As a planet orbits around a star, it blocks out a certain amount of light and causes it to dim from our perspective on Earth. When we looked at Boyajian's star, however, we found that it was dipping by variable amounts. This led to wild speculation that we could have found a "Dyson sphere" – a theoretical megastructure built by an advanced extraterrestrial civilization around a star to harness its energy.
Of course, the explanation turned out to be dust. But in a new preprint paper, which has not been peer-reviewed, astronomers propose that Dyson spheres might not be the best idea for an advanced civilization, and go on to suggest a more viable alternative: moving planets with freaking laser beams.
The team explains that when physicist Freeman Dyson proposed a Dyson sphere, we didn't know of the potentially billions of planets out there even within our own galaxy.
"With such abundance of planets, there would be no need to destroy the entire planetary system to make one sphere," they write in their paper. "Besides, to construct just an 8–20-cm [3-9-inch] thick shell would require all [the] Solar System’s rocky material including Earth, destroying any possible life, which goes against the principle of Planetary Protection – the UN policies governing the preservation of [the] Solar System."
Instead, they looked into the energy required to move planets into different orbits. The idea is that an advanced civilization looking for more resources, or to move to the habitable zone (HZ) of a solar system, could shift whole planets at their leisure.
"These shifts can be performed at a constant low-thrust acceleration using high power directional lasers, resulting in a gradual spiral transfer from one orbit to another."
The method could be used to bring resources of other planets to us, or even to move Mars to the habitable zone, perhaps melting the ice caps and sending water into the atmosphere to help with terraforming. The team calculated that it would require a whole lot less energy and resources than constructing a Dyson sphere.
More excitingly for a species like us not ready to play snooker with planets, if aliens are doing this already it gives us something to look for. The team found that these lasers would be detectable with our current technology, at a relatively low cost. We may even be able to find these civilizations if they are (understandably) trying to hide from the universe.
"If advanced civilizations were capable of moving planets using lasers, it is likely that they would have the technological capabilities to do so without leaving any detectable traces. They may have developed methods of propulsion that do not produce any detectable emissions," the team writes. "So, then how do we know that advanced civilization have moved planets into their HZ? If an advanced species had already moved a planet, then we would see too many planets in the HZ."
Looking at solar systems with high numbers of planets within the habitable zone – such as the TRAPPIST-1 system, which contains four – might be the best way to look for the telltale signatures of a civilization moving planets around with laser beams. The unusual Kepler-20 star system, with six planets close to the host star, may be even more interesting.
"What is unusual is that the planets alternate between small and large with increasing distance from the star. This is totally different compared with the usual situation with small inner planets and exterior gas giants," the team explains, adding "it is quite possible that such planetary systems might been artificially constructed by moving planets intentionally."
The study has been posted to preprint server arXiv.