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: 2340 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.
- Pluto's radius is not well known. Measurements range from 1164 to 1187 km.
- 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.
There is considerable merit to the later position, but historically Pluto
has been classified as a planet and it is likely to remain so.
- 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.
Its orbital inclination is also much higher than the other planets.
Thus though it
appears
that Pluto's orbit crosses Neptune's,
they will never collide.
- Like Uranus, the plane of Pluto's equator is at almost
right angles to the plane of its orbit.
- The surface temperature on Pluto is not well known but is probably between
35 and 45 Kelvins (-228 to -238 C).
- 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.
The bright areas of the surface seem
to be covered with ices of nitrogen with
smaller amounts of (solid) methane 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.
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.
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.
Mike Harvey's planet finder charts 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.
- Charon is Pluto's only known satellite:
- average distance from Pluto: 19,640 km
- diameter: 1260 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.
- Charon's radius is not well known. Measurements range from 590 to 630 km.
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.
- 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.
- 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.
Pictures
- (above) Pluto (constructed from Earth-based occultation measurements)
18k gif
- (above) Charon (constructed from Earth-based occultation measurements)
11k gif
surface "details" (HST)
29k jpg;
62k gif;
194k jpg
(caption)
- Pluto surface map (from above HST images)
108k gif;
218k jpg
(caption)
- two maps of Pluto's surface brightness
131k jpg
Pluto and Charon (FOC/COSTAR)
10k jpg;
19k gif
pre-COSTAR HST image
147k gif;
27k jpg
- Ground based and pre-COSTAR HST views
144k gif
Pluto/Charon from NOT
36k gif
- maps of Pluto and Charon
html
- US Postage stamp: "Pluto Not Yet Explored"
66k gif
Movies
- Rotating Pluto animation (HST)
614k mpg
More about Pluto and Charon
Open Issues
- There considerable uncertainty about the even most basic
properities of mass, radii and density of Pluto and Charon.
- Several Kuiper Belt objects apparently
similar to Pluto have recently been discovered? Are there any
as large as Pluto?
- What are the dark areas on Pluto composed of?
- 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.
- What seasonal changes take place during Pluto's long 'year'.
... Sun
... Neptune
... Nereid
... Pluto / Charon
... Small Bodies
... 
Bill Arnett; last updated:
1996 May 4
