So, then, dwarf or otherwise, isn't Pluto still a planet?
"Yes and no," Morrison said. "The answer is semantic, based on whether dwarf planets are planets, just as dwarf pines are pines. I would say that Pluto is a planet, but it is a dwarf planet, and the first example of the (new) class of trans-Neptunian dwarf planets. Ultimately, the definition of a planet will come through common usage and scientific utility. There is no need to throw away current school texts; Pluto has not gone away."
In the meantime, according to Alan Stern, the New Horizons primary mission to Pluto hasn't changed. When it arrives at Pluto in July 2015 it will unlock one of the solar system's enduring planetary secrets. The spacecraft will skim the orbits of all eight planets, from Earth to Neptune, and then fly by Pluto and its large moon Charon. Seven science instruments on the probe will shed light on the dwarf planets' geology, interior and atmospheres.
What's in the Sky: During the first week of June in the WNW, in the constellation Leo, Regulus and Mars are close together; use these objects to find several stellar clusters. Mars is moving away from Earth; it will become dimmer as summer passes. Seeing Stars' sky chart is courtesy of J. Kirk Edwards.
Lou Varricchio, M.Sc., lives in Vermont. A former NASA science writer, he is Vermont's NASA/JPL solar system ambassador.