"Pluto is (not) a Planet"
Recently Joe Heimlich, co-director of COSI's Center for Research and
Evaluation, fired off a quick question for me: rumors abound that
Pluto may be reinstated as a planet. Is it true? What gives? (I'm
In 2006 the International Astronomical Union adopted the following requirements for planethood: An object must 1) orbit the sun, 2) be large enough that its own self-gravity pulls it into a spherical shape, and 3) clear the neighborhood of its orbit of any debris.
With its large moon Charon, and all the other junk in the outer solar system, Pluto failed requirement 3 and was demoted to "dwarf planet" status.
The decision was met with heavy criticisms within the astronomical community. Why should a definition of an object rely on its environment and not just on properties of the object itself? If the Earth were moved to the orbit of Pluto, we would lose our planet status - how does that make sense? Why weren't all interested parties involved in the vote back in 2006? And so on.
Recently a group of astronomers proposed a new definition of planet: it must be large enough to make it round, and that's it. The new definition covers Pluto and friends, "rogue" planets that aren't bound to any star, and also upgrades some large moons to planetary status.
The debate continues, but one thing is clear: we will never return to a state with exactly 9 planets in the solar system. Either it will continue to be 8 or be around...10,000. We'll see.