Building planetary systems that can last forever

Vacations can be a lot of fun. Visit historic towns, laze in the sun on a tropical beach, or unwind at a relaxing mountain resort. But while the destinations are great, travel itself can be a chore. Crowds, tight flights, delays. It would be nice if there was a shortcut to our destination. Now imagine vacationing in the galactic empire. It’s great to visit the diamond shores in Exoticon 5, but no one enjoys all that stuttering in hyperspace. So why not make these worlds closer to home? That’s the idea behind a study recently published in Monthly Notices of the Royal Astronomical Society. It’s basically looking at how a super-advanced civilization could have populated a group of planets in the habitable zone of a single star.

One downside of habitable zones is that they tend to be fairly narrow in scope. in our area Solar SystemVenus, for example, is too close to the Sun, and Mars is too far away for either to be solidly in the Sun’s habitable zone. If we become a very advanced species in the future, we might push the orbits of Venus and Mars closer to Earth, but that could present some problems of its own. In particular, if the orbits are too similar, gravitational perturbations could make all three orbits unstable over thousands of years, which would destroy the whole goal of re-engineering our system.

Fortunately, there is a way to make two worlds share very similar orbits. We see this with two moons of Saturn, Epimetheus and Janus. Most of the time one of them is a little closer orbit to Saturn, which means that it is going faster until it is almost catching up with the other. Then the two moons do a little gravitational dance, as the outer moon does moon inward, and the inner moon outward. So the two moons never collide, even though they share essentially the same orbit. For Epimetheus, Janus traces a horseshoe-shaped path, which is why they are called horseshoe orbits.

In principle, two Earth-like worlds orbiting a Sun-like star could have common horseshoe orbits, thus sharing a common habitable zone. There are examples of small bodies captured in a horseshoe orbit with Earth, but they tend to be unstable. Orbitals become more stable if they are of similar mass.

For this study, the team wanted to see how many worlds could fit into a similar orbit. They began with the assumption that all worlds would be an Earth mass, and that they would orbit a Sun-like star at 1 au. They found that as you add more planets, the orbits become a bit more variable, but it is possible to group up to 24 worlds from Earth into a stable horseshoe-shaped resonance. The orbits of these worlds will be stable for billions of years with the right setup.

The team went so far as to study what such a system might look like from light years away. If the orbits of such a system are aligned to pass in front of their star from our vantage point, we can detect them as exoplanets using the transit method. Such a strange system could prove the existence of an advanced civilization.

We are unlikely to ever discover such an unusual system, but the thought of this idea is ferocious, especially if you try to imagine what our night sky would look like if it were filled with 23 other planets. It would definitely be worth taking a vacation somewhere with dark skies to enjoy the view.