As I write these words, there are a total of 1,327 confirmed exoplanets that we have detected because they periodically block the light from their host star. Of those 1,327 exoplanets, only 38 orbit their stars in the habitable-zone (i.e., at distances that could allow for liquid water to exist on their surfaces). And of those 38 exoplanets, only 15 of them orbit long-lived, more stable stars like our Sun and its slightly cooler brethren. That’s only fifteen stars; fifteen habitable-zone, transiting planets around cool, stable stars. That’s where the search for intelligent life should begin according to Columbia astrophysicists David Kipping and Alex Teachey. Or at least the search for indirect signs of it.
We never actually see these planets directly. Instead, when a planet crosses in front of its star, we see the light from that star decrease by a measurable amount. How much it decreases, and how long, depends on the star system—and possibly its inhabitants.
NASA’s Kepler Mission is a space-based telescope dedicated to observing the light from over 150,000 stars in a patch of sky the size of your outstretched palm. (Fun fact: If we had 400 Keplers, we could observe the light from stars across the entire sky.) Kepler monitored each of these 150,000 stars for over four years looking for the tell-tale dip in light that could indicate the presence of a planet. That resulted in 150,000 light-curves, one per star.