Pluto
| I | INTRODUCTION |
Pluto, planetary body counted as the ninth planet in
the solar system after its discovery in 1930. In 2006 the International
Astronomical Union (IAU) reclassified Pluto as a dwarf planet. The new IAU
definition of a planet that changed Pluto's status is not accepted by some
scientists, who continue to recognize Pluto as the ninth planet. Pluto revolves
about the Sun once in 247.9 Earth years at an average distance of 5,880 million
km (3,650 million mi). Pluto’s orbit is so eccentric that at certain points
along its path Pluto is slightly closer to the Sun than is Neptune. Pluto is
about 2,360 km (1,475 mi) in diameter, about two-thirds the size of Earth’s
moon. Its composition links Pluto with Kuiper Belt Objects (KBOs), icy bodies
found beyond Neptune in the outer solar system. The first space mission to
explore Pluto, NASA’s New Horizons spacecraft, was launched in January 2006 and
is scheduled to fly by Pluto in 2015. Pluto was named after the god of the
underworld in Roman mythology.
| II | OBSERVATION FROM EARTH |
All the information astronomers have on Pluto
comes from observation through large telescopes. Pluto was discovered as the
result of a telescopic search inaugurated in 1905 by American astronomer
Percival Lowell, who postulated the existence of a distant planet beyond Neptune
as the cause of slight irregularities in the orbits of Uranus and Neptune.
Continued after Lowell’s death by members of the Lowell Observatory staff, the
search appeared to end successfully in 1930, when American astronomer Clyde
William Tombaugh found Pluto near where Lowell predicted another planet. Pluto
became the ninth and most distant known planet in the solar system. However, the
new planet posed a puzzle—Pluto appeared to be too small to affect the orbits of
Uranus and Neptune. Astronomers later detected errors in Lowell’s calculations
and determined that the irregularities Lowell noted in the orbits do not exist.
Clyde Tombaugh’s discovery of Pluto now is seen as a lucky accident that
rewarded hard work.
For many years very little was known about
Pluto, but in 1978 astronomers discovered a relatively large moon orbiting Pluto
at a distance of only about 19,600 km (about 12,180 mi) and named it Charon. The
orbits of Pluto and Charon caused them to pass repeatedly in front of one
another as seen from Earth between 1985 and 1990, enabling astronomers to
determine their sizes accurately. Charon is about 1,200 km (750 mi) in diameter,
making Pluto and Charon the parent-satellite pair closest in size to one another
in the solar system. Some scientists have called Pluto and Charon a double
planet.
Pluto and Charon are tidally locked, meaning
that they always keep the same face toward each other as they rotate. The
lengths of Charon's “day” and “month” (one orbit) are thus the same as Pluto's
“day” (6.4 Earth days). Charon's orbit is also retrograde (clockwise viewed from
Pluto’s north pole). Although Charon is thought of as a satellite of Pluto, both
in fact orbit around a common center of mass (called a barycenter) that is
situated in space between the two bodies.
Astronomers using the Hubble Space Telescope
(HST) detected two more small moons orbiting Pluto beyond Charon in late 2005.
Follow-up observations with the HST in February 2006 confirmed the existence of
the moons, which orbit Pluto in the same plane and same direction as Charon at
distances of about 49,000 km (about 30,400 mi) and about 65,000 km (about 40,380
mi). The two moons later were officially named Nix and Hydra, in outward order.
Additional observations with the HST in March 2006 showed that the two moons are
the same color as Charon. The three moons have a neutral color, like Earth’s
moon, and contrast with Pluto, which has a pinkish hue.
The findings that all three moons orbit in
the same plane and have the same color lend support to the theory that Pluto’s
system formed from the collision of two large bodies about 4.6 billion years
ago. Pluto survived the collision, according to this theory, and the material
that afterward was thrown into orbit around Pluto eventually formed Charon and
the two newly discovered moons.
Every 248 years Pluto’s elliptical orbit
brings it within the orbit of Neptune. Pluto last traded places with Neptune in
1979 and crossed back outside Neptune’s orbit in 1999. No possibility of
collision exists, however, because Pluto's orbit is inclined more than 17.2° to
the plane of the ecliptic (the plane in which Earth and most of the planets
orbit the Sun) and is oriented such that it never actually crosses Neptune’s
path. Neptune completes three orbits around the Sun in the time it takes Pluto
to make two orbits.
