What is a planet?

Old news, but I’ve been following the debate. In astronomy circles, the discussion about what can and can not be called a planet has flared up again with the discovery of Sedna last fall. Pluto, discovered in 1930, has always been an oddity. When it was discovered, it was thought to be about the size of Mercury and followed Bode’s Law, so it deserved “planet status.”

It wasn’t until about 70 years later that some other solid objects like Pluto were discovered beyond the orbit of Neptune. Quaoar, with an orbit that is much like that of the other planets, and most recently Sedna are not as big as Pluto (which itself is much smaller than was thought at its discovery), but they are thought to have more in common with Pluto than Pluto does with the other planets in our solar system.  

So, what is a planet? Pluto’s orbit is much more eccentric than the other planets, but far, far less so than Sedna which varies from 8 billion to 80 billion miles from the sun. Some astronomers think it would be convenient to define a planet by its mass, saying that the threshold for planet-hood would be if it has mass to be shaped like a sphere. That would include Sedna and Quaoar (and some asteroids) if no other tests had to be met. There are still other astronomers that are convinced that any solid object beyond the gas giants is not a planet based on their models for how they think the solar system developed.

When it comes down to it, astronomy is entirely an observational science. Astronomers can’t run experiments in the lab. They can’t travel to the far reaches of the galaxy. They’re stuck here, observing things from a very great distance in order to devise theories that can only be tested by observing more things from afar. Whether or not Pluto and Sedna are planets matters to astronomers. Classifying and categorizing is part of their job. But whether we call them trans-Neptunian objects, Kuiper Belt bodies, Inner Oort Cloud objects, plutinos, or planets won’t affect their existence in the least.

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