WHAT makes a planet a planet? It's nearly 10 years since the International Astronomical Union created a stir by agreeing a new classification system that kicked Pluto out of the club, demoting it to dwarf planet status.
In the intervening years it has become abundantly clear that the solar system's smaller bodies are at least as interesting as those that still enjoy full planetary status. We already think that the moons of Jupiter and Saturn are among the best places to search for alien life. Now, as spacecraft begin to map Pluto and its smaller cousin Ceres, dwarf planets are entering the limelight.
Is another round of planetary hokey-cokey on the cards? The leader of the Dawn mission is on record as saying that Ceres will turn out to be "every bit a planet as its terrestrial neighbours Mars, Earth, Venus and Mercury" (see "New dawn for Ceres"). If Ceres is a planet, so are Pluto, Eris and many others.
It doesn't really matter. In fact, reviving the debate will only detract from the excitement of exploration. Pluto's demotion was keenly felt by a generation of astronomers, both professional and amateur. Ceres has no comparable emotional pull, and nothing to gain from being promoted to planetary status.
This article appeared in print under the headline "Fifth rock from the sun?"
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