Can two planets share the same orbit?

05 April 2016

Question

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?

Answer

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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