When is a planet not a planet?

Thu 24 Aug 2006 01:44 PM

When it's a dwarf planet.

The voting is complete. The definition of 'planet' approved today had an additional clause: A planet must have "cleared the neighborhood around its orbit."

This means that Ceres (in the asteroid belt), Pluto, Charon, and Xena (out in the Kuiper belt) do not count as 'planets' simpliciter. Only the eight biggies are 'planets.' However, Ceres et cetera are 'dwarf planets.' This is roughly what I predicted would happen eventually given the broader definition, but it has happened immediately and officially.

The New York Times story on this ends with a comment from Neil deGrasse Tyson, director of the Hayden Planetarium:

Dr. Tyson said the continuing preoccupation with what the public and schoolchildren would think about this was a concern and a troubling precedent. "I don't know any other science that says about its frontier, 'I wonder what the public thinks,' " he said. "The frontier should move in whatever way it needs to move."

To put it bluntly, this is bupkis. Whether or not Pluto counts as a planet is not a matter of what we know about the universe, but a question of how we choose to express our knowledge. There is a class of entities that meet the criteria agreed upon today, and Pluto is not a member. There is a class of entities that meet the criteria proposed earlier in the month, and Pluto is a member of that.

The word 'planet' predates modern astronomy, and it belongs in part to ordinary language. If scientists were simply to ignore ordinary usage in decide which words to use in designating various kinds, then they would not be moving the frontier of science "in whatever way it needs to move"-- they would be deliberately misusing words.

To put the point with a bit more subtlety: There is an advantage to using existing terms for new or more precise concepts, because people will immediately have a rough idea of what you mean. There is the disadvantage that the connection with previous meanings may be confusing or distracting. If that disadvantage is too great, then coin a neologism. The frontier of science would not be stopped by schoolkids, if it needed to say something schoolkids did not want to hear.

Now we get to watch and see whether people actually accept the outcome of today's vote. Ron is already on board.

UPDATE: More news over at space.com. Without quoting anyone verbatim, they say that the decision is "billed as a victory of scientific reasoning over historic and cultural influences." Piffle, I say. [Repeat the above arguments.]

Yet the opposition is already maneuvering for high ground:

"I'm embarassed for astronomy," said Alan Stern, leader of NASA's New Horizon's mission to Pluto and a scientist at the Southwest Research Institute. "Less than 5 percent of the world's astronomers voted. ... This definition stinks, for technical reasons." ... He expects the astronomy community to overturn the decision.

Of course, that is just what the leader of an expensive project studying Pluto would say. Even though the decision might go either way based on the science, there is prestige and possibly even funding at stake.

ANOTHER UPDATE: Janet's kids are happy to infer from Pluto's being a dwarf planet that it is still a planet. This must be taken with a grain of salt, since they are also prepared both to promote Xena and to demote Pluto. If toy stores invest in redesigning plastic orreries, planetariums change their displays, and textbooks are rewritten, then one usage rather than another may become entrenched without any further centralized decisionmaking.