Pluto

Contents

Introduction

After Percival Lowell established his Flagstaff observatory and published three books on Mars, its canals and supposed inhabitants, he turned his attention to the outer planets. Claims were made that just as the motion of Uranus deviated from predictions, that of Neptune did too. Lowell believed those claims, suspected there may exist one more unknown planet beyond Neptune, and started looking for it. Today, with the mass of Uranus better known, calculations and observations are found to fit each other well, removing the need for "Planet X."

Lowell died in 1916 without discovering any planet, and others continued to maintain his observatory (it still exists). In 1928 the observatory received drawings of Jupiter and Mars by a young amateur astronomer who had built his own telescopes, Clyde Tombaugh (born 1906); he was offered a job, and continued Lowell's search. He discovered a new trans-Neptunian planet in 1930: it was quite dim, but moved too slowly to be an asteroid.

He named it Pluto--the name of the Roman God of the Underworld (Hades to the Greeks), a name which also started with the initials of Percival Lowell.

But something was odd about Pluto: it seemed far too small to perturb a planet as massive as Neptune. For many years it was believed to be the size of the Earth, and later observations found it even smaller, not even the size of our own Moon. One reason it may have seemed larger turned out to be Charon, a big satellite, half the diameter of Pluto, discovered by John Christy in 1977.

[In the Greek, "ch" is the letter chi, pronounced as in German, Scotch and Russian (like "kh"), though US astronomers pronounce it as in "Christie." However, John Christie claims he chose the name because the initials fit those of his wife Charlene, and he pronounces it as in her name.]

Since then two additional small moons (Nix and Hydra) were also observed. Pluto's orbit is odd, too: it intersects the one of Neptune, and its period is in resonance with that of Neptune--three orbits of Neptune equal two of Pluto. Gravity makes such a resonance stable, and the resonance also keeps the two from colliding.

Preview Image

Pluto is mostly brown. The above picture captures the true colors of Pluto as well as the highest surface resolution so far recovered. Although no spacecraft has yet visited this distant world, the New Horizons spacecraft launched early this year is expected to reach Pluto in 2015. Pluto recent reclassification, by the International Astronomical Union, from planet to dwarf planet remains a topic of much debate. The above map was created by tracking brightness changes from Earth of Pluto during times when it was being partially eclipsed by its moon Charon. The map therefore shows the hemisphere of Pluto that faces Charon. Pluto's brown color is thought dominated by frozen methane deposits metamorphosed by faint but energetic sunlight. The dark band below Pluto's equator is seen to have rather complex coloring, however, indicating that some unknown mechanisms may have affected Pluto's surface.  (Source: Pluto in True Color - NASA.)

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