Why are planetary orbits so circular?

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Discussion Overview

The discussion centers on the reasons behind the nearly circular orbits of planets in our solar system. Participants explore various theories and models, including the effects of collisions in the protoplanetary disk, gravitational interactions, and drag from surrounding media. The conversation encompasses theoretical, conceptual, and exploratory aspects of planetary motion.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants suggest that collisions in the protoplanetary disk lead to more circular orbits, as proposed by George Jones.
  • Others argue that the stability of solutions in planetary motion tends toward circular or hyperbolic orbits, with some mathematical background provided.
  • One participant notes that while Mercury and Pluto have eccentric orbits, they can still be described as "nearly circular" due to the small differences between their axes.
  • Another viewpoint emphasizes the role of drag on planetesimals, which could lead to circular orbits as they interact with surrounding dust and gas.
  • Some participants question the extent to which gravitational interactions, particularly with Jupiter, influence the circularity of orbits, suggesting that Jupiter may increase eccentricity rather than decrease it.
  • There is a discussion about the chaotic nature of the solar system and how it affects the predictability of orbits over long timescales.
  • One participant raises the idea that the initial conditions of the solar system may have favored nearly circular orbits, with chaotic influences being less significant over the lifespan of the solar system.
  • Another participant mentions various mechanisms, including inelastic collisions and interactions with smaller bodies, as potential contributors to the circularization of orbits.

Areas of Agreement / Disagreement

Participants express a range of views on the factors contributing to the circularity of planetary orbits, with no clear consensus reached. Disagreements exist regarding the roles of drag, gravitational interactions, and the implications of chaotic dynamics in the solar system.

Contextual Notes

Some claims rely on assumptions about the stability of orbits and the effects of gravitational interactions, which may not be universally accepted. The discussion also touches on the chaotic nature of the solar system and how it influences orbital shapes over time, but these aspects remain unresolved.

  • #31
cmb said:
Not sure I am. The centre of mass of the universe is a point that is equally the age of the universe away from all points in our 3D space, is it not?
By that definition, every point in the universe is the center of the universe. Another way to say it: The universe has no center.

All of the discussion started by post #20 is incredibly off-topic. Start a new thread, or better yet, find an existing thread that is already devoted to this topic.
 
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  • #32
D H said:
What two planets are you talking about? Only one of the eight planets, Mercury, has anything close to an eccentric orbit. Moreover, a big part of Mercury's eccentricity results from Jupiter.
Pluto's is eccentric enough to pass within Uranus'. And it's tilted.
 
  • #33
cmb said:
Not sure I am. The centre of mass of the universe is a point that is equally the age of the universe away from all points in our 3D space, is it not?
As D_H points out, so was my point: all points in the universe meet that criteria.
 
  • #34
DaveC426913 said:
Pluto's is eccentric enough to pass within Uranus'. And it's tilted.
I was being nitpicky. Pluto is no longer considered to be a planet.
 
  • #35
D H said:
I was being nitpicky. Pluto is no longer considered to be a planet.
Is too. Dwarf planets are a subset of planets. :biggrin:
 

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