New Horizons photo of Pluto
New Horizons closeup photo of Pluto showing ice mountains
New Horizons photo of Pluto's moon Charon

New Horizons photo of Pluto

New Horizons closeup photo of
Pluto showing ice mountains

New Horizons photo of
Pluto's moon Charon

God of the Underworld

As we leave behind us the system of Neptune and its moons, we leave the world of the gas giants and approach the extreme outer limits of our solar system. This region is inhabited by many small, rocky worlds. This region is known as the Kuiper belt, and two of its most famous inhabitants are the planet Pluto and its single moon, Charon. The darkness and isolation of these two worlds has likened them to the underworld. In fact, Pluto was named after the Roman god of the underworld. Charon was named after the mythological figure who ferried the dead across the river Styx to Hades. Pluto was discovered in 1930 by Clyde W. Tombaugh at Lowell Observatory in Arizona. It was much too small to be seen by most telescopes of the time. Astronomers first theorized that a ninth planet might exist when they observed that something seemed to be exerting a gravitational pull on the planet Neptune. An in-depth sky survey finally turned up the tiny planet. Pluto is so far away from the Sun that it takes 248 Earth years to complete one orbit. The temperature here averages -382° F (-230° C).

Artist rendering of Pluto and its moon Charon as seen from one of its newly discovered moons

Artist rendering of Pluto and its moon Charon as seen from one of its newly discovered moons.
(NASA, ESA and G. Bacon (STScI))

The Twin Planets

For many decades, Pluto was thought to be alone in its dark corner of our solar system. But in 1978, Jim Christy discovered that Pluto had a companion. This moon, Charon, is more than half the size of its parent planet. In fact, many astronomers once referred to Pluto and Charon as "twin planets" because they are so close to each other in size. Because Charon is so large, it does not actually orbit around Pluto. Rather, the two bodies actually orbit around a common center of gravity somewhere between them. Pluto and Charon are also tidally locked in a synchronous orbit. Pluto's rotational period is 6.3 days. It takes 6.3 days for Charon to make one revolution around Pluto. Thus, the two bodies continuously face each other. In 2005, two additional moons were discovered by the Hubble Space Telescope. Named Nix and Hydra, these tiny moons orbit Pluto at three times the distance as Charon. The orbit of Pluto is so highly eccentric that is actually crosses inside the orbit of Neptune. Because of this, Pluto is actually closer to the Sun than Neptune for 20 years of its 248-year orbit. The 17-degree inclination of Pluto's orbit means that it there is no chance that it will eventually collide with Neptune.

In 2005, two new moons were discovered by the Hubble Space Telescope Pluto Companion Search Team on. Once the orbits were confirmed, the moons were assigned the names Nix and Hyrda. The names were chosen in part because the initials (NH) refer to the New Horizons mission, which is scheduled to orbit Pluto in the year 2015. In Greek mythology, Hydra was the 9 headed serpent who guarded the waters of the underworld and Nix was the goddess of the night. Two additional moons were discovered in 2011 and in 2012, bringing the total to five.


Features of Pluto and Charon

Very little is known about surface of Pluto and Charon. They are so tiny and so far away that even the Hubble space telescope has trouble resolving any surface features on their surfaces. With a diameter of only 1,473 miles (2,370 km), Pluto is not considered to be a planet. Instead, it belongs to a relatively new category of objects known as dwarf planets. Until recently, no one really knew why this small, rocky  object could be found among the giant gas planets of the outer solar system. But recent discoveries of many similar objects have answered that question. There may be hundreds, even thousands of Pluto-like objects lurking in the outer Solar System near the Kuiper belt. Between 1985 and 1990, the Earth was aligned with the orbits of Pluto and Charon in such a way that a series of eclipses would take place. Astronomers were able to take advantage of this situation to accurately determine the size of both Pluto and Charon, and to create the first image of Pluto showing some of its larger surface features. This revealed that Pluto is the second most contrasty body in the Solar System. Astronomers are hoping that a future space probe may one day be able to show us the surface of this strange and distant world. But until then, every new discovery will bring with it a long list of new questions.


The Lost Planet

For years,many astronomers questioned the status of Pluto as one of the nine planets of the Solar System. They believed that Pluto and Charon could be asteroids or comets that somehow got locked into an orbit around the Sun. Soon after the discovery of the Kuiper belt, most astronomers began to think that these two bodies were actually rogue Kuiper objects and should not longer be classified as planets. The debate raged on for decades, supported by the public's fondness for their favorite planet. Finally, on August 24, 2006, the International Astronomical Union (IAU) decided to redefine the definition of a planet. This decision officially reclassified Pluto and Charon as dwarf planets. Only 76 years after its discovery, Pluto was quietly added to the list of minor planets and given the number 134340. It was the end of an era as the Solar System's ninth planet surrendered its title. Today, we know that many similar objects can be found in this region of space. Some are even larger and farther away than Pluto. The future holds the possibility of many exciting new discoveries as we continue to probe the darkness in search of Pluto's cousins.



Statistics for Pluto

Discovered by

Year of Discovery


Known Natural Satellites

Mean Distance from the Sun

Rotational Period

Orbital Period

Orbital Eccentricity

Orbital Inclination

Inclination of Axis

Mean Surface Temperature

Main Atmospheric Component

Apparent Magnitude














Clyde W. Tombaugh


1,473 miles (2,370 km)


3,673,537,000 miles (5,913,520,000 km)

6.3 days

248 years


17.14 degrees

122.5 degrees

-382° F (-230° C)




The Moons of Pluto




Orbital Period














10 - 25

46 - 137

13 - 34

61 - 167

















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