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Pluto 1
Name – In Roman mythology, Pluto (Greek: Hades) is the god of the underworld of
the dead.
Discovery – The idea that a distant, ninth planet might exist emerged shortly after the
discovery of Neptune. The person most identified with a search for “Planet X” was
Percival Lowell. Lowell and his colleagues conducted a photographic search for such
a planet for years without success. The search resumed in 1929, thirteen years after
Lowell’s death. Clyde W. Tombaugh 2, a young astronomer, was hired to take an
exhaustive series of photographs and to examine them for objects that were not stars.
He exposed 14-by-17-inch plates by night, and by day, examined the images in an
instrument called a blink comparator. By rapidly alternating a magnified view of two
photographs of the same area of the sky taken at different times, any object that was
not a distant star appeared to move against the background star field. Clyde
Tombaugh’s diligence and patience was rewarded when a 15th-magnitude speck
moved on photographic plates taken on January 23 and January 29 of 1930 3. The
discovery of the ninth planet was announced six weeks later on March 13 (Percival
Lowell’s birthday).
Spacecraft Exploration – The New Horizons spacecraft, which lifted off from Cape
Canaveral, Florida on January 19, 2006, became the first spacecraft to fly by Pluto on
July 14, 2015.
Location – Pluto’s average distance from the Sun is approximately 3,670,000,000
miles. Seen at this vast the distance, the Sun would be about 1,000 times fainter than
it appears from Earth 4.
Size – Pluto measures 1,473 miles in diameter.
Orbital Period – 247.7 years.
High Orbital Eccentricity and Inclination – Pluto is usually farther from the Sun than
the planet Neptune. However, due to the high eccentricity of its orbit 5, it is closer
than Neptune for about 20 years out of its 247.7-year orbit. Pluto crossed Neptune’s
orbit January 21, 1979, made its closest approach September 5, 1989, and remained
within the orbit of Neptune until February 11, 1999. This will not occur again until
September 2226. Moreover, as Pluto approaches perihelion, it reaches its maximum
distance from the ecliptic due to its 17.14º orbital inclination 6.
Rotational Period – 6.39 days (retrograde).
Inclination of Rotation Axis – 119.6º. Like Uranus, Pluto rotates with its poles
almost in its orbital plane.
Number of Satellites – 5 known moons.
Atmosphere 7
Pluto has a thin atmosphere consisting of nitrogen (N2), methane (CH4), and carbon
monoxide (CO), which are in equilibrium with their ices on Pluto’s surface. The surface
pressure ranges from about one million to 100,000 times less than Earth’s atmospheric
pressure. Pluto’s elliptical orbit is predicted to have a major effect on its atmosphere: as
Pluto moves away from the Sun, its atmosphere should gradually freeze out. When Pluto
is closer to the Sun, the temperature of Pluto’s solid surface increases, causing the ices to
Composition – Pluto’s surface is composed of more than 98% nitrogen ice, with
traces of methane and carbon monoxide. Pluto’s surface is quite varied, with large
differences in both brightness and color. The color varies between charcoal black,
gray, and dark orange. This coloration is a result of hydrocarbons called tholins,
which form when solar ultraviolet light and cosmic rays interact with methane in
Pluto’s atmosphere (which precipitate out onto the surface) and its crust.
Features – Notable geographical features of Pluto’s surface include a large bright area
called Tombaugh Regio (the Heart) 8 and a dark, cratered terrain named Cthulhu
Regio (the Whale) 9. Forming the the left lobe of Tombaugh Region is Sputnik
Planum, a craterless, icy plain. Images taken by the New Horizons spacecraft
revealed a number of features associated with the plain:
Polygonal Structure 10 – Patterns of polygonal ice displaying wind-blown streaks
indicating geysers and pitting resulting from the sublimation of ice.
Nitrogen-Ice Glaciers 11 – Glaciers of what is probably nitrogen ice can be seen
flowing into valleys and craters at the edge of the plain; the valleys appear to have
been formed through erosion.
Water-Ice Mountains 12 – Mountains nearly two miles high have been found along
the southwestern and southern edges of Sputnik Planum. Water-ice is the only ice
detected on Pluto that is strong enough at Plutonian temperatures (nearly -400º F)
to support such heights.
