Ethan R. Siegel
Astrophysicist:  Ph D.: University of Florida
Blog: Starts with a Bang: Forbes, Scienceblogs
Author: Beyond the Galaxy  (WSPC: 2015)
Author: Treknology: The Science of Star Trek from Tricorders to Warp Drive (Voyageur Press: 2017)

How will humanity first discover ET Life?
Best guess: biosignatures (distant exoplanets)

Here in our own Galaxy, we literally have tens of billions of 
Earth-like planets around other stars. Planets that are the 
same size as Earth, that are the same distance from our Earth 
from our Sun, that orbit around a Sun-like star, capable of 
(life-) sustaining temperatures for billions or even tens of 
billions of years. With next generation telescopes, we will 
be able to tell if they are greening and browning with seasons, 
and detect oxygen, nitrogen, and carbon dioxide -- signatures 
of life. This could be our very first hint of life beyond Earth.

Where will we first find Alien Life?
On directly imaged exoplanets

Graph of Size Relative to Earth versus Orbital Period in Days
Earth-like conditions around Sun-like stars shown by the green oval
(presently empty)

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