The Search for Extraterrestrial Intelligence (SETI) is expanding. Dominated for decades by the search for radio transmissions from aliens, SETI has recently included optical searches for laser beams. A third SETI option, the search for extraterrestrial artefacts, is also gathering pace.

Astronomers have conducted searches for Dyson Spheres using data from infrared astronomical satellites. Dyson Spheres are hypothetical shells built around entire solar systems, which could only be constructed by civilizations with highly advanced technologies.

The spheres are designed to capture most of the energy radiated by the sun at the center of the system. Dyson Spheres would be extremely difficult to deploy, but they should, in theory, be fairly easy to detect. They would have a profound effect on the spectrum of energy radiated by a star. One recent search for Dyson Spheres examined stars outside our own galaxy.

Dyson Spheres are probably at the extreme end of possible artefacts that could be constructed by an extraterrestrial civilization. But could it be possible to detect other infrastructure in other star systems?

An upcoming paper in the journal Acta Astronautica has addressed this question. Written by the author of this article, the paper "Reconsidering macro-artefacts in SETI searches" considers what else extraterrestrials could build that could be large enough to be seen by astronomers from Earth.

The list could be extensive. Space colonies could be large and highly conspicuous. Vast solar sails could fly between the planets. Space mining operations could be extensive. Extraterrestrials could construct huge space mirrors to turn night into day on their homeworld, or huge screens to block out solar flares.

Actually taking images of these structures at interstellar distances would be difficult. Current astronomical observatories simply aren't good enough. But it could be possible to detect such objects through spectral observations, just as astronomers have used infrared spectral analysis to search for Dyson Spheres.

Subtle changes in the wavelengths of energy emitted or reflected by these objects could be enough to reveal that something unusual is there.

In the future, large telescopes will be constructed in space. These will probably offer capabilities that make our currently most advanced telescopes look obsolete. It's entirely possible that these advanced telescopes will allow some extraterrestrial constructions to be directly imaged, making detections easier and more credible.

Searching for extraterrestrial macro-artefacts is an exciting proposal, but it's also highly controversial. How can we be sure that something unusual is really artificial, and not a previously unknown natural astrophysical object? When pulsars were first detected, they were briefly suspected by some astronomers to be artificial radio beacons from extraterrestrials. Could this scenario repeat itself in the future?

Some natural objects exhibit the rigid geometries of artificial construction. Some artificial constructions mimic the fluid, organic structures found in nature. Confirming an object as a genuine extraterrestrial artefact would require an extensive amount of data collection, analysis and deduction.

Apart from building infrastructure in space to serve their own purposes, extraterrestrials may also choose to construct large artefacts as deliberate signals of their own existence. These would be like signboards in space, designed to be detected by alien (to them) astronomers in other star systems. These "signboards" would need to be very large, but they could be robotically constructed over long periods.

Technology similar to our own 3-D printers could gradually extrude them. The signs could be constructed with regular geometric shapes to make them distinguishable as artificial objects. Some could even be constructed as large platonic solids.

Nobody really knows when or if we will find evidence of extraterrestrial intelligence. But another question also complicates the quest. What is the best way to really search for them? A multifaceted search, using a variety of techniques, probably offers our best hope of proving that we are not alone.

Research paper