Simulations show planets that jockey for position around their star and appear to bounce off each other could survive in stable configurations - but it is unclear if they could occur naturally.
Planets that orbit at very similar distances from their stars could jockey for position while still remaining in a broadly stable configuration for billions of years – long enough to be spotted by astronomers. Such star systems might be so unlikely that they could only be created artificially, by advanced alien civilisations.
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| Are strange star systems out there waiting to be discovered? Jurik Peter/Shutterstock |
Most planets have their orbits to themselves, but orbital mechanics allows for worlds to be close enough to effectively share an orbit, forcing them into a helter-skelter dance where they swap places with respect to the distance from their star. As well as that orbital change, such worlds would repeatedly get nearer to each other and then recede a bit.
Someone standing on the surface of such a planet would see the other appear to approach and then reverse direction, seemingly tracing out a horseshoe in the sky.
We only know of one such orbit arrangement in our solar system, between two of Saturn’s moons, Janus and Epimetheus, but whether planets can exist in this configuration is unknown. To learn more, Sean Raymond at the University of Bordeaux in France and his colleagues simulated different arrangements of Earth-sized planets sharing an orbit and assessed their stability.
“We don’t know of any planetary systems that have these seemingly crazy orbital architectures, but it’s interesting to see what’s possible,” says Raymond. “The idea behind that is if anything is possible, it’s worth looking for and maybe we can find it.”
Raymond and his team ran simulations of different numbers of planets that start on the same orbit, equally spaced around their star. Many of these planetary systems will stay equally spaced, but some of them will fall into stable horseshoe configurations for billions of years, with the most complex system containing 24 planets all jostling around.
While it is possible that systems with two or three planets may have formed naturally like this, finding one with many more planets is so improbable that it might be evidence of a system designed by aliens, says Raymond. “It’s either a super-duper rare outcome of planet formation, or some civilization has put the planets in that place on purpose.”
The fact that these stable systems exist in simulations is a useful indicator that they might occur for real, says David Brown at the University of Warwick, UK. However, detecting them might be difficult because of how similar the signals we rely on, such as the change in light coming from their stars, might look to other configurations, he says. “Working out that it’s this kind of horseshoe configuration rather than a planet on a shorter period orbit is going to be tricky.”
Reference:arxiv.org/abs/2304.09210 & arxiv.org/abs/2304.09209