An Etymological Dictionary of Astronomy and Astrophysics
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A → dwarf planet in the → solar system which until 2006 was known as the 9th major planet. Pluto revolves around the → Sun in a highly elliptical orbit at a mean distance of 39.5 → astronomical units once every about 248 years. The orbit → eccentricity is 0.25 (compare with the Earth's 0.02) yielding a → perihelion distance of 29.66 → astronomical units and an → aphelion distance of 48.87 AU. Its → orbital inclination is 17 degrees, which is much higher than those of the other planets. Pluto's mass is 1.308 × 10²² kg, that is 0.00218 Earth mass (0.177 Moon mass), its equatorial radius ib 0.19 Earth radius, and its → rotation period is equal to 6.39 Earth days. It has five known → satellites, in order of distance from Pluto: → Charon, → Styx, → Nix, → Kerberos, and → Hydra. Pluto's radius is estimated to be about 1150 km (0.18 Earths). Pluto is smaller than seven of the solar system's satellites (the → Moon, → Io (Jupiter I) , → Europa, → Ganymede, → Callisto, → Titan, and → Triton). Pluto's surface has an estimated temperature of 37.5 K and is composed of more than 98% → nitrogen  → ice, with traces of → methane and → carbon monoxide.

In Roman mythology, Pluto is the god of the underworld and Judge of the dead, from L. Pluto, Pluton, from Gk. Plouton "god of wealth," literally "wealth, riches." Pluto was the son of Saturn. The alternative Gk. name is Hades.

A → radioactive → chemical element, symbol Pu. → Atomic number 94; → mass number of most stable isotope 244; → melting point 640 °C; → boiling point 3,235 °C. It was first synthesized in 1940 by American chemists Glenn T. Seaborg, Edwin M. McMillan, Joseph W. Kennedy and Arthur C. Wahl in the → nuclear reaction: ₉₂U²³⁸ + ₀n¹→ ₉₃Np²³⁹ + β^- (23.5 minutes) → ₉₄Pu²³⁹ + β^- (2.36 days). The → half-life of ₉₄Pu²³⁹ is 2.44 × 10⁴ yr. Plutonium-239 is a → fissile isotope.

The name derives from the planet → Pluto. It was selected because it is the next planet in the solar system beyond the planet → Neptune and the element plutonium is the next element in the → periodic table beyond → neptunium.