August 6, 2003


The smallest and usually the most distant planet in our solar system, Pluto is unique as it is neither a terrestrial planet nor a gas giant. Another surprise is its orbit which, rather than being circular and very near the ecliptic plane like the other planets, is highly elliptical and tilted.
Smaller than our Moon, Pluto is a solid body probably composed of rocks and ice. Its surface is at least partly covered with frozen methane and nitrogen and surrounded by a thin, nitrogen-rich atmosphere that evaporates and thickens as the planet nears the Sun. Atmospheric pressure at the surface is 100,000 times less than on Earth.
Pluto is often likened to asteroids and comets, and even to double planet systems since it is just twice the size and mass of its satellite Charon. Most significant of all is the recent discovery of other, similar bodies in the same region.
These remnants of the solar system’s formation, named Varuna, Ixion and Quaoar, are the largest objects in the Kuiper Belt and Pluto is much more like them than a planet.
Etymology: Son of Saturn and brother of Jupiter and Neptune, Pluto was king of the underworld and god of the dead in Roman mythology.

Discovered: 18 February 1930, by US astronomer Clyde Tombaugh.

Distinctive feature: Pluto is the 1st 'dwarf planet'. Its composition and orbit are unlike any of the 8 planets. It orbit is so elliptical and tilted that it periodically crosses that of Neptune.

Typical one-way journey time to Pluto using current technologies: more than 12 years.

Last updated: May 2003

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