Cryovolcanism – Two possible cryovolcanoes, provisionally named Wright Mons 13
and Piccard Mons, have been identified near the south pole. Both are nearly 100
miles across and at least 2.5 miles high, the tallest peaks known on Pluto at present.
They are lightly cratered and thus geologically young.
Pluto’s size and density (1.87 g/cm3) suggest that it is composed of about 70% rock and
30% ice (mainly water-ice). Though Pluto is a small body, a sufficient amount of heat to
melt the ice for differentiation was most likely generated by the decay of radioactive
material associated with its high rock content. Modeling of Pluto’s internal structure
consists of a rocky core overlain by a mantle and crust of ice 14. It is possible that
sufficient heating continues today, creating a layer of liquid water at the core-mantle
Escaped Moon – In an early hypothesis, Pluto was an escaped moon of Neptune,
knocked out of orbit by its current moon, Triton. However, the 3:2 orbital resonance
between Neptune and Pluto makes this hypothesis implausible. Specifically, Neptune
completes three 165-year orbits as Pluto completes two 247.7-year orbits; and
computer simulations of the motions of these worlds back to the origin of the solar
system show that they have never been close, nor can they ever be.
Trans-Neptunian Object – Researchers have proposed a theory that links Pluto to
trans-Neptunian objects (TNOs). A TNO is any object in the solar system that orbits
the Sun at a greater distance on average than Neptune. They are remnants from the
Solar System’s formation. The majority of TNOs are thought to be low-density
mixtures of rock and frozen volatiles (ices), such as methane, ammonia, and water,
with some organic (carbon-containing) surface material such as tholin. Hundreds of
TNOs have been discovered since the early 1990s 15. Some of these TNOs are quite
large, and a few of them have moons. Like Pluto, approximately one-quarter of
TNOs have 3:2 orbital resonances with Neptune. They are called “plutinos”.
Computer simulations of the formation of the solar system suggest that gravitational
interactions between the planets and a disk of planetesimals caused the planets to
migrate to more distant orbits. In the process, Neptune trapped a number of these
planetesimals into a 3:2 orbital resonance.
Kuiper Belt – The first astronomers to suggest the existence of a “belt” of
trans-Plutonian objects (note: not Neptune) were Frederick Leonard in 1930 and
Kenneth Edgeworth in 1943. However, Gerard Kuiper in 1951 published a paper
mentioning objects beyond Pluto, and because of this paper, the belt became
generally known as the Kuiper belt. Moreover, the bodies that comprise the
region are referred to as Kuiper belt objects (KBOs). The largest KBOs include
Pluto, its moon Charon, Makemake, and Quaoar (about 800 miles in diameter).
Neptune’s moon Triton and Saturn’s moon Phoebe are commonly thought to be
captured KBOs. Studies have revealed the Kuiper belt region is shaped like a
thick disk, extending outward from the orbit of Neptune at 30 AU to 50 AU.
TNO Divisions – The Kuiper belt, scattered disk, and Oort cloud (discussed later)
are names for divisions of TNOs. The scattered disk is a dynamically unstable
region overlapping the Kuiper belt, and thought to have formed when KBOs were
“scattered” by gravitational interactions with Neptune into highly eccentric and
inclined orbits. The TNO, Eris is classified as a scattered disk object (SDO). Its
orbit is quite eccentric (bringing it to within 37.91 AU of the Sun, a typical
perihelion for SDOs) and extremely inclined (tilted at an angle of about 44º to the
Five Moons 16 – Pluto has five known natural satellites: Charon was first identified in
1978 by astronomer James Christy. Nix and Hydra were discovered in 2005 by the
Hubble Space Telescope Pluto Companion Search Team. The team also discovered
Kerberos in July 2011 and Styx in July 2012. Pluto’s moons are believed to have
been formed by a collision between Pluto and a similar-sized body, early in the
history of the Solar System. The collision released material that consolidated into the
moons. However, Kerberos has a much lower albedo than the other moons of Pluto,
which is difficult to explain with a giant collision.
