Which objects meet the definition needs further research, but among the initial candidates will be Mercury, Venus, Earth, our moon, Mars, Phobos, Deimos, Ceres, Vesta, various other asteroids (don’t ask me how many), Jupiter, Ganymede, Callisto, Io, Europa, some number of the 67 other moons of Jupiter, Saturn, Titan, Enceladus (and more: I’m skipping more than 50 other moons of Saturn), Uranus, Titania, Oberon, Umbriel, Ariel, Miranda (don’t make me go through the 22 other moons of Uranus), Neptune, Pluto, Charon, Proteus, Quaoar, Styx, Nix, Kerberos, Hydra, Eris (a strong candidate, having greater mass than Pluto), Dysnomia, Makemake, Haumea, Sedna, Varuna, Orcus, and Ixion (I swear I’m not making these up). It will be quite a party trick to recite all the planets if this proposal is accepted. I recommend “Real Scientists Like Geophysically-defined Solar Systems.” You may need a mnemonic to recall the names of the astronomers proposing the new geophysical definition: K.D. “My Very Educated Mother Just Served Us Nine Pizzas” (or “Mary’s Violet Eyes Make Johnny Stay Up Nights Plenty,” or “My Very Efficient Memory Just Stores Up Nine Planets,” or whatever your favorite may have been) will be obsolete. Simple mnemonics for the planets in order of their sidereal periods will be just a memory. Ganymede and Titan have to make the cut (they’re both larger than Mercury), and there will be scores of others. A planet can orbit the sun, or orbit another planet, or be locked in a group of objects that sort of orbit one another, with mutual tidal locking, like Pluto and Charon.Ī definition like that will let a lot more of the solar system’s bits of rocky debris count as planets. Orbital dynamics would not be relevant under this definition. The geophysical definition would be (roughly) that a planet (i) weighs less than a star, (ii) has never undergone nuclear fusion, and (iii) is ball-shaped because of its own gravity (to be more precise, “has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid” - the paper is here). (It lost that status in 2006 and was reclassified as a dwarf planet.) Oh, no: The lexical-semantic battle over Pluto is on again! I learn from that a research group is going to try to get a new geophysical definition of the word planet approved by the International Astronomical Union, and is drafting it in a way that will allow Pluto to count as a planet once more.
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