Why is Pluto not a planet anymore?

Why is Pluto not a planet anymore?

Space Topics: Pluto

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.