Statistics: 

• Planet Diameter:  2320 km, 0.2 Earth Diameters
• Planetary Mass:  1.29x10²² kg, 0.0025 Earth masses
• Average Distance from Sun:  5.913 billion km, 39.53 AU*
• Length of Day:  about 6.4 Earth Days
• Length of Year:  248.53 Earth Years
• Atmosphere:  very thin, mostly methane (CH₄)
• Day/Night Surface Temperature:  very cold; 54-59 degrees Kelvin

*This denotes that Pluto's orbit is highly eccentric; that is, the orbit of Pluto appears to be lop-sided.  This orbit is so lop-sided that part of its orbit lies within that of Neptune!  In addition, the orbit of Pluto is greatly tilted with respect to the solar plane.

 Discovery of Pluto
 Knowledge of Pluto
 Charon, Pluto's Moon
 

The outermost planet of our solar system, Pluto, is very interesting.  It was discovered by Clyde Tombaugh in 1930; astronomers had been searching for a ninth planet for many years because the deviations in the orbit of Uranus were not completely explained by the presence of Neptune alone.  Thus, they surmised that another planet must exist.  At the great distance that Pluto rests from Earth, there are absolutely no details visible from ground-based observations; as a matter of fact, it is difficult to distinguish between Pluto and the surrounding stars!
 

Knowledge of Pluto

There is very little known about Pluto.  This is because of many reasons... as stated before, Pluto is so far away that detailed astronomical observations cannot be made of the planet itself; and Pluto is the only planet in our solar system that has not been visited by robot probes from Earth.  Therefore, much of what we know about this distant world is guesswork.

In 1979, through a measurement technique known as speckle interferometry, scientists were able to determine the size of Pluto within a reasonable degree of accuracy.  The results of the measurement show us that Pluto is just about the same size as Earth's Moon.  Studies of Pluto occulting background stars has yielded information about the thin atmosphere of this mysterious planet.  Scientists traveled to Australia and New Zealand to observe an occultation in 1988.  The results of the occultation measurements indicate that there exists a layer of smog-like methane 46 kilometers over the hard surface of Pluto.  The atmospheric pressure at the surface of Pluto was determined to be about 0.00001 (0.001%) times that of surface pressure on Earth.

The density of Pluto, which is about 2 grams per cubic centimeter, is too low for there to be much rock in the makeup of the planet.  Therefore, it has been concluded that Pluto, though solid, is made up mostly of ices of various kinds.  Scientists have performed spectrographic studies of Pluto and concluded that there exist some rocky materials (called silicates) in addition to the great amount of methane ice composing Pluto.

Pluto is more similar to many of the satellites of the gas giant planets than the other terrestrial worlds in our solar system.  Thus, scientists have speculated that Pluto (and its moon) were originally satellites of Neptune that somehow got torn away from the planet and ended up on a wildly stable orbit around the Sun.

Charon, Pluto's Moon

In 1978, James W. Christy (a scientist at the U.S. Naval Observatory) was studying photographic plates made of Pluto in the hopes that they would lead to a more precise knowledge of Pluto's orbit.  He noticed that there appeared to be a bump on the side of Pluto in some of the photographs.  Further analysis yielded that the bump was actually a moon of Pluto, Charon.  Analyses of Charon and Pluto have revealed that Charon is composed of water ice, but methane and ammonia ice are absent from the spectrum of Charon.  Charon is about 10% of the mass of Pluto, so some astronomers have stated that Pluto & Charon constitute a binary planet system rather than a planet-moon system.  Further studies of Pluto & Charon are still being carried out in an effort to learn more about the most lonely worlds in our solar system.
 

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