Pluto is the ninth planet in the solar system. Discovered in
1930 and immediately classified as a planet, its status is
currently under dispute [6]. Pluto has an eccentric orbit that is
highly inclined in respect to the other planets and takes it inside
the orbit of Neptune. Its largest moon is Charon, discovered in
1978; two smaller moons were discovered in 2005. Pluto's
astronomical symbol is a P-L monogram, ♇. This represents both the
first two letters of the name Pluto and the initials of Percival
Lowell, the man who lent his name to the observatory that was used
to find Pluto. An alternate symbol resembles that of Neptune, but
has a circle in place of the middle spoke in the top center.
Due to its size and unusual orbit, there has been debate regarding
Pluto's classification as a major or a minor planet, and there is
increasing momentum for recognizing "dual status." Pluto is
classified as a trans-Neptunian object. As of July 31, 2005, one
other trans-Neptunian object, 2003 UB313, had been found that is
larger than Pluto.
Pluto was discovered by the astronomer Clyde Tombaugh at the Lowell
Observatory in Arizona on February 18, 1930 after an extensive
search when he compared photographic plates taken on January 23 and
29. After the observatory obtained confirming photographs, the news
of the discovery was telegraphed to the Harvard College Observatory
on March 13, 1930. The planet was later found on photographs dating
back to March 19, 1915. Tombaugh was searching for a "Planet X" to
explain discrepancies in the predicted orbit of Neptune. It is now
known these discrepancies were an artifact of the slightly
incorrect value then assumed for the mass of Neptune.
In the matter of Pluto, the discretion of naming the new object
belonged to the Lowell Observatory and its director, Vesto Melvin
Slipher, who, in the words of Tombaugh, was "urged to suggest a
name for the new planet before someone else did". Soon suggestions
began to pour in from all over the world. Constance Lowell,
Percival's widow who had delayed the search through her lawsuit,
proposed Zeus, then Lowell, and finally her own first name, none of
which met with any enthusiasm. One young couple even wrote to ask
that the planet be named after their newborn child. Mythological
names were much to the fore: Cronus and Minerva (proposed by the
New York Times, unaware that it had been proposed for Uranus some
150 years earlier) were high on the list. Also there were Artemis,
Athene, Atlas, Cosmos, Hera, Hercules, Icarus, Idana, Odin, Pax,
Persephone, Perseus, Prometheus, Tantalus, Vulcan, Zymal, and many
more. One complication was that many of the mythological names had
already been allotted to the numerous asteroids. Virtually all the
female names had been used up, and male names were usually reserved
for objects with unusual orbits.
The name retained for the planet is that of the Roman god Pluto,
and it is also intended to evoke the initials of the astronomer
Percival Lowell, who predicted that a planet would be found beyond
Neptune. The name was first suggested by Venetia Phair (n¨¦e
Burney), at the time an eleven-year-old girl from Oxford, England
[7]. Over the breakfast table, one morning her grandfather, who
worked at Oxford University's Bodleian Library, was reading about
the discovery of the new planet in the Times newspaper. He asked
his granddaughter what she thought would be a good name for it.
Venetia, who was quite interested in Greek and Roman myths and
legends, suggested the name of the Roman God of the underworld.
Professor Herbert Hall Turner cabled his colleagues in America with
this suggestion, and after favourable consideration which was
almost unanimous, the name Pluto was officially adopted and an
announcement made by Slipher on May 1, 1930.
Pluto's orbit is unlike those of the other planets. It is highly
inclined above the plane of the ecliptic, and highly eccentric
(non-circular). The eccentricity of its orbit is such that it
crosses the orbit of Neptune, and making Pluto only the eighth-most
distant planet from the Sun for part of each orbit; the most recent
occurrence of this phenomenon lasted from February 7, 1979 through
February 11, 1999. Mathematical calculations indicate that the
previous occurrence only lasted fourteen years from July 11, 1735
to September 15, 1749. However, the same calculations indicate that
Pluto was the eighth-most distant planet between April 30, 1483 and
July 23, 1503, which is almost exactly the same length as the 1979
to 1999 period. Recent studies suggest each crossing of Pluto to
inside Neptune's orbit lasts alternately for approximately thirteen
and twenty years with minor variations.
Pluto orbits in a 3:2 orbital resonance with Neptune. When Neptune
approaches Pluto from behind their gravity start to pull on each
other slightly, resulting in an interaction between their positions
in orbit of the same sort that produces Trojan points. Since the
orbits are eccentric, the 3:2 periodic ratio is favoured because
this means Neptune always passes Pluto when they're almost farthest
apart. Half a Pluto orbit later, when Pluto is nearing its closest
approach, it initially seems as if Neptune is about to catch up to
Pluto. But Pluto speeds up due to the gravitational acceleration
from the Sun, stays ahead of Neptune, and pulls ahead until they
meet again on the other side of Pluto's orbit.
Beginning in the 1990s, other trans-Neptunian objects have been
discovered, and a certain number of these also have a 3:2 orbital
resonance with Neptune. TNOs with this orbital resonance are named
"plutinos", after Pluto.
Pluto is not only smaller and much less massive than every other
planet, but at less than 0.2 lunar masses it is also smaller and
less massive than seven moons: Ganymede, Titan, Callisto, Io,
Earth's Moon, Europa and Triton. However, Pluto is more than twice
the diameter, and a dozen times the mass, of Ceres, the largest
minor planet in the asteroid belt, and it was larger than any other
object known in the trans-Neptunian Kuiper belt until 2003 UB313
was announced in 2005. See List of solar system objects by mass and
List of solar system objects by radius.
Pluto's mass and diameter could only be estimated for many decades
after its discovery. The discovery of its satellite Charon in 1978
enabled a determination of the mass of the Pluto-Charon system by
simple application of Newton's formulation of Kepler's third law.
Later Pluto's diameter was measured when it was occulted by Charon,
and its disk can now be resolved by telescopes using adaptive
optics.
Pluto's thin atmosphere is most likely nitrogen and carbon
monoxide, in equilibrium with solid nitrogen and carbon monoxide
ices on the surface. As Pluto moves away from its perihelion and
farther from the Sun, more of its atmosphere freezes. When it
returns to a closer proximity to the sun, the temperature of
Pluto's solid surface will increase, causing the nitrogen ice to
sublimate into gas - creating an anti-greenhouse effect. Much as
sweat evaporating from the surface of human skin, this sublimation
has a cooling effect on the planet and scientists have recently
discovered [9], by use of the Submillimeter Array, that Pluto's
temperature is 10 kelvins less than they expected.
Pluto was found to have an atmosphere from an occultation
observation in 1988. When an object with no atmosphere occults a
star, the star abruptly disappears; in the case of Pluto, the star
dimmed out gradually. From the rate of dimming, the atmosphere was
determined to have a pressure of 0.15 Pa, roughly 1/700,000 that of
earth.
In 2002, another occultation of a star by Pluto was observed and
analyzed by teams led by Bruno Sicardy of the Paris Observatory
[10] and by Jim Elliot of MIT [11] and Jay Pasachoff of Williams
College [12]. Surprisingly, the atmosphere was estimated to have a
pressure of 0.3 Pa, even though Pluto was further from the Sun than
in 1988, and hence should be colder and have a less dense
atmosphere. The current best hypothesis is that the south pole of
Pluto came out of shadow for the first time in 120 years in 1987,
and extra nitrogen sublimated from a polar cap. It will take
decades for the excess nitrogen to condense out of the
atmosphere.
