Space Topics: Pluto Top Ten
Pluto Is Not a Planet
© Neil deGrasse Tyson 1999
excerpt from an opinion written for Natural History Magazine, February
1999
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Neil deGrasse Tyson
Chairman of the Board of The Planetary Society |
In an informal poll of 10,000 junior-high-school children, Pluto was the
overwhelming favorite among the 9 planets. The poll was simply a
measure of how much noise the children made during a tour of the solar
system in a planetarium show I presented live to groups of 500 children
at a time. They consistently cheered the loudest for Pluto, especially
when I recited the planets in sequence, aided by the time-honored mnemonic
My Very Educated Mother Just Served Us Nine Pizzas.
But Pluto has “peculiar” written all over it. Found
by Lowell Observatory astronomer Clyde W. Tombaugh in 1930, Pluto was discovered
the same year that Walt Disney created the lovable, slow-witted bloodhound
that shares its name.
Pluto’s orbit is tilted 17 degrees out of the plane of the solar system,
2 1/2 times that of Mercury. Pluto moves in the most eccentric ellipse
and is the only planet whose orbit crosses that of another planet. Pluto
has tidally locked the rotation of its moon Charon, forcing it to forever
show the same face to Plutonians. Pluto is in good company here. Earth
has tidally locked the rotation of its moon (the Moon) so that it always shows
the same face to Earthlings. The embarrassing part is that Charon is
so large compared with Pluto that its tidal forces have tidally locked Pluto’s
rotation where both moon and planet show the same side to each other as they
waltz forever in space. With a diameter of 1,400 miles, Pluto is, by far,
the smallest planet. Seven moons in the solar system are larger:
Io, Europa, Ganymede, Callisto, Titan, Triton, and of course, Earth’s
Moon (although Mercury is smaller than both Ganymede and Titan). Finally,
neither rocky, nor gaseous, Pluto is the only planet made primarily of
ices.
Maybe Pluto isn’t really a planet.
Dare I have made such a suggestion when Clyde Tombaugh’s body is barely
cold? Tombaugh died in 1997, at the age of ninety, seemingly secure
in his status as the third person ever to discover a planet in our solar system. But
there is no question that if Pluto were discovered today, it would not be
classified as a planet.
William Herschel discovered Uranus in 1781, and Johann Galle discovered Neptune
in 1846. Few people know, however, that Giuseppe Piazzi discovered the
planet Ceres in 1801, orbiting the Sun between Mars and Jupiter. But
astronomers rapidly determined that Ceres was much, much smaller than any
other planet: at 600 miles in diameter, it was dwarfed by Mercury, the
reigning smallest planet. Maybe an object can be too small to be defined as
a planet. Shortly after 1801, other small objects were found in orbits
similar to that of Ceres. A new class of object had been identified:
the rocky asteroids
Ceres was discovered first because it is the brightest and largest. At
twice the mass of all the other asteroids combined, of which there are thousands
known and millions that await discovery, Ceres swiftly went from being the
smallest in the class of planet to being the largest in the class of asteroid.
How about Pluto? The more we learned about Pluto, the more it did not fit
any reasonable classification scheme that applied to the other planets. It
was in a class by itself. But can you have a class of one? Should
you have a class of one?
In 1992, David Jewitt of the University of Hawaii and Jane Luu of Harvard
began to discover icy bodies just beyond the orbit of Neptune. Since
then, nearly a thousand such objects have been discovered with similar properties: They
are small, they are icy, they all orbit just beyond Neptune, they have somewhat
eccentric paths, and their orbits are tipped out of the plane of the solar
system. This new class of objects was duly named the Kuiper belt, in
honor of the Dutch-born American astronomer Gerard Kuiper, who in the 1950s
advanced the idea that such a belt of comets might exist.
Alas, Pluto, which is small and icy and orbits just beyond Neptune and has
an eccentric orbit that is tipped out of the plane of the solar system, is
none other than a Kuiper belt object—a leftover comet from the solar
system’s formation. If Pluto’s orbit were ever altered so
that it journeyed as close to the Sun as Earth, Pluto would grow a tail and
look like a jumbo comet. No other planet can make this (possibly embarrassing)
claim.
I must vote—with a heavy heart—for demotion.
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