Can a star have a pair of planets in the same orbit? originally appeared on Quora, the place to gain and share knowledge, empowering people to learn from others and better understand the world. You can follow Quora on Twitter, Facebook, and Google Plus.

Can a star have a pair of planets in the same orbit?

People often visualize this as two planets orbiting on opposite sides of a star, like an Earth and anti-Earth. It's a fun idea - great for science fiction stories! - but that is an unstable configuration that could not possibly last the age of the solar system.

There are two ways that planets could share an orbit in a stable or quasi-stable way.

One possibility is a planet orbiting in the L4 or L5 Lagrangian point of a larger planet: a region 60 degrees ahead of or behind it in the same orbit around its star. We see this arrangement with Jupiter's Trojan asteroids.

The L4 and L5 points are considered stable if the mass ratio between the two objects is greater than about 25:1. In principle, then, a Jupiter-like planet could have an Earth-size planet sharing its orbit. Some models suggest such a configuration could be stable even for equal-mass planets.

Another possibility is “horseshoe orbits,” in which two bodies orbit at nearly the same average distance and swap distances or eccentricities with each other in such a way that the system remains in equilibrium.

As unlikely as it sounds, this arrangement really exists in our solar system. Astronomers originally thought Saturn's moon Janus and Epimetheus were a single object. Eventually they discovered that there are two moons of similar size doing a stable horseshoe orbit-swap around Saturn. The same mechanism could theoretically work for planet-size objects around another star.

Astronomer Greg Laughlin has analyzed a variety of these orbital possibilities. If you are interested in more technical information.

The big limiting factor with these exotic shared-orbit configurations is perturbations by other planets in the same system. The gravitational pulls from those planets could quickly (in astronomical terms) destabilize arrangements that would be stable on their own.

Update: There is third possibility that I didn't mention because it is rather different than the apparent intention of the question. Two planets could share an orbit around a star if they form a double planet, in a bound orbit around each other. As an example, if the Moon were five times more massive, it would be bigger than Mercury and we might legitimately consider Earth a double planet.

The most intriguing possibility is an equal double with two Earthlike planets. There's nothing remotely like that in our solar system, but the history of exoplanet research has shown us that nature is full of creative ideas, including ones that humans never considered plausible until we saw them in the wild.

This question originally appeared on Quora. More questions on Quora:

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