So You Think You Know From Planets

Pluto and its moonsOne of the more fun debates of 2006 was the IAUs deciding on the definition of “Planet”. Their decision excluded Pluto (and some asteroids like Ceres and Vesta as well as Kuiper Objects Eris, formally UB313, and Quaoar) from the family of planets that encircle the Sun. Historically, it’s interesting because Ceres and Vesta once, in the early 19th century, were designated as planets. Astrologically speaking, it’s critical, of course. After all, Pluto has definitely ruled my underworld for centuries…

AARRRGGG! My snark key is stuck again.

Anyway, there is some scientific usefulness in deciding that Mercury, but not similarly sized Pluto, is a planet, because definitions are a good way to denote real differences. Much like biological taxonomy, it’s a basis for understanding, and subject to revision.

Unfortunately, much like biological taxonomy, it’s really, really subject to revision.

Take this, for instance from Alan Boyle reporting for MSNBC.

The growing consensus is that it’s wrong to divide the planetary lineup into first-class and second-class worlds. Planetary scientists say it’s better to think of rocky, terrestrial planets (such as Earth, Mars, Venus and Mercury); gas giants (Jupiter, Saturn, Uranus and Neptune) and dwarfs (Pluto, Eris and Ceres, for example).

The difficulty is that planetary scientists don’t necessarily say that. Some contend that Uranus and Neptune are intrinsically different (in a major way) from Jupiter and Saturn, so rocky planet/gas giant planet/dwarf planet is too simplistic a scheme. In other words, sooner or later you’re going to have as many classifications as you have planets, and that isn’t scientifically helpful. We wouldn’t (and don’t) do that for moons (sheesh – when I was a boy, Saturn had nine moons, Jupiter twelve, and Pluto none. Pluto has three now, and it may not even be properly classified as a planet!). Any individually distinguishable object circling Jupiter is one of it’s moons, and there are more than 60 such objects known already.

You can see that the argument isn’t exactly over, especially for the Alan Stern types, who just happen to be interested in Pluto. So the “Great Planet Debate” is going to be reopened in August at the Johns Hopkins University Applied Physics Lab (my old stomping grounds) in Laurel Maryland. From the workshop’s website:

During the first two days of the conference, we will present what we have learned about planetary bodies over more than 40 years of robotic exploration of the Solar System and what we are learning about planets around other stars. The IAU’s dynamical definition of a planet will be presented, as well as an alternative geophysical definition. The utility of each will be debated, along with other potential planet definitions. A public lecture and panel discussion, featuring scientists who are prominent in the debate on planet definitions, is planned for the evening of the second day, following a reception that concludes the scientific portion of the conference.

Clyde Tombaugh’s ashes may get to visit his planet yet.

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One Comment on “So You Think You Know From Planets”


  1. Clyde Tombaugh’s ashes will most definitely visit the PLANET he discovered. I attended the Great Planet Debate, and unlike the IAU General Assembly, which made its decision in a closed and surreptitious process, more concerned with hurrying things along than with accuracy, this conference involved genuine, in depth discussion of the complexities surrounding planet definition. It was open to anyone who wanted to participate, unlike the IAU’s meeting. The IAU may be fine with allowing four percent of its members, most of whom are not planetary scientists, to issue a decree and force it on the world. That decree does not change the reality of what is out there. They messed up by creating a nonsensical definition that states dwarf planets are not planets at all and one that classifies objects solely by where they are while ignoring what they are. The scientific method is not one in which reality is determined by someone in authority issuing a decree and expecting everyone to blindly follow. The fact that this debate continues shows that people from astronomers to lay people are not blindly following.


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