Pluto's Demotion
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As of today, the Solar System officially (thank goodness)
contains 8 “classical” planets.
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The body that governs the naming of astronomical objects has
officially decided:
"A planet is a celestial body that (a) has sufficient mass for its
self-gravity to overcome rigid-body forces so that it assumes a
hydrostatic-equilibrium (nearly round) shape, and (b) is the
dominant object in its local population zone, and (c) is in orbit
around the Sun."
The Discovery of the Asteroids (ca. 1800)
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On January 1, 1801 an object (Ceres), much smaller than
the Earth's Moon, was discovered orbiting the Sun
between the orbits of Mars and Jupiter.
At first it was thought to be the 8th planet (only 7 were
known at the time), but...
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On March 28, 1802 another object (Pallas) was discovered in
nearly the same orbit, and another (Juno) in 1804, and yet
another (Vesta) in 1807.
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In 1828 textbooks listed 11 planets, however by 1851 twelve(!)
more of these objects had been found.
Finally, in 1852, astronomers recognized these
“asteroids” as a separate class of object and went back
to a Solar System with eight planets (Neptune having
been discovered in 1846).
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The asteroids were tiny compared with the “classical” planets
See http://aa.usno.navy.mil/hilton/AsteroidHistory/minorplanets.html
The Discovery of Pluto in 1930
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Pluto was discovered in 1930 during a deliberate search
for a planet beyond Neptune.
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When it was discovered it was thought to be larger than the
planet Mercury.
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All hailed it as the Ninth Planet in the Solar System.
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That designation stuck until 1992 when the first of hundreds of
similar objects were found orbiting the Sun beyond Neptune.
Pluto's Stock Plummets
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By 2000 it was apparent that Pluto was simply the
largest object yet discovered in an outer (icy) asteroid
belt.
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Astronomers counted the days until an object larger than Pluto
was discovered in this region.
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It happened with the discovery of 2003UB313 (nicknamed
Xena) in 2003.
Astronomers now had to cope with the issue. Was
2003UB313 Planet 10, or was it time to demote Pluto to
non-planetary status.
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Yesterday, the vote was official – The Solar System has Eight
planets.
Why Pluto Never Had a Chance
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Bottom line – the Solar System has Four well
defined zones
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The rocky terrestrial planets – Mercury,
Venus, Earth, Mars
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The asteroid belt
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The immense gas giant worlds – Jupiter,
Saturn, Uranus, Neptune.
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The outer icy asteroid (Kuiper) belt –
containing Pluto and hundreds of
thousands of other objects.
But Pluto's Status as an Interesting
World has not Changed
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Pluto has a large Moon, Charon, and an atmosphere.
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Two new smaller moons were discovered just last year.
The “New Horizons” spacecraft (the fastest thing ever
launched by Humans) will arrive in 2015.