Remember a few years back when there was a brouhaha over whether Pluto should be considered a planet? The International Astronomical Union, the body that decides, among other things, on the official names of astronomical objects and features, has been working on a new definition of the word “planet”. The results of their deliberations should be known by the end of September.

The reason there is some confusion is because we have discovered a population of icy worlds orbiting beyond Neptune that have much more in common with Pluto than Pluto does with any of the other planets. In fact, some of these new worlds (prosaically called Kuiper Belt Objects or KBOs), are in the same special orbital relationship with Neptune that Pluto is. These have been dubbed Plutinos. Recently a KBO was discovered that is bigger than Pluto, but on an orbit that is even more unusual (as far as planet orbits go, which is to say, nearly circular) than Pluto’s. The matter is complicated by the discovery over the course of the past decade or so of scores of extrasolar planets, that is, objects orbiting stars other than the Sun. Some of these blur the line between planet and star just as some of the KBOs blur the line between planet and ice cube. To make things even more fun, when KBOs occasionally get scattered into the inner solar system they start to evaporate as they near the Sun, and we call these objects comets. So should we call KBOs comets? Should we call Pluto a KBO? Should we call of them planets? Or come up with another name for the KBOs (for one thing, there are certainly KBOs around other stars, and it would be a bit peculiar naming all of them after our own backyard Kuiper Belt).

Personally, I favor demoting Pluto from the ranks of the planets, and I expect that the IAU definition will do just that. There is simply no sensible physical definition of a planet that includes Pluto and does not also include countless other objects in the outer solar system. Let’s reserve “planet” for the big guys. But, Pluto by any other name is still just as interesting a place to explore, and when the New Horizons spacecraft goes zipping by in 2015 it will undoubtedly teach us a lot about the origins of our solar system, planets, ice cubes, and all.

When I was a graduate student, not that long ago (it seems), there were no confirmed worlds orbiting other stars. Now there are over 100. Most of these are larger than Jupiter and frequently closer to their stars than Mercury is to the Sun. Both factors make these strange new worlds intriguing, but inhospitable to say the least. New telescopic missions on the horizon, such as the Kepler mission, promise to expand our list of distant planets to include Earth-like worlds. In the meantime, a study of a multi-planet system suggests that one of its planets may be habitable: not too warm, not too cold, and with a solid surface beneath a reasonable atmosphere. Sean Raymond, whose office is just next door to mine, participated in this study and does computer simulations of the formation of habitable planets around other stars. While his research suggests that only a small fraction of the currently discovered extra-solar planetary systems may have habitable planets, the total number in the Milky Way, even in our neck of the woods, could still be, well, astronomical.