Martin J. Rees
Lord of Ludlow
Astronomer Royal of the UK: Since 1995
Emeritus Professor of Cosmology & Astrophysics: University of Cambridge
Chair of Advisory Group: Breakthrough Listen Project
Author: Our Cosmic Habitat†

How will humanity first discover ET Life?
Best guess: biosignatures for life in general, 
but technosignatures for intelligent life (although low probability) 

About biosignatures: [Microbes] could be freeze-dried on the 
Red Planet, [or] it cold be something under the ice of Enceladus, a moon of Saturn.

About technosignatures: If the emergence of intelligence on another world 
had followed the similar evolution to what has happened here, 
then were we to detect ET, we would be most unlikely to catch it in the 
brief sliver of time when it is still organic. For our Earth, between 
four and a half billion years before technology emerged, [lifeís] been 
around for a few millennia, maybe one more millennium, itíll be 
superseded by electronic intelligence which will then go on for 
billions of years. So that means that itís far more likely if we 
detected something artificial, it would be not an organic civilization 
but something which was created by a long dead organic civilization.

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