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Author Topic: QotW - 16.04.05 - Can two planets share the same orbit?  (Read 3293 times)

Offline thedoc

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Is it technically possible for two planets to share the same orbit, as in exact polar opposites to each other travelling at the same speed?
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« Last Edit: 19/04/2016 10:03:20 by _system »


 

Offline alancalverd

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Re: QotW - 16.04.05 - Can two planets share the same orbit?
« Reply #1 on: 07/03/2016 13:31:00 »
Yes
 
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Offline SeanB

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Re: QotW - 16.04.05 - Can two planets share the same orbit?
« Reply #2 on: 07/03/2016 18:45:10 »
But not if there are other large bodies in the same system. Then the disturbance from gravity from them will destabilise the orbits in a few million years quite a lot. This, like a Lagrange point, is only true in a system with 3 masses and is stable. Add more and the stability is not true over long periods.
 

Offline JoeBrown

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Re: QotW - 16.04.05 - Can two planets share the same orbit?
« Reply #3 on: 07/03/2016 19:11:47 »
Haha, funny, is this a trick question?

Aren't the Earth and the Moon too planets sharing the same orbit?
 

Offline evan_au

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Re: QotW - 16.04.05 - Can two planets share the same orbit?
« Reply #4 on: 07/03/2016 19:56:13 »
Quote
Aren't the Earth and the Moon too planets sharing the same orbit?
It is true that the Moon orbits the Sun as much as it orbits the Earth - I understand that the Moon's orbit is always concave towards the Sun, which is not the case if the Moon were primarily orbiting the Earth.

Quote
is this a trick question?
The trick is in the definition of "Planet".

Although the Moon is spherical (one of the criteria for a planet), it does not dominate the mass in its orbit - the Earth is clearly dominant, which is why the Moon is a moon, not a planet.
 

Offline cheryl j

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Re: QotW - 16.04.05 - Can two planets share the same orbit?
« Reply #5 on: 08/03/2016 07:56:03 »
Would it matter how big two planets were in the same orbit?

Why are the planets such different sizes to begin with and in no particular size order?
This has been split off to a separate topic, see: http://www.thenakedscientists.com/forum/index.php?topic=66060.new#new

These are probably dumb questions, but I've just never considered it before.
« Last Edit: 08/03/2016 09:17:05 by evan_au »
 

Offline Colin2B

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Re: QotW - 16.04.05 - Can two planets share the same orbit?
« Reply #6 on: 08/03/2016 08:32:19 »
I understand that the Moon's orbit is always concave towards the Sun, which is not the case if the Moon were primarily orbiting the Earth.
It's more a loop the loop but the concave towards the sun is much greater than the convex period. Newton said it did his head in (I paraphrase) trying to work it out.
 

Offline cheryl j

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Re: QotW - 16.03.08 - How do pheromones work in humans?
« Reply #7 on: 13/03/2016 17:55:43 »
how did this thread get under pheromones?
 

Russell

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« Reply #8 on: 09/04/2016 17:36:59 »
The scientists seem to have answered a different question to the one asked. The questioner  asked if two planets could share the same orbit, not if they could orbit around each other. I think the questioner may have been wondering if a second planet could share the same orbit as Earth but be on the other side of the Sun for example
 

PhilJ

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« Reply #9 on: 14/04/2016 11:24:14 »
There are a couple of naming errors to point out here: Pluto's moon is Charon, not Sharon. And most importantly, the twin-sunned word in Star Wars was called Tatooine, not Dantooine :)
 

Offline puppypower

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Re: QotW - 16.04.05 - Can two planets share the same orbit?
« Reply #10 on: 14/04/2016 13:21:53 »
One way to answer the question of whether two planets can use the same orbit, is to first look at the planet Saturn and its moons. The moons of Saturn do not use the same orbit, but the rings of Saturn do use the same orbit. The difference, I see, has to do with the amount of gravity in the sub-units of the entries in the same orbit. The ring is composed of small units, each of which have limited gravitational impact on each other. We have tons of space debris orbiting the earth some of which uses the same orbit. The moons of Saturn have more gravitational influence on each other.

Say we start with two planets going around the sun, both in the same orbit. The rest of the planets of our solar system will periodically appear on one side of the sun, closer to one of the two planets. This will deflect its path, more than the planet that is on the opposite side; farther away. Now the two orbits will not longer be exactly the same. By the time the second planet reaches that same spot, the planets have moved, causing a slightly different deflection. Eventually we get two separate orbits.

In the unique case of a star with only two planets, both in the same orbit, this may scenario may last longer. However, both planets will need to be nearly identical in terms of size and mass, so there is gravitational symmetry.  If it is asymmetry, due to two difference sizes or mass, then the orbits will begin to depart.

Say we started with hundreds of planets around a sun; hypothetical. This would be a very violent place of planetary collisions as orbits are deflected into the path of others. In the end, there may still be a few planets left, on paths that remain distinct.
 

Offline PmbPhy

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Re: QotW - 16.04.05 - Can two planets share the same orbit?
« Reply #11 on: 04/05/2016 01:54:30 »
Quote from: SeanB
But not if there are other large bodies in the same system.
In reality that is correct. However that's only when we don't use the model where the mass of the other planet is small enough so as not to perturb the orbit of Earth significantly. When physicists calculate the orbits of planets they can often ignore such small perturbations because it doesn't effect the results of their calculations too much.

Quote from: SeanB
Then the disturbance from gravity from them will destabilise the orbits in a few million years quite a lot.
We're not certain if the OP had in mind exact calculations. It's rare not to use a model because its too difficult to take everything into account to get an exact answer, mainly because an exact answer is not required. In fact no calculation which uses data from measurements taken from physical objects in nature has ever been exact. Physicists always use a model of some sort.

In that spirit the answer to the question is yes.
 

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Re: QotW - 16.04.05 - Can two planets share the same orbit?
« Reply #11 on: 04/05/2016 01:54:30 »

 

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