Percival Lowell noticed irregularities in Neptune’s orbit as early as 1905, and began a search for another planet beyond all previously studied that would be influencing its orbit. An overview of the planetary orbits may be found at the University of Maryland website on the nine planets.

Pluto is so small and distant, however, that it was not discovered until 1930 by Lowell’s assistant, Clyde Tombaugh. Even after its discovery, little was known about this far away world, including its diameter, density, and whether or not it had any satellites.Not until 1978 was Charon (pronounced KEHR-on), Pluto’s only moon, discovered by high powered, Earth-based telescopes.

Now the Hubble Space Telescope has finally given us better pictures and information about this bizarre planet/moon system.  NASA launched a mission to Pluto on January 19,2006 called the New Horizons or Pluto-Kuiper Express.  The probe will get a gravity assist from Jupiter in February 2007, take an interplanetary cruise until June 2015 and reach the Pluto-Charon system in July 2012.

Pluto has a diameter of only 2300 km, similar to Neptune’s largest moon, Triton. Charon is over half the size of Pluto, with a diameter of nearly 1200 km. It also orbits very close to Pluto, at a distance of only 20,000 km. Charon might have been formed during a violent impact on Pluto, which ejected material to form the moon. Until the discovery of Charon, it was believed that the Earth's moon was the only satellite planetary body formed by this mechanism.

Charon keeps the same face toward Pluto and, uniquely, Pluto keeps the same face toward Charon. This means that someone living on the far side of Pluto would never even know they had a moon!

Composition:

Once accurate measurements were determined for Pluto’s size and density, hypotheses about its internal structure were developed. With such a high density when compared to the gas giants, Pluto is thought to consist of between 70 and 80% rock. The outer shell of the planet is probably made up of ices of methane, nitrogen, and water.

Origins:

The origin of this unique system is still unknown, though several theories exist. One is that Charon was formed when another body collided with Pluto, ejecting material from the planet and allowing it to re-form as a satellite. This should sound familiar, as it is the same theory for the formation of the Earth’s Moon. Charon has a low density (1.3 g/cm3) when compared to that of Pluto (2.02 g/cm3), which suggests it may be just a mass of fractured ice and rock. A large collision is supported by the fact that Pluto is inclined nearly 122 degrees from its orbital plane. Therefore, similar to Uranus, Pluto keeps its poles toward the Sun. How did Pluto form in the first place? To determine this answer we must first examine the composition and atmospheric conditions of this tiny, cold planet.

Atmosphere:

Pluto’s tenuous atmosphere of N2 and CO gas probably gives the planet a pink, wispy appearance. There may be seasonal changes within Pluto’s atmosphere as it orbits around the Sun. At the closest point of its 248 year "year", the atmosphere may become thick with N2 gas, but at aphelion, the atmosphere may re-freeze and fall to the surface. Charon is a dusky grey color and might have a surface made up of water ice. A bizarre atmospheric transfer between Pluto and Charon causes this difference between the two. During Pluto’s summer, methane ice changes to gas on both Pluto and Charon. Charon is so small, however, it does not have the gravitational pull to retain the methane gas and loses it to Pluto.

Get to know this distant planetary body -- it really is considered by many the last frontier of our Solar System. Visit the Nine Planets website to understand more about the Pluto-Charon pair. Learn about how Pluto was discovered by accident, and named as the god of the underworld, or perhaps the "PL" in Pluto are simply the initials of Percival Lowell. Several other

Pluto's moon Charon remains an enigma -- at least until the New Horizons spacecraft encounters the pair and we learn more about it's composition and, hence, it's origin. Charon was only recently discovered in 1978 by Jim Christy, but still little is known about the size other than it is the largest moon relative to it's primary planet in the Solar System. Charon might be similar in composition to Rhea or other icy moons of Saturn. While visiting the Nine Planets website, read about some of the open issues concerning the Pluto-Charon system (especially the on-going debate over whether Pluto really is a planet at all -- why or why not?).

Comets:

The appearance of a bright comet in the night sky filled our ancestors with awe and dread.  Change in the unchanging sky portended plagues and death at the very least.  And indeed, in 1066, Halley's comet accompanied the bloody beginning of Norman rule in England, an event recorded for us by the needles of the women who embroidered the Bayeux Tapestry.  Halley's Comet as depicted on the Bayeux Tapestry, which commemorates the Norman Conquest of England. In the upper left corner of the image.

Perhaps the most important thing to learn about comets is where they live most of the time. Study the Kuiper Belt and the Oort Cloud and learn what they are and where they are relative to other parts of the Solar System.

Be sure to visit the SEDS site that ourlines the importance of comet Shoemaker-Levy 9! Recall that this comet encountered the Roche Limit of Jupiter, broke into 21 fragments, and impacted Jupiter while people on Earth watched!

Today we know that these icy visitors arrive and depart as dictated by physical laws.  They do bring material from the outer reaches of the solar system to the planets.  Read this PDF document by NASA "Facts: Asteroids, Comets, and NASA Research" (you probably will want to open this in a new window) -- or visit the "Deep Space 1" site which is all about new missions to study comets in greater detail - http://nmp.jpl.nasa.gov/ds1/

In 1950, Jan Oort made a statistical analysis of the orbits of long-period comets. Read this PDF document discussing Oort cloud. You can also visit "Hubble Identified a Long Sought Population of Comets Beyond Neptune" - http://www.jpl.nasa.gov/sl9/news65.html

After studying comets and their origins, we really do begin to understand how the Solar System was formed and what components are important in forming planets, moons, asteroids and comets. Treat this exercise as one the begins your extended learning experience and think about this learning process when New Horizons makes news in 2015.

Glossary

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