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Pluto and its Kuiper belt companions
continue to make news here on Earth.
Is Pluto the tiniest planet or the second
largest member of the Kuiper belt? Is Charon just a really large moon, or are Pluto and Charon a double planet? What does
this distant pair of icy bodies look like up close? There are far more questions than there are answers about a pair of objects
that mark the outer edge of the orderly Solar System, and the inner edge of a vast region of frigid worlds, the Kuiper belt
and Oort cloud, home of comets and, possibly, of many more planet-sized bodies.
Pluto is small relative to the
other planets. In fact, there are a total of 15 bodies in the solar system larger than Pluto -- all eight of the other planets,
plus the moons Ganymede, Titan, Callisto, Io, Earth's Moon, Europa, and Triton. But Pluto makes up for its small size by having
the most gargantuan moon -- compared to its planet -- in the solar system. Charon is a whopping half the size of Pluto. (The
next largest duo is the Earth-Moon pairing, but our Moon is only a quarter the size of Earth, and even that is a standout
among moon-planet pairs in the solar system.) Tidal effects that Pluto and Charon exert on each other have locked the two
bodies into mutual spin-orbit resonance, meaning that one face of Charon always looks at Pluto while one face of Pluto always
looks at Charon; it's as though the two bodies form a dumbbell spinning in space. Recently, two more small moons, Nix and
Hydra, were discovered orbiting Pluto and Charon in their own (likely) resonant orbits.
Discovery photographs
of PlutoIn the 1840s, using Newtonian mechanics, Urbain Le Verrier predicted the position of the then-undiscovered planet
Neptune after analysing perturbations in the orbit of Uranus. Subsequent observations of Neptune in the late 19th century
caused astronomers to speculate that Uranus' orbit was being disturbed by another planet besides Neptune. In 1906, Percival
Lowell, a wealthy Bostonian who had founded the Lowell Observatory in Flagstaff, Arizona in 1894, started an extensive project
in search of a possible ninth planet, which he termed "Planet X". By 1909, Lowell and William H. Pickering had suggested
several possible celestial coordinates for such a planet. Lowell and his observatory conducted his search until his death
in 1916, but to no avail. Unbeknownst to Lowell, on March 19, 1915, his observatory had captured two faint images of Pluto,
but did not recognise them for what they were.
Due to a ten-year legal battle with Constance Lowell, Percival's
widow, who attempted to wrest the observatory's million-dollar portion of his legacy for herself, the search for Planet X
did not resume until 1929, when its director, Vesto Melvin Slipher, summarily handed the job of locating Planet X to Clyde
Tombaugh, a 23-year-old Kansas man who had just arrived at the Lowell Observatory after Slipher had been impressed by a sample
of his astronomical drawings.
Pluto's volume is about 0.66%
that of EarthPluto's mass is 1.31×1022 kg, less than 0.24 percent that of the Earth, while its diameter is 2,306 (+/-
20) km, or roughly 66% that of the Moon. Astronomers, assuming Pluto to be Lowell's Planet X, initially calculated its mass
based on its presumed effect on Neptune and Uranus. In 1955 Pluto was calculated to be roughly the mass of the Earth, with
further calculations in 1971 bringing the mass down to roughly that of Mars. However, in 1976, Dale Cruikshank, Carl Pilcher
and David Morrison of the University of Hawaii calculated Pluto's albedo for the first time, finding that it matched that
for methane ice; this meant Pluto had to be exceptionally luminous for its size and therefore could not be more than 1 percent
the mass of the Earth.
The discovery of Pluto's satellite Charon in 1978 enabled a determination of the mass of
the Pluto–Charon system by application of Newton's formulation of Kepler's third law. Once Charon's gravitational effect
was measured, Pluto's true mass could be determined. Observations of Pluto in occultation with Charon allowed scientists to
establish Pluto's diameter, while the invention of adaptive optics allowed them to determine its shape accurately.
Among
the objects of the Solar System, Pluto is smaller and much less massive than the terrestrial planets, and at less than 0.2
lunar masses it is also less massive than seven moons: Ganymede, Titan, Callisto, Io, Earth's Moon, Europa and Triton. Pluto
is more than twice the diameter and a dozen times the mass of the dwarf planet Ceres, the largest object in the asteroid belt.
However, it is smaller than the dwarf planet Eris, a trans-Neptunian object discovered in 2005.
CRIRES model-based
computer-generated impression of the Plutonian surface by ESO—L. Calçada, with atmospheric haze, and Charon and
the Sun in the sky.Pluto's atmosphere consists of a thin envelope of nitrogen, methane, and carbon monoxide gases, which are
derived from the ices of these substances on its surface. Its surface pressure ranges from 6.5 to 24 µbar. Pluto's elongated
orbit is predicted to have a major effect on its atmosphere: as Pluto moves away from the Sun, its atmosphere should gradually
freeze out, and fall to the ground. When Pluto is closer to the Sun, the temperature of Pluto's solid surface increases, causing
the ices to sublimate into gas. This creates an anti-greenhouse effect; much as sweat cools the body as it evaporates from
the surface of the skin, this sublimation cools the surface of Pluto. Scientists using the Submillimeter Array have recently
discovered that Pluto's temperature is about 43 K (-230 °C), 10 K colder than would otherwise be expected.
The
presence of methane, a powerful greenhouse gas, in Pluto's atmosphere creates a temperature inversion, with average temperatures
36 K warmer 10 km above the surface. The lower atmosphere contains a higher concentration of methane than its upper atmosphere.
