Here are some of the unexpected ways that we might detect the hand of an extraterrestrial civilization.
Looking for Dyson Spheres and Similar Structures
The physicist Freeman Dyson has proposed that a highly advanced extraterrestrial civilization might aim to capture all or most of the solar energy coming from the star around which its planet revolves. The civilization could do this by building a sphere of solar energy collectors around the star. Such a sphere is called a Dyson Sphere. If such a sphere were built, the heat from it would give off lots of infrared radiation which scientists might be able to detect. Scientists such as Richard Carrigan have looked for sunlike stars with excess infrared radiation that might be the result of a Dyson Sphere. While some interesting candidates have been collected, nothing conclusive has been detected.
Looking for Artificial Planets
Let us imagine an extraterrestrial civilization many thousands of years more advanced than ours, or possibly millions of years more advanced than ours. Such a civilization might have the ability to move, break up and reorganize planets or moons within its solar system. The civilization might be able to do tasks such as (1) breaking up a gas giant and making it into smaller planets, which might be moved to a location closer to the local sun; (2) moving moons from a gas giant far away from the sun, to positions closer to the sun; (3) combining asteroids together to make a new artificial planet. Such activities might be possible to any alien civilization armed with an army of super-proficient robots.
NASA's Kepler Space Telescope has been very successful in detecting hundreds of extrasolar planets through a technique called the transit method, which tracks how the light of a star dims at periodic intervals when a planet passes between the star and an observer on Earth. Conceivably analysis of the Kepler data (or the data of its successor mission) might reveal indications of an artificial planet.
In fact, one could argue that what we have already discovered is a bit suspicious. The strangest finding regarding extrasolar planets is an unusually high number of “hot Jupiters” – Jupiter-sized planets that are very close to the stars they orbit (much closer than the orbit of Mercury). Could some of these be artificial planets – perhaps giant solar power collectors?
Imagine if an extraterrestrial civilization wanted to let other civilizations on other planets know that it existed. Besides sending out radio signals, the civilization could create a few artificial planets and position them in a way that indicated an artificial origin. One way to do that would be to make the relative distance of the planets from their sun follow an artificial pattern, such as 2, 3, 5, 7, 11, 13, and 17, the first seven prime numbers. Of course, it would be fantastically expensive to create such an arrangement, but once it was created, it would serve its purpose for billions of years. Any advanced civilization within 1000 light years (armed with its own version of the Kepler Space Telescope) might be able to detect the pattern, and know that intelligent life had existed on this planet.
It is conceivable that we might one day find evidence of extraterrestrial life by finding a planet pattern created by some alien civilization eager to broadcast its existence to its galactic neighbors.
Sniffing for Signs of Alien Life
One of the most powerful astronomical techniques is spectroscopy, which allows astronomers to detect elements in distant stars or planets. Spectroscopy involves passing the light from a distant light source through a prism. The prism breaks up the light into a series of colored bands. By analyzing those bands, scientists can tell what elements are in a distant star or planet.
We may soon be able to detect life on distant planets by using spectroscopy to analyze the light from planets detected through the Kepler Space Telescope. If we detect large amounts of oxygen in the planet's atmosphere, this would be an indication of abundant plant life. Most of the oxygen on our planet was produced by plants.
We might also be able to detect signs of intelligent life through such a technique. If we detected signs of chlorofluorocarbons (CFCS), this would be a telltale sign of extraterrestrial intelligence, as no natural process produces these molecules (produced on Earth through industrial processes).
Alien Gamma Ray Signals
An advanced civilization trying to get the attention of everyone in the galaxy could create a beacon using gamma rays, the most intense form of electromagnetic radiation. Astronomers have detected various gamma ray bursts from far away in space, most lasting less than a few minutes. Many of these are unexplained. It is easy to imagine us one day detecting a gamma ray burst with a deliberately encoded message, or with a deliberately artificial pattern.
Neutron Stars as Artificial Beacons
When the first neutron star or pulsar was detected, it was named LGM-1, with the LGM standing for Little Green Men. The pulsar made such an unusual radio source that astronomers initially wondered whether it was a radio beacon produced by an alien civilization. Today most astronomers think pulsars are purely natural phenomena, although Paul A. LaViolette has written a book arguing that some pulsars are artificial radio beacons. In any case, it is easy to imagine a superadvanced extraterrestrial civilization having the ability to manipulate a pulsar to use it as a signal transmitter. Such a technique might be worthwhile because of the great attention that pulsars attract (as they are a rare and interesting astronomical phenomena). Further analysis of the signals from neutron stars might one day turn up a telltale sign of an extraterrestrial intelligence.