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Planetary Paths





Dennise
1. Are all known planetary orbit paths elliptical?

2. Why is this so?

3. While a circle is really just a special case of an ellipse, are there any known planets with circular orbit paths?

............... just would like to know.
ocalhoun
Dennise wrote:

3. While a circle is really just a special case of an ellipse, are there any known planets with circular orbit paths?

There probably are a few somewhere, but it is very unlikely, since even the slightest deviation would make a circle into an ellipse.
silver67
All planetary orbits are elliptical. Even the ones that are circular. Now I may be completely wrong but the reason that planets orbit in elliptical patterns is because the mass of the planet shifts the center of mass of the Sun-planet system away from the center of the sun. Essentially, if the planets were mass less then they would orbit in a circular pattern.

Yeah. Don't count on that explanation to be true though.
ocalhoun
silver67 wrote:
if the planets were mass less then they would orbit in a circular pattern.

If the planets were mass-less they wouldn't orbit at all.
They wouldn't have the momentum to keep moving, nor would they have the weight to draw them towards the star they orbit.
Indi
Dennise wrote:
1. Are all known planetary orbit paths elliptical?

Yes and no. Yes, in theory, all orbits are elliptical. But in practice, orbits are affected by more than a single gravitational source, and by gravitational decay. That means that they change all the time, and, eventually decay into a spiral. The Earth's orbit, for example, varies between around 0.06 and 0.003 over a period of around 40,000 years (right now, it's around 0.017).

Dennise wrote:
2. Why is this so?

i presume you mean: why elliptical and not circular?

Solely because perfection is rare in nature.

There's not much particularly special about the circular case (except that it's the lowest energy stable orbit). The chance that the eccentricity of a randomly selected orbit is 0 is the same chance that it is 1, 0.4, 25 or 0.00000000325235; that is, infinitesimal. When you say "why aren't there more circular orbits?", you're really saying "why aren't there more orbits with an eccentricity of 0?". But you could just as easily say "why aren't there more orbits with an eccentricity of 0.2456923562473734?". There's really nothing special about a circular orbit, except in the human mind: we have a need for order and patterns and regularity, so something "seems" more significant about a circular orbit (an orbit with an eccentricity of 0) than an orbit with an eccentricity of 0.2456923562473734.

So there's your first problem: there are an infinite number of possible eccentricities - from 0 to infinity - and a finite (albeit enormous) number of orbiting bodies in the universe. The chance of finding an orbit to your exact specification (which, in your case, happens to be exactly 0, rather than exactly some other number) is infinitesimal.

Your second problem is because nature never sleeps. No orbit just stays the same... they're always changing, due to the effect of other bodies, and due to simple gravitational decay. So... even if you manage to find an orbit that is exactly 0... it won't be for long. Either it will be shifted into a more elliptical orbit, or it will decay and spiral into oblivion.

Dennise wrote:
3. While a circle is really just a special case of an ellipse, are there any known planets with circular orbit paths?

Venus has a very low eccentricity. Neptune's also pretty circular. Even the Earth isn't so bad. If you want to count moons, Triton has an extremely low eccentricity... damn near circular to like one part in a million.

silver67 wrote:
All planetary orbits are elliptical. Even the ones that are circular. Now I may be completely wrong but the reason that planets orbit in elliptical patterns is because the mass of the planet shifts the center of mass of the Sun-planet system away from the center of the sun. Essentially, if the planets were mass less then they would orbit in a circular pattern.

Yeah. Don't count on that explanation to be true though.

Good guess, but not quite.

What happens with two objects of comparable mass is that they both orbit a central point, called the barycentre... but both of those orbits can be circular. To picture this, imagine two "planets" of identical mass, with no other sources of gravity and no orbital decay: they will both spin around the point in the exact centre between them as if they were attached, which amounts to a circular orbit for both.

ocalhoun wrote:
silver67 wrote:
if the planets were mass less then they would orbit in a circular pattern.

If the planets were mass-less they wouldn't orbit at all.
They wouldn't have the momentum to keep moving, nor would they have the weight to draw them towards the star they orbit.

... not quite. Massless objects can orbit a massive source, and massless objects can have momentum. Hint: event horizon.
Voodoocat
I thought that momentum was mass times velocity?

P = mv
Bikerman
Voodoocat wrote:
I thought that momentum was mass times velocity?

P = mv

Not really - that is a simplified version which is good enough for Newtonian calculations but breaks-down at higher velocities.
Relativistic momentum is given by

though it is more usual, in relativistic mechanics, to use the expression:


For massless objects like the photon, we use:
chatrack
I think , since the center , that is sun is also floating in space. If Sun was at rest some were
, planets may have revolved it in good circular paths. Am I right?
kelseymh
chatrack wrote:
I think , since the center , that is sun is also floating in space. If Sun was at rest some were
, planets may have revolved it in good circular paths. Am I right?


You're wrong. The general case for a central 1/r^2 potential (like gravity) is elliptical orbits with the central source at one of the foci. A circular orbit is a special case where the ellipse has ecentricity of exactly 1.00000000.....
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