Planets Galore
We finally have a definition of planet, and it’s an expansive one. Space.com calls it controversial. I don’t really see that.
It’s weird, not thinking of the nine we grew up with, and knowing that there are 12, should be 53 after a while, and may be many more. At the same time, by defining a subcategory of “plutons,” one can distinguish mostly more traditional planets from the rest, almost creating a small set similar to what we’d have had if the definition had gone the other direction and evicted Pluto.
For those who want the definition without reading either article:
“A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet.”
Thus our moon is not a “planet,” though it’s often been called that because of its size and its ever so slow drift out of Earth’s gravitational control, because it orbits a planet, not the sun. Thus Ceres is a planet because it has enough gravity and orbits the sun.
Next entry: Salmon
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