One assumption scientists make is that advanced alien societies will need a lot of energy. There's something called the Kardashev Scale, which categorises potential alien civilisations based on their energy use:
- a Type I civilisation, also called a planetary civilisation, uses all the energy that falls on its planet from the star the planet orbits (~10^16 watts),
- a Type II civilisation, also called a stellar civilisation, uses all the energy of its parent star (~10^26 watts),
- a Type III civilisation, also called a galactic civilisation, uses all the energy of the galaxy it's in (~10^36 watts).
For comparison, the civilisations on Earth use about 4×10^12 watts, which means we've got a few orders of magnitude to go before we become a Type I. We're still just mildly intelligent apes.
A Type II civilisation would be harnessing the entire output of its parent star. One way they could do that is via a Dyson Sphere, which is a load of solar panels surrounding the star to trap the energy it emits. This would be a massive technological undertaking and any civilisation that could do such a thing would be formidable.
It is reasonable to assume we could spot a Type II civilisation by observing the means by which it harnesses the energy of its parent star, which means looking to see if it has a Dyson Sphere. The back panels of their Dyson Sphere would lose heat in the infrared spectrum, so we'd be looking for stars whose visible light is diminished — because the Dyson Sphere is blocking it — but which also have a strong infrared reading that cannot be explained in other ways.
Upcoming technologies, such as the James Webb Space Telescope, are thousands of times more powerful than the technologies we've used to date. Some scientists think there's a good chance we'll discover an extraterrestrial civilisation within the next 20 years, assuming there's one to be discovered in our galaxy.
I have my doubts to be honest.
: Not to be confused with the Kardashian Scale, which measures the amount of celebrity one acquires in relation to the size of one's bottom.
: And presumably their main source of telephone spam is from solar panel salesmen.