Space Facts

Pluto the Planet that Wasn't

Is Pluto a Planet?

The clearest view yet of ex-planet Pluto and its moon, Charon, as revealed by NASA's Hubble Space Telescope. Taken February 21, 1994, at 4.4 billion kilometers (2.6 billion miles).

Pluto is planet number nine of the Solar system. Oops! Sorry, Pluto, you've been demoted to "dwarf planet." The word of the International Astronomical Union (IAU) may not be final, but for now that's the way it stands. Oh, well.

Named for the Roman god of the underworld, the name seems somehow fitting. Pluto, in its dark, lonely orbit so far from the sun, seems as uncomfortable and distant as death might. In the mythology of Pluto and Proserpina (Pluto's queen), the god of the underworld fell in love with her, kidnapped her, but later struck a bargain allowing her to return to the surface for six months of the year. Each year, when she went underground, winter descended on the mortal world, above. So, the connection to cold is there. The planet's name found favor also because the first two letters matched the initials of the discovering observatory's founder, Percival Lowell.

Pluto was discovered in 1930 by Clyde Tombaugh as part of the observatory's program to find the elusive "Planet X." Yet, Pluto (even with Charon) is too light-weight to account for the discrepancies astronomers had found in the orbit of Uranus. Better estimates of the mass of Neptune have erased the apparent discrepancy and with it the need for a Planet X. Finding Pluto was merely a happy coincidence.

The planet (dwarf) Pluto is about 2,390 kilometers (1,485 miles) in diameter. The diameter of its companion, Charon, is 1,186 kilometers (737 miles). Surface gravity on Pluto is only about 6% that of Earth. That means, if you weigh 100 units (pounds, kilograms, or whatever) on Earth, you would weigh 6 of the same units on Pluto.

We don't know much about Pluto and its moon, Charon, because the two are so far away. Even our best photograph, taken by the Hubble Space Telescope, is blurry and lacking much detail. All that will change in 2015, when the New Horizons spacecraft is expected to arrive at the underworld pair.

Size comparison of Pluto, Charon, the terrestrial planets and Earth's Moon (left to right): Mercury, Venus, Earth, Luna, Mars and the dwarf pair, Pluto and Charon shown at their orbital distance to scale with the planets.		The first four planets of the Solar system are called "terrestrials" because of their rough similarity to Earth. The other type of planet is called "gas giant," because the bulk of each such planet is gaseous. The Solar System Facts table gives more comparisons between the planets. But wait! What about Pluto? It is now referred to as a "dwarf planet."

Could Pluto or Charon Ever Sustain Life?

The answer is an emphatic "no!" Our sun is little more than a bright star in their "day time" skies. Talk about cold! These two worlds suffer temperatures a stone's throw from absolute zero. Add to that the fact that both worlds are smaller than our moon. Even if the two worlds were more massive — enough to retain an atmosphere — that air would quickly freeze. The surface is currently ice made from nitrogen, methane and carbon dioxide. The temperature is about -230 °C (43 ° Kelvin or 43 degrees above absolute zero). What atmosphere Pluto has is a very rarified nitrogen — a pressure approximately 1/700,000 that at Earth's sea level. Even the length of a day works against the unhappy duo — 153 hours. Unless the New Horizons spacecraft finds something far more interesting when it arrives in 2015, the Pluto Tourist Bureau may have no luck attracting visitors, much less settlers.

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