Searching for Exoplanets

Re “Life Among the Stars” (cover story, March 18): There may be trillions of planets in the universe. The important question to consider is what conditions should exist to make a planet habitable for animal life. Peter Ward and Donald Brownlee in their book Rare Earth: Why Complex Life Is Uncommon in the Universe show that more than 100 narrowly tuned constraints have to be satisfied for an Earth-like planet to exist. If so, this could bring the probability to one, i.e., us! Astrophysicist Frank Drake arrived at the same conclusion 40 years ago.