A short article in the April 29 issue of Science News (originally published on line March 23, and yes, I am indeed behind in my reading) reports that a team of planetary scientists involved in NASA’S New Horizons mission to Pluto want to simplify the definition of “planet” in a way that would include Pluto and about 110 other bodies in our solar system.
Incidentally, New Horizon flew past Pluto two years ago Friday. I really need to time these posts better.
The authors of the proposal want to define a planet as any round object in space smaller than a star. That would restore planetary status to Pluto and also Ceres, which was originally considered a planet, then for a very long time an asteroid, and more recently a dwarf planet. It would also promote to planetary status large moons (including our own) and a lot of objects in the Kuiper belt and probably the Oort cloud.
The current official International Astronomical Union definition of a “planet,” which applies only in our solar system, requires that a planet orbit the Sun (and not another planet), be massive enough to assume a nearly round shape, and clear its orbit of debris. The last part of the definition has caused the most objections. The authors of the definition had a specific meaning in mind for clearing its orbit, but that meaning isn’t in the definition, and taken literally as written it would disqualify not just Pluto but Jupiter (look up “Trojan asteroids“) and most or all other planets.
Besides the virtue of simplicity, another advantage of the alternative definition is that it would apply not just in our solar system but everywhere, including exoplanets orbiting other stars or other planets and even rogue planets wandering freely through space, of which there are probably a vast number.
The proposal, presented at the Lunar and Planetary Science Conference in Woodlands, Texas, March 21, was authored by Kirby Runyon of Johns Hopkins University, S. Alan Stern and Kelsi Singer of the Southwest Research Institute, Tod Lauer of the National Optical Astronomy Observatory, Will Grundy of the Lowell Observatory, and Michael Summers of George Mason University.
See also this article at Science Daily.