Did you know...
...that Charon, Pluto's moon, is nearly half as large as Pluto? This is the largest satellite/planet ratio in the solar system.
...that due to its large orbital eccentricity, Pluto is closer to the Sun than Neptune during part of its orbit? In March 1999, Pluto will become the farthest planet again.
When Neptune was discovered in 1846, the universe was thought to be complete. However, questions remained. Strange disturbances in the orbits of some of the outer planets baffled astronomers of the day. The observations seemed to suggest that another planet was at work, using its gravitaty to alter the other planets' paths. Some early astronomers were convinced that a ninth planet must exist, but nothing was found for years. In 1929, astronomers at Lowell Observatory decided to resume the hunt for the hidden ninth planet. Images taken of a starfield within the constellation of Gemini revealed that, indeed, a planet previously unknown did exist. It was named Pluto, after a god of the underworld and of darkness. The name seemed to fit, since Pluto is so far away that it receives very little of the Sun's light, making it a pretty dark place to be.
Some odd facts about the newfound Pluto were uncovered. First, it had an odd orbit. Pluto's orbit is so eccentric that it actually comes closer to the Sun than Neptune at times! (Currently, Pluto is the closer of the two, and it will remain closer until about 1999.) Don't worry, though, Neptune and Pluto will not crash into each other. Their paths do not actually cross because Pluto's orbit is highly inclined compared to the orbits of the other planets. Another weird discovery was that Pluto is a small planet. In fact, it is the smallest of the nine in the solar system. The astronomers had expected a larger planet, because a planet Pluto's size could not possibly cause the orbital disturbances which led to the search for a ninth planet in the first place.
Not surprisingly, speculation about a tenth planet arose. Percival Lowell, the man who had found Pluto at his namesake observatory, did much searching, but found nothing. Eventually, measurements from Voyager 2 gave new data that showing that the disturbances in the orbits of the outer planets were simply caused by incorrect masses of the outer planets. Our old measurements were off, making the orbital calculations wrong, which made it appear like some outside force (like a planet) was at work. But Voyager 2's data makes the numbers show no disturbance, and thus decreases the likelihood of a tenth planet.
Some astronomers say that Pluto should not even be considered a planet at all, merely a large asteroid. A major speculation is that Pluto is simply the largest of a huge belt of asteroids outside the nine planets. The distinctions become blurred; what exactly qualifies as a planet? The decision is mostly arbitrary, but size is the key factor.
We don't know too much about Pluto as compared to the closer planets. We do know that it has an icy surface and a coating of methane frost. The Sun's light only shines about 1/1000th as brightly on Pluto as it does on Earth, making it too cold for any real atmosphere. Pluto also has a moon named Charon which is very close to Pluto and has about one third the of Pluto's diameter.
APOD Index - Solar System: Pluto - Astronomy picture of day page
Uranus, Neptune and Pluto: The Outer Worlds of the Solar System - Outer planets info
Is Pluto a Planet? - Discussion about Pluto
* Photo credit - Dr. R. Albrecht, ESA/ESO Space Telescope European Coordinating Facility, STScI, NASA