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PLUTO - Planet

   Pluto is the farthest planet from the Sun (usually) and by far the smallest. Pluto is smaller than seven of the solar system's moons (the Moon, Io, Europa, Ganymede, Callisto, Titan and Triton).

        orbit:    5,913,520,000 km (39.5 AU) from the Sun (average)
        diameter: 2274 km
        mass:     1.27e22 kg
   In Roman 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. There is no tenth planet.

   Pluto is the only planet that has not been visited by a spacecraft. Even the Hubble Space Telescope can resolve only the largest features on its surface (left and above). There is a planned mission called New Horizons that will launch in 2006 if it gets funded.

   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.

   Pluto's radius is not well known. JPL's value of 1137 is given with an error of +/-8, almost one percent.

   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 basic physics) 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).

   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 (also known as Trans-Neptunian Objects). There is considerable merit to the latter position, but historically Pluto has been classified as a planet and it is very likely to remain so.

   Pluto's orbit is highly eccentric. At times it is closer to the Sun than Neptune (as it was from January 1979 thru February 11 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. Its orbital inclination is also much higher than the other planets'. Thus though it appears that Pluto's orbit crosses Neptune's, it really doesn't and they will never collide. (Here is a more detailed explanation.)

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

   The surface temperature on Pluto varies between about -235 and -210 C (38 to 63 K). The "warmer" regions roughly correspond to the regions that appear darker in optical wavelengths.

   Pluto's composition is unknown, but its density (about 2 gm/cm3) indicates that it is probably a mixture of 70% rock and 30% water ice much like Triton. The bright areas of the surface seem to be covered with ices of nitrogen with smaller amounts of (solid) methane, ethane and carbon monoxide. The composition of the darker areas of Pluto's surface is unknown but may be due to primordial organic material or photochemical reactions driven by cosmic rays.

   Little is known about Pluto's atmosphere, but it probably consists primarily of nitrogen with some carbon monoxide and methane. It is extremely tenuous, the surface pressure being only a few microbars. Pluto's atmosphere may exist 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. NASA mission planners want to arrive at Pluto while the atmosphere is still 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 idea is that Triton, like Pluto, once moved in an independent orbit around the Sun and was later captured by Neptune. Perhaps Triton, Pluto and Charon are the only remaining members of a large class of similar objects the rest of which were ejected into the Oort cloud. 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 not easy. There are several Web sites that show the current position of Pluto (and the other planets) in the sky, but much more detailed charts and careful observations over several months will be required to actually find it. Suitable charts can be created with many planetarium programs such as Starry Night.


   Charon ( "KAIR en"  ) is Pluto's only known satellite:
        orbit:    19,640 km from Pluto
        diameter: 1172 km
        mass:     1.90e21 kg
   Charon is named for the mythological figure who ferried the dead across the River Acheron into Hades (the underworld).

(Though officially named for the mythological figure, Charon's discoverer was also naming it in honor of his wife, Charlene. Thus, those in the know pronounce it with the first syllable sounding like 'shard' ("SHAHR en").

   Charon was discovered in 1978 by Jim 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.

   Charon's radius is not well known. JPL's value of 586 has an error margin of +/-13, more than two percent. Its mass and density are also poorly known.

   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. (This makes the phases of Charon as seen from Pluto very interesting.)

   Charon's composition is unknown, but its low density (about 2 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. Interestingly, this is quite different from Pluto.

   Unlike Pluto, Charon does not have large albedo features, though it may have smaller ones that have not been resolved.

   It has been proposed that Charon was formed by a giant impact similar to the one that formed Earth's Moon.

   It is doubtful that Charon has a significant atmosphere.

Open Issues

 last updated: 12/23/10 03:07:32 PM

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Symbols for the Planets - Including Pluto

Several people have asked me "what is the symbol for Pluto?". The Pluto symbol is made from the initials of Percival Lowell, the astronomer who searched for a ninth planet for many years and after whom the Lowell Observatory (where Clyde Tombaugh discovered Pluto) is named.

The symbols for all the planets are

And here is a site with nicely drawn graphics for the symbols - and much more.

