Use This Definition of a Planet: Big Enough to Have a Round Moon

Big hubbub in astrononomy about how to define planets. Say Pluto’s just an iceball, and we know of only 8. Say a planet’s any solar-orbiting object big enough for its gravity to crunch it into a ball, and now Ceres, which would fit between Boston and Washington, D.C., would count, along with dozens of other farout iceballs.

But a planet’s a world, a place big enough for people to build a base, or waystation, or maybe a vacation home. It’s not some orb that would be dwarfed by Earth’s Moon. It’s a place big enough to have its own real moon. Proposal: Make that the definition of a planet. A star-orbiting world not only massive enough to be round, but massive enough to have a round moon. (Or gravity-grabbed partner, if you want to be a stickler in the case of Pluto and Charon.) Viewed that way, Pluto’s a planet, and so is anything bigger. But not a rock like Ceres where you’d only weigh four pounds, and where a good swing could probably knock a golf ball over the horizon.        


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