from Lake Afton Public Observatory
On February 18, 1930, a 24-year-old from Burdette, Kansas made a startling discovery. While using a device called a blink comparator, which switches viewing back and forth between two different photographs, Clyde Tombaugh noticed one small point of light that would change positions. By carefully ruling out all other possibilities, the answer to the riddle of the jumping light was obvious. On March 13, the 149th anniversary of William Hershel's discovery of Uranus, the discovery was announced to the world. Two months later the new, ninth planet was christened Pluto, after the mythological god of the underworld.
The outer solar system is a realm of giant, gaseous, ringed planets, each possessing a myriad of moons. Pluto just does not fit this mold. In the first place, Pluto is the smallest planet in the solar system. The most recent estimates on the size of this frozen world give it a diameter of approximately 2,385 kilometers (1,483 miles), making it smaller than Earth's moon. Secondly, it has only one moon. Pluto's illusive companion remained unnoticed for almost 50 years, until James Christy noticed a bump on the side of a high resolution photograph of the planet in 1978. The moon was named Charon, after the Greek mythological boatman who ferried the dead across the river Styx to Pluto's grim empire. And finally, while the orbits of the rest of the planets of the solar system never stray farther than about 7x from the plane of Earth's orbit around the Sun (called the ecliptic), Pluto's orbit is inclined a full 17x. Pluto's orbit is also highly eccentric (an elongated ellipse or oval). This "wild' orbit takes Pluto from as close as 4.4 billion kilometers (2.75 billion miles), to as far as 7.4 billion kilometers (4.5 billion miles) from the Sun. As a matter of fact, from 1979 to 1999, Pluto is closer to the Sun than Neptune is.
Charon is about half the size of Pluto. This means that Pluto and Charon comprise essentially a double planet system. Since their respective masses are relatively close (much closer than any other planet-moon combination), Pluto and Charon continually orbit about each other--locked in a mutually synchronous orbit (ie. they always keep the same face pointed toward each other). Discovering this moon has given astronomers a remarkable amount of information such as discerning the sizes of Pluto and Charon, establishing the rotation period (one Plutonian day is 6.4 Earth days long), determining that the axis of rotation of Pluto is tilted at an angle of 122x with respect to its orbit, and estimating the surface color and make-up of the two bodies.
The remarkable tilt of Pluto's axis (and the corresponding orbit of Charon about Pluto's equator) has presented astronomers with an extremely fortunate circumstance. Twice every 248 years (the period of Pluto's orbit around the Sun), the orbit of Charon and Pluto align toward Earth so that the dance can be viewed "edge on". In February of 1985, the two bodies began alternately passing in front of and behind each other. (The last time this happened was during the Civil War.) Since one disappears behind the other, each individual spectrum can be recognized. By carefully studying the spectra of each in turn, the composition and surface color can be ascertained.
If you were actually able to see the "dancing" pair of worlds, what you would most likely see is a planet somewhat reddish in color with white, frozen methane polar caps that extend about ~ the way to Pluto's equator. Pluto probably has a very thin, methane rich atmosphere that is about 100,000 times less dense than Earth's. The density of Pluto (and Charon), about 2 grams per cubic centimeter, implies a rocky core surrounded by thick water ice. The reddish surface of Pluto is due to methane ice reacting with the light from the distant Sun. One interesting possibility is that as Pluto's orbit goes toward aphelion (the farthest point in its orbit about the Sun), the entire methane atmosphere may condense and fall as snow coating the planet in a new methane skin.
Charon, on the other hand, does not show any signs of methane. This is probably due to its smaller size. Methane would simply sublimate (change directly from a solid to a gas state) and escape the moons weak gravity. As a result, Charon would appear dull grey in color, probably from water ice, and have no atmosphere.
At the incredible distance that the pair of worlds is from the Sun, a look at the daytime sky would look much the same as a night on Earth with a very bright star in the sky (the Sun) about as bright as a quarter moon. It would be a decidedly chilly - 215x Celsius. Also, the position of Charon would never change. Depending upon your position on Pluto the moon would either always be up or never be visible.
Although, we think we know much about this denizen of the far reaches of the solar system, many puzzles still remain. Why are Pluto and its moon so much different than the rest of the outer solar system's planets? What happened to Pluto and Charon to tip them over at such an angle? Why does Pluto have such an eccentric orbit about the Sun? All of these are great questions to ask. Understanding this most peculiar of planets will require careful study and a long hard look at the edge of the Sun's family home.
Astronomy Magazine, June '92, "How Big is Pluto".
Scientific American Magazine, June '90 "Pluto".
Time-Life Books, Voyage Through the Universe, "The Far Planets", 1988.
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