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ellipse --> circle ?



Gary Karshner wrote:

... The orbits of the planets are nearly circular as a result of the
accretion process that formed them. That is the dust in elliptical
orbits is more likely to collide with things since they pass through
the circular orbit material and can collide with other elliptically orbit
dust traveling in different directions as well. By an averaging process
the "preferred material" will be that in circular orbits. The nearly
circular orbiting dust clouds can become concentrated by perturbations
from other clouds in similar orbits and the Poynting-Robertson effect
acting on the dust.
Gary

P.S. Periapse is when the object is closest to the star, i.e. Perihelion
in the case of the sun, perigee for the earth. Today perturbations of
other bodies and tidal effects are the main causes of the changes in
orbital parameters in the solar system.

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1) Thanks for being helpful. Small particles can grow by condensation, as
rain drops. No gravitational attraction is involved (only thermal motion
and molecular forces). Large bodies attract gravitationally and this may
lead to their concentration in space. What is the *accretion* process?
May I assume it is the same thing as *condensation* except that particles
are bigger? (Thermal motion leads to collisions; then molecular forces do
the binding. Gravitational attraction is not significant until the objects
are large. Accretion is non-gravitational accumulation dust.)

2) Leigh wrote that *tidal dissipation* can possibly be responsible for
the prevailing tendency of planets to orbit along circles rather than
along ellipses. Is this the same thing as the "Poynting-Robertson effect
acting on the dust"? How can tides be significant on small particles?
I understand that a large cloud would experience tides in the presence of
another cloud, or a star, and how some of its energy can be dissipated.
But I do not see a connection between the circularity of orbits and tides.

Ludwik Kowalski
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