Charon 17 – Charon has a diameter of about 750 miles (slightly more than half as
large as Pluto) 18. By no means the largest planetary satellite in the solar system, it
has the distinction of being the largest satellite as compared to its primary. Charon
appears not to have an atmosphere. Its surface is covered in large part by water-ice,
making it quite different from that of Pluto. New Horizons revealed very few craters
on Charon, indicating that it is geologically active and young. Its surface also has
several canyons up to 5 miles deep and cliffs extending nearly 600 miles. However,
the most intriguing feature is a depression with a peak in the middle of unknown
origin 19. Charon is mainly an icy body and contains less rock by proportion than
Pluto. This supports the idea that Charon was created by the impact of another body
into Pluto’s icy mantle. There are two conflicting theories about Charon’s internal
structure: some scientists think it to be a differentiated body like Pluto, with a rocky
core and an icy mantle, whereas others think it to be uniform throughout.
Name – In Greek mythology, Charon is the boatman who ferries souls across the
river Styx to the underworld. The ch is pronounced with an initial k sound, like in
chaos. However, James Christy himself pronounced the ch in the moon’s name as
sh, after his wife Charlene. The sh pronunciation is common among astronomers
who speak English, and is the prescribed pronunciation at NASA and of the New
Horizons mission team.
Synchronous Rotation and Revolution 20 – Charon orbits Pluto in the same 6.39
days that Pluto takes to rotate, indicating that Pluto and its satellite are locked in
what is called synchronous rotation and revolution. Like two ballroom dancers,
Pluto and Charon face each other as they waltz around the Sun. It appears that
Charon was able to pull Pluto into a tidal lock because of its large relative mass.
If hypothetical Plutonians happened to live on the hemisphere directed away
from Charon, they wouldn’t even know that Pluto had a moon. Conversely,
for inhabitants of the Charon-facing side, the moon would always be in the
sky at the same place. Some artificial satellites are given “geosynchronous”
orbits so they can function as television relays.
Dwarf Planet 21 – On August 24, 2006, the International Astronomical Union (IAU)
passed a resolution reclassifying Pluto as the ninth planet from the Sun to a new
category of celestial bodies in the solar system referred to as dwarf planets. The IAU
has also classified four other bodies of the solar system as dwarf planets: the asteroid
Ceres (discussed later) and the TNOs Eris, Makemake, and Haumea. The resolution
describes a dwarf planet as an object that: (1) is in orbit around the Sun, (2) has
sufficient mass for its gravity to overcome rigid body forces so that it assumes a
spherical shape, and (3) has not “cleared the neighborhood” around its orbit. The
phrase “clearing the neighborhood” refers to an orbiting body “sweeping out” its
orbital region over time by gravitationally interacting with smaller bodies nearby. In
other words, over many orbital cycles, a large body (planet) will tend to cause small
bodies either to accrete with it, or to be disturbed to another orbit. As a consequence,
it does not then share its orbital region with other bodies of significant size, except for
its own satellites.
Eris 22 – Because Eris appeared to be larger than Pluto (in fact, Eris is slightly
smaller), NASA initially described it as the Solar System’s tenth planet. This,
along with the prospect of discovering other objects of similar size in the future,
motivated the IAU to define the term “planet” more precisely and introduce the
new category of dwarf planet. Eris has an orbital period of 558.04 years. Eris’
average distance from the Sun is 67.78 AU, with a maximum possible distance
(aphelion) of 97.65 AU.
Haumea 23 – Although evidence suggests that it is not spherical, Haumea is
thought to have sufficient gravity to overcome rigid body forces. Its suspected
“ellipsoidal” shape is believed to be a result of an unusually rapid rotation (about
4 hours); similar to a water balloon stretching out when tossed with a spin.
Other Candidates – It is suspected that about 150 known TNOs are dwarf planets.
Furthermore, scientists speculate that thousands of TNOs may exist that can be
classified as dwarf planets.
Charon’s Status – On account of its large mass relative to Pluto, Charon revolves with
Pluto around a center of mass (barycenter) located in space between Pluto and Charon
rather than around a point located within Pluto 24. Since neither object truly orbits the
other, it has been argued that Charon should be considered to be more than just a
satellite of Pluto. Under the IAU draft proposal in 2006, Charon would have been
classified as a planet, since the draft explicitly defined a planetary satellite as one in
which the barycenter lies within the major body. In the final definition, Pluto was
reclassified as a dwarf planet, but the formal definition of a planetary satellite was not
decided upon. Therefore, the status of Charon remains uncertain, as there is presently
no clear definition of what distinguishes a satellite system from a “binary system”.