There is a problem with our approach at the moment when looking for alien civilisations. SETI looks for signals that are sent by the electromagnetic spectrum and their are a lot of frequencies to search. If you pair this with the size of the universe you can see how SETI is really looking for a needle in a haystack. SETI is still a good idea as one needle in one haystack is all that is needed.
There are other ideas one being the search for alien artefacts. Alien artefacts such as spacecraft, probes, dead aliens, abandoned bases, spanners etc would give a positive indication that ET had visited our solar system.
The Earth and the moon have been around for about 4.5 billion years compared to the search for SETI since 1960 when Frank Drake did his first Seti experiment. As you can see 50 years of searching with SETI compared to 4.5 billion years gives a large possibility of an alien leaving an artefact in our solar system.
So where would be the best places to look for alien artefacts? Earth is being searched all the time in our everyday lives as we go about our business and we haven’t found anything we can say is an artefact. The oceans are mainly unexplored as the finding of hydrothermal vents and life thriving around them has shown.
Our planet is not static but travels through the Galaxy as a spacecraft passing other stars and other stars passing us. With the discovery of exoplanets we may soon know how common oxygen containing atmospheres are. If these types of atmosphere are rare then the Earth would stand out to any alien civilisation around another star passing our position.
If a rich alien did notice our planet and decide to investigate the moon would be a much better place to observe the Earth than in orbit or actually on the Earth itself. The moon’s advantages include no atmospheric erosion of, geological problems or annoying apes/humans interfering with any equipment.
The advantages the moon brings to equipment placed for Earth monitoring when compared to an object in orbit around the Earth are protection from meteorites and radiation, fuel not needed to maintain its orbit, a source of materials for repair, heat control and support for life. If they fancied hiding the moon would obviously be a much better place than advertising their presence around the Earth in orbit. The moon’s much lower gravity may also be preferred.
Aliens would probably look at a landing on the moon as essential just as we are looking at Phobos as a base for missions on Mars. Therefore the moon could be full of artefacts. Large artefacts over time would probably disappear due to meteorites but the smaller bits and pieces such as parts of old destroyed artefacts may still exist beneath the surface covered by dust and meteorite impacts where they would be protected from further bombardment.
If an alien is thinking the same way as us it may well think that the moon is a likely place for alien artefacts and leave its own artefacts that we could eventually find.
Unless the aliens are very green they will probably pollute the space in our solar system. This could mean that artificial alien satellites have disintegrated into debris, each piece of debris having a different chemical composition which could be shown in multicoloured bodies. There is so much junk in orbit around the Earth that it would take quite a long time to search. Much further out where our pollution hasn’t affected the solar system we may have more luck.
If we look for debris that has fallen to Earth it is very easy to mistake objects and make false claims as many people do without significant evidence. Most are meteorites and lots of objects are classed as UFO’s but are actually objects that just have not been identified and are not spaceships etc. It would be very odd that they have flown all the way just to annoy us and fill up the Internet with lots of UFO pictures.
Rubbish from an alien civilisation could leak into the interstellar medium and eventually reach us. This could be due to light pressure which could expel extremely small debris particles from such things as rocket engines. An artefacts interaction with a planet may cause it to assume an asteroid or comet like motion due to gravity and therefore be flung away from a planetary system possibly towards our solar system. Computer simulations have been carried out and about 10 to 30% of small bodies are predicted to leave the solar system. Explosions of artefacts such as satellites and spacecraft could propel artefacts away from a planetary system.
Obviously these artefacts are going to take a long time to travel to us from even the closest star but don’t forget that the solar system has been around for about 4.5 billion years, it just depends on when the artefacts were expelled.
The search for alien artefacts may become more relevant in the future as we explore more and more of the solar system and as time goes by. The attitude towards this type of search for intelligent life signs doesn’t go down that well in most scientific circles although if you look at Seti it was once called pseudoscience but has been more widely accepted today. With the invention of astrophysics and astrobiology years ago people are beginning to accept that it is worth looking for signs of alien life. If basic life is found in the solar system on, say, Europa then the search may gain momentum and finding signs of intelligent life or complex life itself may become a reality.