Why planets are spheres

  1. Why planets are "spheres"

    I was reading the specifics on why large bodies in space, such as planets, are pulled into spherical shape and they had a very technical term for the process.

    Does anyone know what I'm talking about?
     
  2. jcsd
  3. Chronos

    Chronos 9,757
    Science Advisor
    Gold Member

    Gravity is the short answer. A sphere produces maximum volume with the least possible surface area. A planet forms via accretion. Small particles clump together, most likely by electrostatic attraction. As the clump grows gravity takes charge. Gravity is an equal opportunity employer and each particle attracted tries to get as close to the center of gravity as possible. This forces the planet to become spherical over time. The high spots eventually migrate into the lowest available spots. A certain amount of oblateness is present in all planets due to centrifugal force.
     
  4. I understand the forces at work when it comes to the shape. I'm just trying to track down the term that was used that I can't seem to find.

    I believe term was three words and was something along the lines of "gravitational ____________ __________"

    I apologize that I can't be more descriptive on what I'm looking for.
     
  5. DrClaude

    DrClaude 1,964
    Science Advisor

  6. That's it! Thanks
     
  7. They aren't all spheres. Jupiter is significantly wider at the equator than pole to pole. Look at Haumea, a dwarf planet in the Kuiper belt. It has a highly ecliptic shape!
     
  8. I know there's an equation to find out how much an object shifts from being a true sphere. I forgot it though :(
     
  9. Well, Jupiter would be a sphere but the force of its rotation causes the oblateness. That's going to be true of any rotating sphere, even a relatively solid one such as the earth.
     
  10. In addition, the fact that a spherical shape minimizes surface area is key when it comes to understanding planet formation. Planets are more easily formed when there are a bunch of ice particles floating around (they accrete material more easily), so minimizing surface area minimizes exposure to sunlight, which minimizes the amount of ice that gets melted, which makes it easier for the planet to from. But yeah, gravitational compression is going to be the main cause for anything larger than a few tens of kilometers in radius.
     
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