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
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
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
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
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
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.
Bill Arnett; last updated: 2002 Nov 1