It's official. Pluto is not a red-blooded planet. As decreed in August by a vote of the General Assembly of the International Astronomical Union, Pluto is now a dwarf.
At first, the IAU seemed ready to defend Pluto. On August 16, the union's seven-member Planet Definition Committee released a draft Planet Definition Resolution, which stated that round objects in orbit around the sun are planets. Pluto is a round object in orbit around the sun. Therefore, Pluto is a planet. This definition would have given everyone the right to utter "Pluto" and "Jupiter" in the same breath, even though Jupiter is 250,000 times larger. The draft resolution would also have opened the door to granting planet status to at least three objects that had, until recently, been considered unworthy.
Plutophiles had about a week to rejoice before the astronomers returned from their deliberation with a change of heart. According to the final IAU definition, a planet must still be round but must also dominate the mass of its orbital zone. In other words, a full-fledged planet must have no competitors in its zone.. Poor Pluto is crowded by thousands of other icy bodies in the outer solar system, some bigger than Pluto itself, so it fails the test. To soothe Pluto's boosters, the IAU elected to call it a dwarf planet, without entirely quantifying what that is.
All this embarrassment stems from a simple problem. The term planet had not been defined since the time of ancient Greece, when the label originated. The word simply meant "wanderer" and referred to the seven prominent celestial objects that moved against the background of stars. They were Mercury, Venus, Mars, Jupiter, Saturn, the sun, and the moon. So influential were these celestial travelers on classical culture that the names of our seven days of the week can be traced back to them.
Life got more complicated in 1543, when Nicolaus Copernicus described a newfangled, heliocentric universe. Instead of remaining stationary in the middle, Earth moved around the sun just like the other bodies. From that moment onward, planet had no official meaning, and astronomers tacitly agreed that whatever orbits the sun is a planet and whatever orbits a planet is a moon.
This would not be a problem if cosmic discoveries had ended in 1543. But shortly thereafter, we learned that comets orbit the sun, too, and are not, as long believed, local atmospheric phenomena. Comets are icy objects on elongated orbits that throw off a long tail of gases as they near the sun. Are they planets too? How about the craggy chunks of rock and metal that orbit the sun in a region between Mars and Jupiter? When Ceres, the first of these objects, was sighted by Giuseppi Piazzi in 1801, everyone called it a planet. With the discovery of dozens more, however, it became clear that this new community of objects deserved its own classification. Astronomers called them asteroids and now have cataloged tens of thousands of them.
Even the traditional planets don't fit into one neat category. Mercury, Venus, Earth, and Mars form a family because they are relatively small and rocky, while Jupiter, Saturn, Uranus, and Neptune are large, gaseous, have many moons, and bear rings.
The story took another twist in 1992, when David C. Jewitt of the University of Hawaii and Jane Luu of MIT began to detect frozen objects on the solar system's fringes, out beyond Neptune. They found a new swath of space traffic, akin to the asteroid belt discovered two centuries before. Known as the Kuiper belt, in honor of the Dutch-born American astronomer Gerard Kuiper, who predicted its existence, this region of the solar system contains Pluto, one of its largest members. But Pluto has been called a planet since it was discovered in 1930. So should all Kuiper belt objects be called planets?