Pluto Scientists calculated that Neptune

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PlutoScientists calculated that Neptunes
gravity was not enough to cause all of the
irregularities of Uranus orbit.
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They also found that something else was
perturbing Neptunes orbit.
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Percival Lowell calculated the probable location
of this third body and searched for it in the
decade prior to his death in 1916. He never
found planet X.
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In 1930 Clyde Tombaugh found the planet only 6
away from Lowells predicted location. He
announced his discovery on March 13, 1930. (March
13 was Lowells birthday)
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Neil deGrasse Tyson with Clyde Tombaughs Wife
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These are the actual photos Clyde Tombaugh used
to locate Pluto.
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Actually Pluto is too small to cause the
perturbations calculated for Uranus and Neptune.
Also, these calculations were later found to be
in error, so the discovery of Pluto was nothing
more than dumb luck.
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Pluto in bulkSemi-major axis 39.5
A.U.Eccentricity 0.25 (very large)
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Plutos orbit is inclined 17.2 to the ecliptic,
more than any other planet. Plutos orbital
period is 248.6 years (1.5 times Neptunes).
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Plutos orbital path carries it inside the orbit
of Neptune at perihelion. So, at times, Pluto is
the eighth planet from the Sun rather than the
ninth, as it was from 1979 to 1999.
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Pluto has three known moons, Charon was the
first one discovered. The other two, Nix and
Hydra have been discovered very recently.
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Charons orbit is inclined 118 to Plutos
orbital plane, so Charon orbits at a right angle
to Plutos orbit around the Sun.
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Plutos rotation angle is tilted at 120, so the
moon still orbits over the equator of Pluto.
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Charons orbital period was used to find the mass
of Pluto. Charons orbit was lined up such that
there were a series of Charon-Pluto eclipses
from 1985 to 1991. (This only happens once
every 124 years.)
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These eclipses helped astronomers to find the
mass, radii and other information about Pluto and
Charon.
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Plutos mass is 0.0025 Earth masses. Remember,
Earth is one of the smaller planets so, in
size, Pluto is more like a moon than a planet.
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Pluto is smaller than seven solar system moons
Earths Moon plus the moons Io, Europa, Ganymede,
Callisto, Titan, and Triton.
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Plutos diameter is 2300 km. (one-fifth that of
Earth)Charons diameter is 1300 km. This is
over half the diameter of Pluto.
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Pluto and Charon are tidally locked. The same
side of Charon always faces Pluto, and the same
side of Pluto always faces Charon.
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Charons orbital period is 6.4 Earth days.
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Charons mass is about one-sixth that of Pluto.
Were Pluto still a planet, that would be the
largest satellite to planet ratio in the solar
system. Now that honor belongs the the Earth
and Luna.
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Pluto is similar in mass and radius (and
therefore, probably in composition) to the
Neptunian moon Triton.
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Pluto has a high albedo, 0.6, which aided in its
detection.
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Frozen CH4 is a major surface constituent. Pluto
is the only planet cold enough to have solid CH4.
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Pluto is so different from the Jovian planets and
is so much like the moons of these planets, it
was thought to be an escaped moon of one of the
gas giants.
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This was a good explanation, until Charon was
found. It is unlikely that two lost moons would
come together to form the Pluto-Charon system.
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There may have been lots of ice chunks like Pluto
orbiting the Sun at one time. Many of them
eventually captured to form Jovian moons while
another formed Charon. But, all of this is just
speculation.
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Pluto was removed as a planet when the definition
of a planet was officially defined. A planet
must (1) orbit the Sun, (2) be large enough to
be formed into a sphere by gravity, and
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(3) it must have enough gravity to clear its
orbit of other large objects. Pluto fails to meet
this last qualification.
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Many trans-Neptunian objects have been found at
orbits beyond Neptunes. One, Eris, is actually
larger than Pluto.
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The discovery of Eris was the last straw in the
reassignment of Pluto to dwarf planet status.
(One asteroid, Ceres, is also considered a
dwarf planet.)
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New Horizons launched in 2006, scheduled to
arrive at Pluto in July, 2015.
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