Building Planetary Systems That Could Last Forever

Vacations can be quite enjoyable. Visiting historic cities, lounging in the Sun on a tropical beach, or snuggling up at a cozy mountain resort. But while the destinations are great, traveling itself can be a chore. Crowds, cramped flights, delays. It would be great if there were some short cut to our destination. Now imagine the vacationers of a galactic empire. It’s great to visit the diamond shores of Exoticon 5, but nobody enjoys all that mucking about in hyperspace. So why not bring these worlds closer to home? That’s the idea behind a study recently published in MNRAS. It basically looks at how a super-advanced civilization might pack a bunch of planets into the habitable zone of a single star.

One of the downsides of habitable zones is that they tend to have a fairly narrow distance range. In our solar system, for example, Venus is too close to the Sun, and Mars is too distant for either of them to be solidly in the Sun’s habitable zone. If we become a super advanced species in the future, we might nudge the Venusian and Martian orbits closer to Earth, but that could raise some problems of its own. In particular, if the orbits are too similar, gravitational perturbations could make all three orbits unstable over the course of thousands of years, which would ruin the whole point of re-engineering our system.

Fortunately, there is a way to have two worlds share very similar orbits. We see this with two moons of Saturn, Epimetheus and Janus. Most of the time one of them has a slightly closer orbit to Saturn, which means it speeds along faster until it almost catches up to the other. The two moons then do a little gravitational dance, where the outer moon is pulled inward, and the inner moon outward. So the two moons never collide, even though they essentially share the same orbit. Relative to Epimetheus, Janus traces a horseshoe-shaped path, which is why these are called horseshoe orbits.

How multiple planetary horseshoe orbits might evolve over time. Credit: Raymond, et al

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

For this study, the team wanted to find out how many worlds could be packed into a similar orbit. They started with the assumption that all worlds would be Earth-mass, and 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 pack as many as 24 Earth worlds into stable horseshoe resonances. The orbits of these worlds would be stable for billions of years with the right set-up.

The team went further to study how such a system might look from light-years away. If the orbits of such a system were aligned to pass in front of their star from our vantage point, we could detect them as exoplanets using the transit method. Such an odd system could prove the existence of an advanced civilization.

We aren’t likely to discover such an unusual system, but it is a wild idea to contemplate, particularly if you try to imagine what our night sky would look like if it were filled with 23 other Earths. That would definitely be worth a vacation to a dark-sky location to take in the view.

Reference: Raymond, Sean N., et al. “Constellations of co-orbital planets: horseshoe dynamics, long-term stability, transit timing variations, and potential as SETI beacons.” Monthly Notices of the Royal Astronomical Society 521.2 (2023): 2002-2011.