attributed to the gravitational effects of an unknown planet. Unfortunately, he died before finding it. In 1930, Clyde Tombaugh discovered the tiny planet from the Lowell Observatory in Flagstaff, Arizona. Venetia Burney, an 11-year-old English girl, suggested the name Pluto to her grandfather, in honor of the Roman god of the underworld. Her grandfather forwarded the name to the observatory, and Tombaugh eventually chose it. The name also honors Percival Lowell, in that the first two letters represent his initials.
Pluto's Characteristics
- Pluto's diameter is less than a fifth of Earth's, and roughly two-thirds of the moon's. Scientists believe it has a rocky core surrounded by water ice. It may also have a light covering of methane and nitrogen frost. Pluto has a highly eccentric orbit, meaning that its distance from the sun varies greatly. At its closest approach to the sun, Pluto is actually within the orbit of Neptune. At its furthest point, the dwarf planet is more than 49 times farther from the sun than Earth is. The length of Pluto's orbit is 248 Earth-years; its day is equal to 6.4 Earth-days.
Pluto's Moons
- In 1978, astronomers discovered that Pluto had a moon. They named it Charon, after the demon who ferries souls to the underworld in Greek mythology. Charon is nearly half the size of Pluto, which has led many astronomers to consider them a binary system, or double dwarf-planet system. In 2005, the Hubble Space Telescope helped astronomers identify two additional moons, now called Nix and Hydra.
Dwarf Planet Status
- Pluto's status as a planet has always been subject to debate. In 2005, astronomers discovered Eris, another distant object that is about the same size as Pluto. The question of whether Eris was a tenth planet reignited the debate over Pluto. In 2006, the International Astronomical Union devised the "dwarf planet" classification for objects that are more developed than asteroids, but not quite large enough to be planets. The primary difference is that a dwarf planet does not dominate its "neighborhood," meaning it lacks the gravitational strength to sweep its orbit clean of other objects.

Astronomy