Naked Science Forum
General Science => Question of the Week => Topic started by: thedoc on 19/04/2016 10:03:20
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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?
Asked by Jonathan
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Yes
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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.
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Haha, funny, is this a trick question?
Aren't the Earth and the Moon too planets sharing the same orbit?
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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.
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.
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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.
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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.
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how did this thread get under pheromones?
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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
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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 :)
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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.
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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.
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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NO! Each planet has it's own orbit.
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NO! Each planet has it's own orbit.
If you use an orbit simulator, and try to set up two planets in the same orbit, you will see that eventually they deflect each other into different orbits; this can happen fairly quickly if there are other planets in the same planetary system.
Eg see: http://www.testtubegames.com/gravity.html
There is another scenario where two objects may orbit the Sun in (roughly) the same period, and at the same average distance from the Sun, without colliding with each other. This is called a "horseshoe orbit". There are several asteroids in horseshoe orbits which share Earth's orbit.
See: https://en.wikipedia.org/wiki/Horseshoe_orbit