Last August members of the International Astronomical Union voted to define a planet as a body that orbits a star, is large enough to be round, and has cleared other bodies out of its neighborhood. The definition was intended to bring closure to a long-standing debate but instead seems to have poured fuel on the fire.
Critics have called the definition arbitrary and imprecise, but the charge is unfounded. The solar system divides cleanly into eight bodies massive enough to dominate their orbital zones and swarms of smaller ones that occupy intersecting orbits. This pattern appears to reflect the way the solar system formed and evolved.