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Pluto's Moons


Pluto is perhaps best characterized by its history in astronomical and popular culture. After Uranus was discovered, tiny changes in its observed orbit could not be accounted for by the gravitational effects of the other planets. The end result was that another planet was theorized to exist, and it was discovered to be Neptune. Additional perturbations were then found to Neptune's orbit in 1905, and this created the idea that there should be an additional planet beyond Neptune.

Many searches were conducted, but the "missing planet" was not found. But in 1930, Clyde Tombaugh discovered what appeared to be the key, and the body was confirmed to be a large object beyond Neptune's orbit, and it was granted planetary status and named Pluto. At the time, it didn't matter that this planet was in the "wrong" place to account for the perturbations, and it was later determined that the differences in Neptune's orbit could be accounted for by other things.

Because of its huge distance from the Sun (at that time it was beyond Neptune's orbit), very little could be discerned about the distant world. Some believed it was larger than Earth, and some thought it was as large as Neptune or Uranus. Its orbit was worked out and it was discovered that it was highly elliptical, like a comet. But it was by far the biggest comet anyone had ever encountered, so its planetary status was not questioned. Its eccentric orbit takes it inside of Neptune's by less than 8 million km (5 million miles) but as far out as 160% as far as Neptune is from the Sun.

Most recently, Pluto crossed Neptune's orbit in 1999 and became the farthest planet from the Sun. Pluto and Neptune, however, will never collide. For every 3 years on Neptune, Pluto has 2 years, and this resonance will prevent them from ever hitting each other.

Another orbital anomaly is that Pluto orbits in a plane that is tilted 17° from the plane Earth orbits in. Other than Mercury, which orbits 7° from Earth's plane, all the other planets orbit within 2.5° of Earth's plane. The only objects that have an orbital inclination like Pluto's are comets and asteroids.

A Moon

In 1978, the discovery of the moon Charon was announced by Captain J.C. Smith of the U.S. Naval Observatory, where it was discovered by James W. Christy. It was soon determined that twice in the 248-year orbit of Pluto, Charon transits, or crosses in front of Pluto, as seen from Earth. By a lucky chance, one set of transits occurred between 1985-1990. If Charon has been discovered just a few years later, we would have to wait until 2109 before we could see this.

When Charon crosses in front of Pluto, and when Pluto passes in front of Charon, the total amount of light that we receive from the system dims. The amount of dimming depends upon exactly which part of the planet or moon is blocked from Earth's view. By making many observations throughout the set of transits, a map of the reflectivity of the planet and moon could be built up. These are the best maps we have of these two objects.

Pluto's Status

The information above presents the case that, at the very least, Pluto is an odd planet that has many characteristics of a planet. It was discovered at a time when people were looking for another planet in that region of space, and when they were not looking for a large belt of cometary objects, as we are now (see the discussion of the Kuiper Belt). In recent years, giant objects several hundreds of kilometers have been discovered in the same region of space as Pluto. In the Summer of 2005, it was announced that an object 25% larger than Pluto was discovered, which has since been named "Eris."

If Pluto were discovered today, there is no doubt that it would be considered a Kuiper Belt Object and not a planet. However, historically and popularly, Pluto is a planet. When the director of the Rose planetarium in New York did not have a display for it in its planets area, there was a public outcry.

Because of the changing nature of our understanding of the outer solar system, the International Astronomical Union (IAU) formed a committee that worked for about two years to study what the definition of a planet "should" be in scientific terms. They presented their recommendations at the August 2006 meeting of the IAU in Prague. Their recommendations were not greeted with much favor - in part because it seemed as if the definition had been construed to keep Pluto a planet - and subsequent changes were made and ratified. This created a new class of planets called "Dwarf Planets," of which Pluto was now a member.

Other Features

Pluto has many unique attributes. First, its moon, Charon, is fully half as large as its companion. For this reason, Pluto and Charon are sometimes considered to be a double planet system. Charon is so close to Pluto and Pluto's atmosphere is so thin and extended that it actually engulfs Charon.

Pluto's atmosphere grew as it approached the sun. But, as it is moving farther away, its atmosphere is freezing, and falling as snow. This is another reason why many astronomers think that Pluto may not be a planet at all, but that it is a giant comet. (As a comet approaches the sun, its gases begin to melt and surround it in what is called a coma. When it goes farther away, the gases re-freeze and are either lost in space, or rejoin the nucleus. This seems to be what is happening to Pluto.)

Satellites that Have Visited or Will Visit

  • New Horizons - NASA flyby mission with a planed launch date in 2006

Mythology and Naming Schemes

Pluto is named after the ancient Roman god of the underworld, which is a fitting name for a planet that lies so far from the warm Sun, receiving only 1/16,000 of the light we receive on Earth. It was not named after the Disney-created dog -- rather, the dog was named for the planet. Additionally, the name "Pluto" was chosen so that the first two letters, "P" and "L," in order to honor its discoverer, Percival Lowell.

Pluto's largest moon, Charon, is named after the figure who ferried dead souls across the river Styx into the underworld.

The two smaller moons have been named Hydra and Nix. The moon Nix would have been named "Nyx," but that name is taken by asteroid 3908, so the Egyptian equivalent was chosen. Nix is the goddess of darkness and night, and she was the mother of Charon. Hydra was the monster from Greek mythology that had the body of a serpent and nine heads. Additionally, the names were chosen to honor the New Horizons craft (so they begin with "N" and "H") that is currently en route to Pluto, and also to honor the Hubble Space Telescope (Hydra beginning with "H"), which was used to verify the moons' existence.

Since no craft nor telescope has ever seen features on Pluto, a naming convention for them has not yet been assigned.

Data for the Planets:

Ceres Pluto Eris
Mean Orbital Distance (106 km)
Average Orbital Velocity (km/s)
Orbital Inclination (from Earth's Orbit)
Orbital Eccentricity
Equatorial Radius (km)
2400 ± 100
Polar Radius (km)
2400 ± 100
Mass (1024 kg)
Density (water=1)
Escape Velocity (km/s)
Gravity (m/s2)
Length of Day (hours)
> 8?
Tropical Orbital Period (days)
Average Surface Temperature (Celsius)
Number of Moons
Absolute Magnitude
Guiseppe Piazzi
Clyde Tombaugh
Mike E. Brown, C.A. Trujillo, and D.L. Rabinowitz
Discovery Date
January 1, 1801
February 18, 1930
October 21, 2003

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