Just the Facts

Radius of Pluto = 1145 to 1200 km
Radius of Charon = 600 to 650 km
Mass of Pluto = 1.3 x 1022kilograms
Mass of Charon = 0.16 x 1022kilograms
Density of Pluto = 1.92 to 2.06 gm/cc
Density of Charon = 1.51 to 1.81 gm/cc
Surface composition of Pluto = H2O, N2, CH4, CO ices
Surface composition of Charon = H2O ice
Atmospheric composition of Pluto = N2, CH4, CO
Surface temperature of Pluto = 40 Kelvin (= -233 C)
Spin period of Pluto = 6.39 days
Spin period of Charon = 6.39 days
Orbital period of Charon around Pluto = 6.39 days
Orbital period of Pluto and Charon around Sun = 248 years
Average distance from Sun = 39.5 x distance of Earth from Sun (Astronomical Units)
Eccentricity of orbit around Sun = 0.249
Inclination of orbit atound Sun = 17.14 degrees
Tilt of spin axis relative to ecliptic plane = 119.6 degrees

From Pluto and Charon, edited by Alan Stern and Dave Tholen, University of Arizona Press (chapter by Dave Tholen and Marc Buie).

Pluto's Orbit. Here is a diagram showing Pluto's orbit with the orbits of all the other planets in the solar system. (The figure comes from an excellent introductory astronomy textbook - Cosmic Perspectives). You may also be wondering why Pluto does not collide with Neptune, since their orbits are close. This is because their orbits are in resonance. Pluto orbits the Sun twice for every 3 orbits of Neptune. As a result, Pluto and Neptune are never very close to each other. Here is a diagram which illustrates the resonance of their orbits. The upper diagram is the usual map of the solar system looking down from above the Sun. The lower diagram has been made by keeping Neptune and the Sun fixed - the planets are then allowed to follow their orbits over thousands of years. You will see Pluto's eccentric orbit maps out a strange, curly path - but never comes close to Neptune. (From the chapter by Malhotra and Williams in the book Pluto and Charon, edited by Alan Stern and Dave Tholen).

Pluto's gravity. Since the mass of Pluto is 1/455 that of Earth, you might first guess that the gravity would be 455 times less. But you also need to take into account the fact that the size of Pluto is 1/5.5 times less than the Earth so that when you stand on Pluto you are 5.5 times closer to the center of mass. Since the force of gravity decreases with the square of the distance from the center of mass, the gravity on Pluto will be changed by a total of (5.5 x 5.5) / 455 = 0.07 or 1/15. This means that a person on Pluto would weigh 1/15 what they weigh on Earth (nice way to lose weight - but a chilly way to do it!). For comparison, the astronauts on the Moon weighed 1/6 what they weighed on Earth.

HST Images of Pluto

Hubble Space Telescope images of Pluto, showing its surface, have been released (March 8, 1996). These are the first images ever taken which directly resolve Pluto's surface. They show 12 major "regions" of light and dark features. These images were taken with Hubble's Faint Object Camera. The investigators include Dr. Alan Stern of Southwest Research Institute, Dr. Marc Buie of Lowell Observatory and Dr. Laurence Trafton of the University of Texas.

These images and a map of Pluto's surface are available on the here.

So - is Pluto a planet?

My view is that a useful working definition of a planet is that

  1. a planet orbits the Sun (rather than a moon that orbits a planet) and
  2. it must be big enough that its own gravity pulls it together into a sphere - i.e. it must be round.

From looking at many objects in the solar system, we can generally say that objects greater than about 400 km across tend to be more or less spherical (rather than irregular or 'potato-shaped'). Pluto, although smaller than several moons, qualifies under this definition. About a handful of asteroids also qualify. I would call all of these minor planets.

The debate has really come about because of recent discoveries of 30 or so new objects in the outer part of the solar system - (these are thought to be part of a large collection of objects, Pluto-sized and smaller, that form a disk-shaped cloud beyond Pluto's orbit - called the Kuiper Disk - see below). But these new Kuiper Disk objects are about 10 times smaller than Pluto, less than the nominal 400km to be round, so that they certainly do not qualify as planets.

Therefore, I am sticking with Pluto as a planet. Now, there may be more planets than the usual nine, if we count the larger asteroids and the possibly 100s more Pluto-sized objects in the distant Kuiper Disk that have not been discovered - but just because Pluto is small (about the size of the USA - see below) that does not mean we should 'demote' it.

Fran Bagenal

Naming of Pluto and Charon

PLUTO: Discovered in 1930 by American astronomer Clyde W. Tombaugh at Lowell Observatory in Flagstaff, AZ during a systematic search for a trans-Neptune planet predicted by Percival Lowell and William H. Pickering. Named after Roman god of the under- world who was able to render himself invisible. Here is more about how Pluto was named.

CHARON: Discovered in 1978 by 2 American astronomers, James W. Christy and Robert S. Harrington. Named after the mythological boatman who ferried souls across the river Styx to Pluto for judgement.

Information on Pluto: