The mysterious Wow! signal could have come from a civilisation moving to a new star system 'aboard' a free-flying planet - dark and almost invisible.
No star can glow forever, endlessly maintaining stable conditions on its planets. A civilisation that manages to survive and evolve long enough will sooner or later run into this problem and will need to 'relocate' to a new suitable location. Science fiction often builds huge interstellar ships for this purpose, but you can generally get by with a more economical means of transportation - free-flying planets. By moving from one star system to another, they can deliver entire civilisations to a new home. Such a scenario is discussed in an article published in the International Journal of Astrobiology.
Orphan planets are not uncommon in our Galaxy. A random play of gravity can eject them away from their parent system, sending them on a long, free flight. Such planets are not illuminated by nearby stars and remain almost invisible to our instruments, so even estimates of their numbers remain very rough. Nevertheless, they can sometimes be spotted, for example, as the planet passes against a background of more distant luminaries. Orphan planets may retain satellites and oceans beneath the surface, so they could theoretically even be inhabited.
In new work, astrophysicist Irina Romanovskaya of Houston Community College has suggested that an alien civilisation preparing to "relocate" could take advantage of a passing orphan planet to relocate to it and embark on a long journey to a new star system. Suitable worlds may also appear in the parent system itself, if a change in a dying star throws one of its planets away. Alternatively, a fully developed civilisation may create an orphan planet of its own, simply by using one of the large objects on the periphery of the planetary system, giving it the appropriate acceleration.
In the absence of radiation from the parent star, controlled thermonuclear fusion may be the primary energy source on such a planet. By constructing housing and infrastructure beneath the surface, you can protect yourself from the planet's killer cosmic radiation. Once you reach your destination, you may be able to relocate to a more suitable planet, or bring your orphan planet into a stable orbit around the new star.
The search for such travelling civilisations should differ from traditional observations in search of alien intelligence. It is theoretically possible to spot technological evidence of their existence, but not to see the planet itself. Romanowska recalls the famous Wow! radio signal, which was recorded in 1977 and has never been repeated since. The nature of the signal remains a mystery and even the source of the signal is not exactly localised. Perhaps precisely because it is almost indistinguishable and moves quickly.