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Down the Rabbit Hole One More Time -- What is a Planet?

02/22/2018 03:55PM

Down the Rabbit Hole One More Time -- What is a Planet?

What is a planet? For generations the answer was easy -- A big ball of rock or gas that orbited the Sun. And there were nine of them in our Solar System. But then astronomers started finding more Pluto-sized objects orbiting beyond Neptune. Then they found Jupiter-sized objects circling distant stars. First by the handful and then by the hundreds. Suddenly the answer wasn't so easy. Were all these newly found things planets? The International Astronomical Union (IAU), who is in charge of naming newly discovered worlds, tackled the question at their 2006 meeting. They tried to come up with a definition of a planet that everyone could agree on. But the astronomers couldn't agree, so they voted and picked a definition that they thought would work. The results have been mixed. In the end, the IAU did accomplish one thing -- They figured out a way to turn something simple into something complex.


If you want to define a planet, first you should ask the question - What is the purpose of your definition? Or more to the point, how do you want to distinguish planets from other celestial bodies? I can think of four.<br><br>1) Geophysical - Compositional. (as per Alan Stern, et. al.) If isolated in space, what are the INTERNAL properties that separate a planet from an asteroid, or a Brown Dwarf?<br><br>2) Primordial. How was it formed? (Kevin Schlaufman) Condensed gas cloud, or accreting around a rocky core? Formed in an independent orbit around a star, or in isolation from any larger body? Was it ripped from the body of a larger object in a collision, or captured?<br><br>3) Dynamical (Mike Brown, IAU 2006) How does it interact with other bodies in its Solar System? Has it "Cleared its orbit"? Does it's planethood depend on WHERE it was found, and the company it keeps?<br><br>4) Cultural (School children, retrophiles, etc.) When I was a child there were 9 planets! In my heart, Pluto will always be a planet! <br><br>Until you agree on the purpose of your definition, these arguments are somewhat pointless.<br><br>My two bits,<br>- Saipan SkyLobster
After some more thought, I came up with a definition that might at least somewhat satisfy the first 3 purposes of a Planet definition. As for the 4th purpose, at least I don't demote Pluto, even if I do put it in the back of the class.<br><br>Keith's Planet Definition:<br>A PLANET is a celestial body that has insufficient mass to engage in any form of nuclear fusion, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a nearly round shape, and has formed through a process of accretion rather than condensation.<br><br>Several QUALIFIERS could be added to describe the dynamical sub-category of a particular planet, as we do with stars.<br>Here are my proposed sub-definitions: <br><br>a MAJOR PLANET orbits a SUN or a BROWN DWARF, and has cleared, or is in the process of clearing, the neighborhood around its orbit.<br>a MOON PLANET orbits a larger planet, such that the barycenter is within the surface of the larger planet.<br>a DOUBLE PLANET is in mutual orbit around another body of similar size, such that the barycenter is in empty space between the two worlds.<br>a DWARF PLANET orbits a SUN or a BROWN DWARF, but is dynamically incapable of clearing its orbit of other bodies.<br>a FREE PLANET is not in orbit around a SUN, but travels independently through interstellar space.<br><br>- Keith<br>