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Astronomy

Planets

I love all things astronomy, but I confess there is a special place for planetary astronomy. This spring I'm looking forward to taking a planetary science course. For some reason, universities keep sticking planetary science-y stuff into the "Earth Science" department.

I feel it necessary to address the planet definition controversy here directly. As a linguist and an astronomer, I'm frankly appalled. It's a linguistic nightmare. Take the term "dwarf planet". Even though it has planet in the name, it's not actually a planet according to the IAU, and furthermore, it has nothing whatever to do with the size, but how clear the orbit is. It's not even a category of planet. This is pure linguistic insanity. To make matters worse, while everyone agrees that the Earth is certainly a planet, if an object the size of Earth were to be found at the distance of Pluto (about 40 AU), or further (say, 100 AU) it is unlikely to have the gravity necessary to have cleared its vast orbit. Indeed, the current definition proposed in 2006 essentially requires that the further you go from the star you orbit, the larger your gravity needs to be in order to meet the definition. And we can't actually tell if any of the exoplanets we've found even meets the definition.

This clearly must be changed. Whatever you think of little Pluto, fine. But if the concern indeed is that it's too "small", come up with a definition based on gravity, roundness, melting of the core, mass limit, whatever, that will do the job and justify it in science. Science is riddled with arbitrariness, so there is no real reason to shy away from a reasonable, but otherwise arbitrary definition. Otherwise, it's a planet, and a third category of planet, next to terrestrial planets and jovian planets. And if the solar system ends up with 25 planets, who says you have to memorize all of them? Have you memorized more than the ten closest stars? Have you even done that? There is nothing special about planets except that we happen to live on one.

This argument is made in detail in one of my presentations. See the link at top right.

That having been said, the pages below contain links on the various planetary bodies in the solar system (and beyond). I've tried to divide them up in a sensible way, and I've included objects that would be included in a broader definition of planet that I subscribe to, absent a better definition. (Since the normally sane IAU has chosen to behave insanely, I will ignore them until they come back to their senses.) In general, if you are looking for links on, say, Mars, check both Mars and the general Planets link. If you are looking for Titan, check Saturn, Moons and the general Planets links. I will do my best to be sensible, but I'm not going to post every link about all nine planets on every single page.

Incidentally, despite my objections, links are on each page both as though these objects were planets, and as though they were not, so it is possible to scroll through the whole list page by page and skip over Ceres, for instance.