Science Questions

Can two planets share the same orbit?

Tue, 5th Apr 2016

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Jonathan asked:

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?


Graihagh Jackson put this question to Dr Stuart Higgins from the University of Cambridge...

Stuart - In theory... yes.

Graihagh - Well, now that we’ve got that straight… Only joking. Astronomers actually have a special name for these things…

Stuart - Astronomers call such systems where two objects are orbiting around each other with a common centre between them - binary systems.  You might have recently about a binary system of two black holes whose fingerprint was discovered in gravitational waves, measured by the LIGO experiment.  In that case, the two black holes were spiralling into each other, merging but could it be possible for a less destructive scenario to occur with two planets.

Well, first of all it’s actually worth noting that when the Moon orbits the Earth, it’s not just the Moon moving.  The mass of the Moon has enough gravitational pull to also influence the motion of the Earth.  However, if you imagine drawing a line between the centre of the Moon and the centre of the Earth, the point on that line about which the Moon and Earth are rotating is located deep inside the Earth, very close to the centre in fact.  So, while the Moon does cause the Earth to wobble about a bit, because the Earth is so much bigger than the Moon, it’s essentially as though the Moon is just moving around the Earth.

Whereas, when we think of a binary system, say between two more equally matched objects.  If you were to draw an imaginary line between these, the point on that line about which they were rotating wouldn’t be inside either of the objects, they’re both rotating round a point of empty space.  And a classic example of this is Pluto and its moon, Charon.  Because they have roughly similar masses (Charon is about 12% the mass of Pluto), the impact it has on Pluto’s orbit is much greater than say the Moon’s orbit on the Earth.  This means that Pluto and Sharon slowly rotate around a point in space.  It looks a bit like an adult swinging a child around in the playground.  The adult’s feet remain at the centre of the rotation but, as they lean back, their head is also rotating around their feet as is the child.

Graihagh - Okay. Spinning children till their sick is one thing but I wanted to know is have we ever actually seen the rocky equivalent out there in the universe?

Stuart - Well, in 2012, astrophysicists using the KEPLER space telescope observed something even more complicated than that.  A pair of planets orbit round a pair a stars.  Imagine two suns orbiting orbiting closely around each other and then two planets at different distances orbiting around those rotating suns.  If you were standing on the surface of one of those planets and looked up at the sky, you would see two suns like the famous fictional planet of Dantooine from Star Wars.

Graihagh - Science fiction turns to science fact.  Great, I love it when that happens but that’s only one example.  Is there any other evidence that hint at these binary planets.  Well according to Stuart, we should be thanking our lucky stars...

Stuart - In 2014, scientists from the California Institute of Technology developed a computer simulation that binary systems of two earth-like planets are also possible.

Graihagh - There you have it Jonathan.  In theory yes but we’re yet to find too many examples.  So, watch this space. A big thank you to Stuart Higgins who helped us out with this one. Next time on question of the week we’re hot on the trail of Lebohang’s predicament.


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Yes alancalverd, Mon, 7th Mar 2016

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. SeanB, Mon, 7th Mar 2016

Haha, funny, is this a trick question?

Aren't the Earth and the Moon too planets sharing the same orbit? JoeBrown, Mon, 7th Mar 2016

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.

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. evan_au, Mon, 7th Mar 2016

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:

These are probably dumb questions, but I've just never considered it before. cheryl j, Tue, 8th Mar 2016

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. Colin2B, Tue, 8th Mar 2016

how did this thread get under pheromones? cheryl j, Sun, 13th Mar 2016

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 Russell, Sat, 9th Apr 2016

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 :) PhilJ, Thu, 14th Apr 2016

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. puppypower, Thu, 14th Apr 2016

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.

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. PmbPhy, Wed, 4th May 2016

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