WEEKLY WHINE

Spot the planet

So now there's 2004DW, an all new Kuiper Belt object that appears to be orbiting in an orbit similar to Pluto's. But they want everyone to know that it's not a planet. Got that? Not a planet. Anyone who calls it a planet will have to answer to Al Gore.

You may be asking, "Well, wait a minute. Who gets to say what's a planet and what's not a planet anyway?" That would be the International Astronomical Union.

Commission 16 at the IAU, the Physical Study of Planets and Satellites commission, and Commission 15, the Physical Study of Comets and Minor Planets commission, would seem to be the ones who get to decide. So if Commission 15 wants to study it, it's a minor planet. If Commission 16 wants to study it, it's a major planet. If neither wants to study it, it might be any number of things. Like maybe a toaster.

But can we really make a definitive argument about what is a planet? Sure we can. Getting others to agree... there's the rub.

Note: All of these definitions would include the Sun. Just ignore that.

R >= 1000 KM

A bunch of people seem to be willing to go with this definition. If that's the case, there are 16 known planets: Mercury through Pluto, the Moon, Io, Europa, Ganymede, Callisto, Titan, and Triton.

Maybe. There are a few other trans-Neptunian objects that have such uncertain albedos that they may in fact be larger than a thousand kilometres in radius.

VERDICT: Seems to be an okay definition.

ANYTHING WITH A MOON

Again, now we have to define what a moon is. If we say that a planet is anything that's orbited by something not built by humans, we'd have twenty or thirty planets: Earth through Pluto, plus minor planets like 243 Ida and 45 Eugenia. Mercury and Venus would have to be considered minor planets. In Venus's case, that's one hell of a minor planet.

VERDICT: Don't think so.

ANYTHING INTERESTING

Well, that only includes the entire freaking solar system.

Okay, the really interesting things are the ones with atmospheres or really varied surfaces. Trouble is, we have to visit something to figure out whether it's interesting. [And usually whenever we do, we find out that it is interesting.]

VERDICT: Wouldn't change anything. The definition of a planet is already arbitrary.

ANYTHING WE TARGET SPECIFICALLY

This defintion would declare as a planet anything to which we send spacecraft specifically. Example: The numerous Pluto and/or Kuiper Belt missions that have been proposed were mostly interested in Pluto particularly. They also wanted to visit a Kuiper Belt object, but - crucially - it could be any old Kuiper Belt object. Ask them to go to, say, 28978 Ixion instead of Pluto, and they'd say something along the lines of: "No. Shut up."

So by this definition: Mercury was a target of Mariner 10. Venus has been a target of numerous spacecraft. Earth has been a target of bazillions of spacecraft. Mars has been a target of numerous spacecraft. Jupiter was a target of Pioneers 10 and 11, Voyagers 1 and 2, Ulysses, Galileo, and Cassini. Saturn is a target of the Voyagers and Cassini. Uranus and Neptune were both targets of Voyager 2. Pluto would be a target of the aforementioned Kuiper Belt missions.

But there were also missions targeted for Comet 1P/Halley. So that means we have ten planets.

VERDICT: The more missions we fly, the more planets we have.

So what's the answer? How's about we say that anything that measures more than 2,000 kilometres across is a planet. As long as we didn't build it. And as long as it's not the Sun.