Pluto orbits beyond the orbit of Neptune (usually). It is much smaller than any of the official planets and now classified as a "dwarf planet". Pluto is smaller than seven of the solar system's moons (the Moon, Io, Europa, Ganymede, Callisto, Titan and Triton).

Planet Profile

orbit: 5,913,520,000 km (39.5 AU) from the Sun (average)
diameter: 2372 km
mass: 1.303e22 kg

History of Pluto

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 Planet X. But that doesn't mean there aren't other objects out there, only that there isn't a relatively large and close one like Planet X was assumed to be. In fact, we now know that there are a very large number of small objects in the Kuiper Belt beyond the orbit of Neptune, some roughly the same size as Pluto.

 Until 2015 even the Hubble Space Telescope was able to resolve only the largest features on its surface (left and above).  On 14 July 2015 the New Horizons spacecraft did a flyby of  Pluto after being launched.

Pluto has five moons:  Charon, Hydra, Nix, Kerberos, Styx.

Charon - 1,208 km
Hydra  - 55 km x 40 km
Nix - 42 km x 36 km
Kerberos - 12 km x 4.5 km
Styx - 7 km x 3 km

Family of Moons

Until the visit by New Horizons the individual masses of Pluto and Charon could not be determined with great accuracy.

Pluto and Charon size comparisons by New Horizons:

                    Diameter        Mass            Density
Pluto            2372 km        1.303e22 kg        1.860 +/- 0.013 g/cm
Charon        1208 km        1.586e21 kg        1.702 +/- 0.021 g/cm

The Pluto-Charon pair orbit about each other around a common center of mass called the barycenter.

Pluto has an atmosphere consisting of mainly  nitrogen extending to 1,600 km above the surface. Methane is another constituent of the atmosphere and it is likely caused by sunlight breaking down methane gas particles into ethylene and acetylene, which were also discovered by New Horizons. As the ethylene and acetylene sink into the atmosphere they condense and create a haze.  In fact two distinct layers of haze surround the planet, one starting at about 80 km above the surface and extending to 130 km, the other is lower at an altitude of 50 km.  New Horizons captured this image of the haze layers.

The ultraviolet sunlight also acts on the haze converting it to tholins which are dark hydrocarbons and gives Pluto its characteristic color.


Pluto has a heart shaped region called Sputnik Planum.  The region is composed of nitrogen, carbon monoxide and methane ices.  These ices flow like glaciers in the minus 234 C environment.  The glaciers are thought to flow just as glaciers here on Earth do, although water ices on Pluto is very hard and virtually immovable.

New Horizons sent back images from the edge of Sputnik Planum showing the tell-tale signs of glacial flow.

Question: Pluto has a surface pressure over 10,000 times less than Earth, it has a atmosphere consisting of 98 percent Nitrogen.  It has been determined Pluto is losing Nitrogen from its atmosphere at a rate in the order of tons per hour, yet we have nitrogen glaciers.  Where does all the nitrogen come from?

Could cometary impacts be replenishing the nitrogen directly or possibly excavating enough nitrogen ices for replenishment?  It could also be enough residual heat internal to Pluto and geologic activity  is  releasing nitrogen from within Pluto itself.  As more data from New Horizons is downloaded we may find out.

Pluto is the second most contrasty body in the Solar System (after Iapetus).

There has been controversy about the classification of Pluto. It was classified as the ninth planet shortly after its discovery and remained so for 75 years. But on 2006 Aug 24 the IAU decided on a new definition of "planet" which does not include Pluto. Pluto is now classified as a "dwarf planet", a class distinct from "planet". While this may be controversial at first (and certainly causes confusion for the name of this website) it is my hope that this ends the essentially empty debate about Pluto's status so that we can get on with the real science of figuring out its physical nature and history.

Pluto has been assigned number 134340 in the minor planet catalog.

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.

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 days will be required to reliably find it. Suitable charts can be created with many planetarium programs.


Charon ( "KAIR en" ) is Pluto's largest satellite:

        orbit:    19,500 km from Pluto(average)
        diameter: 1208 km
        mass:     1.586e21 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 is a little over half the diameter of Pluto (50.9 percent) and has a very different surface composition than Pluto.  Pluto is mainly nitrogen, methane and carbon monoxide ices, Charon is more water ice.

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.

Charon has a massive "canyon" across more than 1,600 km its equatorial region almost as if the moon was nearly split in two.  It has been proposed that Charon was formed by a giant impact similar to the one that formed Earth's Moon

The New Horizons team also discovered the plains south of the canyon called the Vulcan Plaum have noticeably fewer craters than the north so they are younger and the smooth plains along with the grooves and faint ridges are tell-tale signs of wide scale resurfacing.  Could this be explained by cryovolcanism? Possibly further data from New Horizons will shed light on this mystery.


More about Pluto and Charon