Pluto

Pluto Facts

• Pluto is the farthest planet from the Sun (usually) and by far the smallest:
  • average distance from Sun: 5,913,520,000 km (39.5 AU)
  • diameter: 2320 km
  • mass: 1.32e22 kg
  Pluto is smaller than seven of the solar system's moons: the Moon, Io, Europa, Ganymede, Callisto, Titan and Triton.

• In Greek mythology, Pluto (Greek: Hades) is the god of the underworld. The planet received this name (after many other suggestions) perhaps because it's so far from the Sun that it is in perpetual darkness and perhaps because "PL" are the initials of Percival Lowell.

• Pluto was discovered in 1930 by a fortunate accident. Calculations which later turned out to be in error had predicted a planet beyond Neptune, based on the motions of Uranus and Neptune. Not knowing of the error, Clyde W. Tombaugh at Lowell Observatory in Arizona did a very careful sky survey which turned up Pluto anyway.

• After the discovery of Pluto, it was quickly determined that Pluto was too small to account for the discrepancies in the orbits of the other planets. The search for Planet X continued but nothing was found. Nor is it likely that it ever will be: the discrepancies vanish if the mass of Neptune determined from the Voyager 2 encounter with Neptune is used.

• Pluto is the only planet that has not been visited by a spacecraft. Even the Hubble Space Telescope cannot resolve features on its surface.

• Fortunately, Pluto has a satellite, Charon. By good fortune, Charon was discovered (in 1978) just before its orbital plane moved edge-on toward the inner solar system. It was therefore possible to observe many transits of Pluto over Charon and vice versa. By carefully calculating which portions of which body would be covered at what times, and watching brightness curves, astronomers were able to construct a rough map of light and dark areas on both bodies.

• Though the sum of the masses of Pluto and Charon is known pretty well (it can be determined from careful measurements of the period and radius of Charon's orbit and Kepler's Third Law), the individual masses of Pluto and Charon are difficult to determine because that requires determining their mutual motions around the center of mass of the system which requires much finer measurements -- they're so small and far away that even HST has difficulty. The ratio of their masses is probably somewhere between 0.084 and 0.157; more observations are underway but we won't get really accurate data until a spacecraft is sent.

• Pluto is the second most contrasty body in the Solar System (after Iapetus). Exploring the origin of that contrast is one of the high-priority goals for the proposed Pluto Express mission.

• There are some who think Pluto would be better classified as a large asteroid or comet rather than as a planet. Some consider it to be the largest of the Kuiper Belt objects.

• Pluto's orbit is highly eccentric. At times it is closer to the Sun than Neptune (it has been so since 1979 and will continue until 1999). Pluto rotates in the opposite direction from most of the other planets.

• Pluto is locked in a 3:2 resonance with Neptune; i.e. Pluto's orbital period is exactly 1.5 times longer than Neptune's. This prevents them from colliding.

• Like Uranus, the plane of Pluto's equator is at almost right angles to the plane of its orbit.

• Pluto's composition is unknown, but its density (about 2 gm/cm3) indicates that it is probably a mixture of 80% rock and 10% water ice much like Triton. Its surface seems to be covered with ices of methane, nitrogen and carbon dioxide.

• Little is known about Pluto's atmosphere. The material that makes up Pluto's atmosphere exists as a gas only when Pluto is near its perihelion; for the majority of Pluto's long year, the atmospheric gases are frozen into ice. Near perihelion, it is likely that some of the atmosphere escapes to space perhaps even interacting with Charon. The Pluto Express mission planners want to arrive at Pluto while the atmosphere is unfrozen.

• The unusual nature of the orbits of Pluto and of Triton and the similarity of bulk properties between Pluto and Triton suggest some historical connection between them. It was once thought that Pluto may have once been a satellite of Neptune's, but this now seems unlikely. A more popular theory is that Triton, like Pluto, once moved in an independent orbit around the Sun and was later captured by Neptune. Like the Earth's Moon, Charon may be the result of a collision between Pluto and another body.

• Pluto can be seen with an amateur telescope but it is very difficult. Mike Harvey's planet finder charts show the current position of Pluto (and the other planets) in the sky, butmuch more detailed charts and careful observations over several months will be required to actually find it.

Charon

• Charon is Pluto's only known satellite:
  • average distance from Pluto: 19,640 km
  • diameter: 1270 km
  • mass: 1.47e21 kg

• Pronounced "KAIR en"

• Charon is named for the mythological figure who ferried the dead across the River Styx into Hades (the underworld).

• Charon was discovered in 1978 by J. Christy. Prior to that it was thought that Pluto was much larger since the images of Charon and Pluto were blurred together.

• Charon is unusual in that it is the largest moon with respect to its primary planet in the Solar System (a distinction once held by Earth's Moon). Some prefer to think of Pluto/Charon as a double planet rather than a planet and a moon.

• Pluto and Charon are also unique in that not only does Charon rotate synchronously but Pluto does, too: they both keep the same face toward one another.

• Charon's composition is unknown, but its low density (1.4 gm/cm3) indicates that it may be similar to Saturn's icy moons (i.e. Rhea). Its surface seems to be covered with water ice.

Pictures

1. (above) Pluto (constructed from earth-based occultation measurements) 18k gif
2. (above) Charon (constructed from earth-based occultation measurements) 11k gif

3. Pluto and Charon (FOC/COSTAR) 10k jpg; 19k gif

4. HST image of Pluto (pre COSTAR) 147k gif; 27k jpg

5. Ground based and pre-COSTAR HST views 144k gif

6. Pluto/Charon from NOT 36k gif

More about Pluto and Charon

• Excellent Pluto page from the University of Colorado
• from ASU
• from LANL
• from JPL
• from RGO
• The Struggles to Find the Ninth Planet by Clyde W. Tombaugh
• from NASA Spacelink
• Dr. Cain's class notes (scroll down for Pluto)

Open Issues

• There considerable uncertainty about the mass, radii and densities of Pluto and Charon.
• Several Kuiper Belt objects apparently similar to Pluto have recently been discovered? Are there any as large as Pluto?
• What is the nature of Pluto's atmosphere?
• What "geologic" features and processes exist on Pluto and Charon?
• Will the Pluto Express mission be funded? If we miss this chance to observe Pluto near its perihelion our next chance won't come until the 23rd century.