Two weeks ago, the International Astronomical Union formally approved the name “Plutoids” for small but near-spherical bodies like Pluto that orbit the Sun beyond Pluto. To stimulate discussion on the controversial issue of what constitutes a “planet,” SkyandTelescope.com is running the following article by the eminent planetary scientist Alan Stern. The opinions expressed by Dr. Stern do not necessarily reflect those of Sky & Telescope or its individual staff members. We welcome all readers to express their own opinions by clicking on the Post a Comment link at the bottom of this article.

    — Robert Naeye, Editor in Chief, Sky & Telescope

Classification is an important and productive scientific tool that is employed in many branches of science, from biology to geology to chemistry and astronomy.

Planetary science today faces a significant classification challenge: defining what objects are and are not “planets.” This challenge has come to the fore owing to the discovery of numerous dwarf planets in the outer solar system, the recognition that Ceres is a dwarf planet (a fundamentally different body than the smaller asteroids), the discovery of planets around a pulsar, and the numerous discoveries of hot Jupiters orbiting other stars.

Geophysicists have come up with a planetary definition that makes a lot of sense. They define a “planet” as a natural object in space that is massive enough for gravity to make it approximately spherical, but not so massive that it has generated energy by internal nuclear fusion. This definition nicely separates planets (i.e., objects larger than a few hundred kilometers across) from both smaller bodies that are too small to be fundamentally shaped by gravity, and larger bodies (very many times the mass of Jupiter) that manifest themselves as brown dwarfs and stars.

Scientists and the public would be much better off if we adopted a comprehensive planetary definition that is a self-consistent and allows astronomers to reliably and consistently sort objects into “planetary” and other categories. The geophysical definition does just that because it allows scientists to reliably categorize bodies based on a single, simple, robust observable property—their known or estimated mass.

The geophysical planetary definition avoids the severe difficulties associated with other concepts. Some definitions depend on how objects affect their orbital zones. But these definitions result in identical objects being classified differently depending on their circumstance. Earth, for example, would not be considered a planet if it orbited the Sun beyond Neptune, because its gravitational influence would be insufficient to clear out the Kuiper Belt. Definitions based on origin are problematic because we can rarely determine how an object formed, especially if it’s outside the solar system. Definitions based on the presence of an atmosphere or satellites are also problematic, since they can be exceedingly difficult to determine observationally, and each of these factors would rule out various objects commonly regarded as textbook examples of planets in our solar system.

The geophysical planetary definition does not tilt the population of planets in a system based on scientific biases such as preference for a limited number of planets in our solar system. Instead, it embraces the diversity of planetary types being discovered in our solar system and around other stars.

Unfortunately, the International Astronomical Union (IAU), populated primarily by astronomers who do not even study planets, has resisted the geophysical planetary definition that is popular among planetary scientists. The IAU’s president has recently said that few scientists or laypeople are unhappy with the IAU’s planetary definition, which excludes dwarf planets. But this statement is false. Public polls like this one produced many tens of thousands of votes, slanted heavily in favor of dwarf planets being full-fledged planets. Further, more planetary scientists pledged not to use the IAU’s definition than were even in the IAU meeting room in Prague when the IAU voted on this matter.

If you are interested in this subject, consider attending the Great Planet Debate this August in Maryland. You can also voice your opinion at here.