Yes (maybe) 2 planets can share an orbit

by | July 19, 2023, 12:00 PM | Stars & Nebulae

A planet and its Trojan orbiting a star in the PDS 70 system (annotated) Credit: ALMA

A whole fair amount of astronomy consists of theorists coming up with mathematically valid ideas of what is possible and then everyone debating if the universe would actually do something that wild. This is a common issue in particle physics, where researchers have predicted all manner of particles through theories such as supersymmetry. In that particular case, the universe seems to have responded with a solid “Nah. Not gonna do that.”

Other theories, however, do eventually get proven true, and a lot of nah sayers are left going “huh” as the universe has once again shown us that if you create enough things, pretty much anything can happen.

One of the theories a lot of us were willing to discount was published by Gregory Laughlin and John E. Chamber in 2002. In their paper, they suggested that if two planets had just the right separation, and were just about the same mass, they should be able to share an orbit.

This particular separation takes advantage of the gravitational balancing points that exist wherever a small object orbits a large orbit.

These points are called the Lagrange Points and we have them in the Earth- Sun system, and we use them as holding zones for satellites like the JWST and Solar Dynamic Orbiter, and with the Jupiter – Sun combination we find Trojan asteroids hanging out in the leading and trailing L4 and L5 Lagrange points.

In Laughlin and Chamber’s paper, they proposed that two worlds could orbit in one anothers Lagrange points if they were about the same mass. Essentially, the leading planet would sit in the leading, L4, Lagrange point of the trailing planet, and that Trailing planet would sit in the trailing, L5, Lagrange point of the leading plant.

The math all works out, but the probability of two similar sized planets landing in just this position just seemed impossible, so we haven’t exactly been going out of our way to find a solar system with this particular geometry.


Balsalobre-Ruza, O., de Gregorio-Monsalvo, I., Lillo-Box, J., Huélamo, N., Ribas, Á., Benisty, M., Bae, J., Facchini, S. and Teague, R., 2023. Tentative co-orbital submillimeter emission within the Lagrangian region L5 of the protoplanet PDS 70 b. Astronomy & Astrophysics675, p.A172. 10.1051/0004-6361/202346493