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Question How would you (personally) define a planet?

Oct 23, 2019
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Just for the fun of it suppose it was up to you to define what is and what is not a planet. How would you do it? What criteria would you choose and why?

Below (for reference) you will find the current IAU definition of a planet.

(1) A "planet" is a celestial body that: (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit.

(2) A "dwarf planet" is a celestial body that: (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighborhood around its orbit, and (d) is not a satellite.

(3) All other objects, except satellites, orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".


This definition is not without problems. If it was strictly adhered to then the earth itself would be disqualified from begin a planet. Why, because the orbit of the moon is at all points concave to the sun, technically making the sun, not the earth, its primary. And as the earth has not yet cleared its orbit of this co-orbital planetary sized body, then strictly speaking the earth by the IAU definition cannot be called a planet. It is too big to be called a dwarf planet and does not really fit in the “Small Solar System Bodies” category either. Therefore, technically, by IAU definition, both the earth and the moon remain unclassified.

So, go ahead, have some fun, and give us your new and improved definition of a planet.
 
Oct 25, 2019
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(1) A "planet" is a celestial body that orbits a star, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has gravity sufficient to hold an atmosphere with a density at mean ground level or sea level if a sea is present at least one-half that of Earth at sea level without regard to whether or not it actually possesses such an atmosphere. 2.) A "dwarf planet" is a celestial body that: (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) and does not have gravity sufficient to hold an atmosphere with a density at mean ground level or sea level if a sea is present at least one-half that of Earth at sea level without regard to whether or not it actually possesses such an atmosphere. 3.) Whether or not celestial bodies in categories 1 and 2 are satellites or twin bodies is irrelevant except that when such bodies are satellites they shall be called "Moon Planets" or "Moon Dwarf Planets" and when twin bodies shall be called "Twin Planets" or "Twin Dwarf Planets.' 4.) All other objects, except satellites of planets or dwarf planets, orbiting a star shall be referred to collectively as "Small Stellar Orbiting Bodies."
 
Oct 23, 2019
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Double Planet Definition

Type "A" Double Planet: Two co-orbital planetary bodies both having orbits at all points concave to their sun, and each of which are also gravitionally pulled into a hydrostatic equilibrium shape. Example: the Earth/Moon system.

Type "B" Double Planet: Two co-orbital planetary bodies, each of which are gravitionally pulled into a hydrostatic equilibrium shape, whose center of gravity is outside in space between the two bodies. Example: the Pluto/Charon system.

Type "C" Double Planet: A contact binary consisting of two bodies each of which are gravitionally pulled into a hydrostatic equilibrium shape. No known example.
 

rod

Oct 22, 2019
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"How would you do it?" My answer, I would observe the Sun using my telescopes with good solar filters and compare the telescope views with objects like Venus, Mars or Jupiter for example. You can clearly see the differences very quickly in the eyepiece field of view between these objects, as well as asteroids like 4 Vesta coming to opposition on 12th of November near magnitude 6.5. :)
 
Oct 23, 2019
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Single Planet definition:

Without regard to any sub-classification such as rocky, gas, ice, giant, dwarf, etc., a planet is a non-stellar body that (1) is in hydro-static equilibrium, and (2) whose orbit is at all points concave to its sun.
 

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