In 1988 astronomers discovered that Pluto has
a thin atmosphere consisting of nitrogen with traces of carbon monoxide and
methane. Atmospheric pressure on Pluto’s surface is about 100,000 times less
than Earth’s atmospheric pressure at sea level. Pluto’s atmosphere is believed
to freeze out as a snow on its surface for most of each Plutonian orbit. During
the decades when Pluto is closest to the Sun, however, the snows sublimate
(evaporate) and create the atmosphere that has been observed. In 1994 the HST
imaged 85 percent of Pluto’s surface, revealing polar caps and bright and dark
areas of startling contrast. Astronomers believe that the bright areas are
likely to be shifting fields of clean ice and that the dark areas are fields of
dirty ice colored by interaction with sunlight. These images show that extensive
ice caps form on Pluto’s poles, especially when it is farthest from the Sun.
Pluto only receives about one-thousandth the amount of light from the Sun that
the Earth does. Its surface temperature, according to measurements reported by
astronomers in January 2006, is around -230°C (-382°F).
| III | ORIGIN OF PLUTO |
With a density about twice that of water,
Pluto is apparently made of a much greater proportion of rockier material than
are the giant planets of the outer solar system. This may be the result of the
kind of chemical reactions that took place during Pluto’s formation under cold
temperatures and low pressure. Many astronomers think Pluto was growing rapidly
to be a larger planet when Neptune’s gravitational influence disturbed the
region where Pluto orbits, a region known as the Kuiper Belt, stopping the
process of planetary growth there. The Kuiper Belt is a ring of material
orbiting the Sun beyond the planet Neptune that contains millions of rocky, icy
objects like Pluto and Charon. Charon could be an accumulation of the lighter
materials resulting from a collision between Pluto and another large Kuiper Belt
Object (KBO) in the ancient past. Scientists are interested in Pluto and other
KBOs because they likely represent the type of primitive bodies and material
from which the solar system formed.
| IV | CLASSIFICATION OF PLUTO |
In recent decades some scientists began to
challenge Pluto’s classification as a planet. Its much smaller size, icy,
cometlike composition, and association with the Kuiper Belt set it apart from
the other recognized planets in the solar system. The term ice dwarf was
suggested to distinguish Pluto from the terrestrial planets (Mercury, Venus,
Earth, and Mars) and the gas giants (Jupiter, Saturn, Uranus, and Neptune).
Arguments in support of continuing to call Pluto a planet despite its small size
included its spherical shape, its differentiated internal structure with a rocky
core surrounded by layers of ice, the presence of an atmosphere, and a set of
moons. The strongest argument, however, may have been historical: People have
called Pluto a planet for more than 75 years.
In 2005 astronomers announced the discovery
of an icy planetlike body slightly larger than Pluto and with a more distant
orbit around the Sun. The new object (now officially called Eris) was given the
temporary scientific designation 2003 UB313 and the popular nickname Xena. 2003
UB313 was the largest known of a series of planetlike KBOs found in recent years
and was an indication that the outer reaches of the solar system may contain
hundreds of such Pluto-like planetary bodies. If Pluto ranked as the ninth
planet, then the larger 2003 UB313 logically could qualify as the tenth planet.
Whether it would be officially recognized as the tenth planet depended on how
Pluto was classified.
The status of Pluto drew world attention in
2006 when the official body that governs the naming of astronomical objects, the
International Astronomical Union (IAU), voted for an official definition of the
term planet. According to the standards adopted, a “classical planet”
must orbit the Sun, must have a rounded shape from effects of its own gravity,
and must be the dominant object in its region of space, having cleared the
neighborhood of its orbit of other bodies as it formed. The IAU erected a new
category called dwarf planets for bodies that orbited the Sun and had a rounded
shape from their own gravity, but were not massive enough to have cleared other
objects from around their orbits.
The new IAU definitions removed Pluto from
the solar system’s “classical planets,” now reduced to eight, and made it a
dwarf planet along with Eris (2003 UB313) and Ceres (formerly the largest
asteroid). Pluto and Eris orbit through the Kuiper Belt, while Ceres orbits in
the asteroid belt between Mars and Jupiter, a zone scattered with thousands of
small rocky bodies. The IAU also made Pluto the prototype of a new class of
trans-Neptunian objects (TNOs) that includes Eris, Sedna, Quaoar, and a number
of other major KBOs. In a further move, the IAU gave Pluto the official minor
planet catalog number 134340.
The IAU rulings came after much debate,
including a scrapped proposal to recognize Pluto, Charon, Ceres, and Eris as
true planets. The results of the final IAU vote were controversial and met
strong opposition from a range of astronomers and planetary scientists, some of
whom refused to recognize or use the new definitions. Possible future actions
may modify the IAU rulings on Pluto’s status.
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