The first evidence of Pluto's atmosphere was found by the Kuiper Airborne Observatory in 1985, from observations of
the occultation of a star behind Pluto. When an object with no atmosphere moves in front of a star, the star abruptly disappears;
in the case of Pluto, the star dimmed out gradually. From the rate of dimming, the atmospheric pressure was determined to
be 0.15 pascal, roughly 1/700,000 that of Earth. The conclusion was confirmed and significantly strengthened by extensive
observations of another similar occultation in 1988.
Pluto Size: 9th largest planet - 2,306 kilometers - 0.181 Earths
Charon Size: 22nd largest body in the solar system - 1,205 kilometers - 0.094 Earths
Calendar: 1 Pluto/Charon
year = 248.02 Earth years; 1 Pluto/Charon day = 6.387 Earth days
Orbit: 5,906,380,000 kilometers - 39.482 Earth orbits
Axial tilt: 122.53 degrees (retrograde, and tilted 9.08 degrees more than Earth's)
Number of moons: 2 or 3, depending
on whether Pluto-Charon is a double planet or Charon is Pluto's moon
Tombaugh's task was to systematically image the night sky in pairs of
photographs taken two weeks apart, then examine each pair and determine whether any objects had shifted position. Using a
machine called a blink comparator, he rapidly shifted back and forth between views of each of the plates, to create the illusion
of movement of any objects that had changed position or appearance between photographs. On February 18, 1930, after nearly
a year of searching, Tombaugh discovered a possible moving object on photographic plates taken on January 23 and January 29
of that year. A lesser-quality photograph taken on January 21 helped confirm the movement. After the observatory obtained
further confirmatory photographs, news of the discovery was telegraphed to the Harvard College Observatory on March 13, 1930.
The object was officially named on March 24, 1930. Each member of the Lowell Observatory was allowed to vote on a
short-list of three: Minerva (which was already the name for an asteroid), Cronus (which had lost reputation through being
proposed by the unpopular astronomer Thomas Jefferson Jackson See), and Pluto. Pluto received every vote. The name was announced
on May 1, 1930. Upon the announcement, Madan gave Venetia five pounds as a reward. The name was soon embraced by wider culture.
The Disney character Pluto, introduced in 1930, was named in the object's honour. In 1941, Glenn T. Seaborg named the newly
created element plutonium after Pluto, in keeping with the tradition of naming elements after newly discovered planets, such
as uranium, which was named after Uranus, and neptunium, which was named after Neptune.
This 'side view' of Pluto's
orbit (in red) shows its large inclination to Neptune's orbit (in blue). The ecliptic is horizontal
This diagram shows
the relative positions of Pluto (red) and Neptune (blue) on selected dates. The size of Neptune and Pluto is depicted as inversely
proportional to the distance between them to emphasise the closest approach in 1896.Pluto's orbital period lasts for 248 Earth
years. Its orbital characteristics are substantially different from those of the planets, which follow nearly circular orbits
around the Sun close to a flat reference plane called the ecliptic. In contrast, Pluto's orbit is highly inclined relative
to the ecliptic (over 17°) and highly eccentric (elliptical). This high eccentricity means a small region of Pluto's orbit
lies nearer the Sun than Neptune's. Pluto was last interior to Neptune's orbit between February 7, 1979 and February 11, 1999.
Detailed calculations indicate that the previous such occurrence lasted only fourteen years, from July 11, 1735 to September
15, 1749, whereas between April 30, 1483 and July 23, 1503, it had also lasted 20 years.
Although this repeating
pattern may suggest a regular structure, in the long term Pluto's orbit is in fact chaotic. While computer simulations can
be used to predict its position for several million years (both forward and backward in time), after intervals longer than
the Lyapunov time of 10–20 million years, calculations become speculative: Pluto's tiny size makes it sensitive to unmeasurably
small details of the solar system, hard-to-predict factors that will gradually disrupt its orbit. Millions of years from now,
Pluto well may be at aphelion, at perihelion or anywhere in between, with no way for us to predict which. This does not mean
Pluto's orbit itself is unstable, but its position on that orbit is impossible to determine so far ahead. Several resonances
and other dynamical effects keep Pluto's orbit stable, safe from planetary collision or scattering.
Pluto's rotation
period, its day, is equal to 6.39 Earth days. Like Uranus, Pluto rotates on its "side" on its orbital plane, with
an axial tilt of 120°, and so its seasonal variation is extreme; at its solstices, one hemisphere is in permanent daylight,
while the other is in permanent darkness.
Hubble map of Pluto's surface, showing great variations in color and
albedo. Three hemispheres of PlutoPluto's distance from Earth makes in-depth investigation difficult. Many details about Pluto
will remain unknown until 2015, when the New Horizons spacecraft is expected to arrive there.
Pluto's visual apparent
magnitude averages 15.1, brightening to 13.65 at perihelion. To see it, a telescope is required; around 30 cm (12 in) aperture
being desirable. It looks star-like and without a visible disk even in large telescopes, because its angular diameter is only
0.11".
RockObservations by the Hubble Space Telescope place Pluto's density at between 1.8 and 2.1 g/cm³, suggesting
its internal composition consists of roughly 50–70 percent rock and 30–50 percent ice by mass. Because decay of
radioactive minerals would eventually heat the ices enough for the rock to separate from them, scientists expect that Pluto's
internal structure is differentiated, with the rocky material having settled into a dense core surrounded by a mantle of ice.
The diameter of the core should be around 1,700 km, 70% of Pluto's diameter. It is possible that such heating continues today,
creating a subsurface ocean layer of liquid water some 100 to 180 km thick at the core–mantle boundary. The DLR Institute
of Planetary Research calculated that Pluto's density-to-radius ratio lies in a transition zone, along with Neptune's moon
Triton, between icy satellites like the mid-sized moons of Uranus and Saturn, and rocky satellites such as Jupiter's Europa.
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