How many civilizations in the Milky Way could be sending us messages right now? Play with the variables.
Frank Drake wrote his famous equation in 1961 as a conversation starter for a meeting at Green Bank Observatory. It multiplies a series of factors to estimate the number of detectable civilizations in the Milky Way: stars per year, fraction with planets, fraction where life emerges, fraction that becomes intelligent, and how long civilizations broadcast.
The problem is that every variable is a guess. We know the Milky Way forms about 3 new stars per year. We know about 50% of stars have planets. But we have no idea what fraction of planets develop life, or how long civilizations last. Some astronomers think the answer could be as low as 1 (us). Others estimate thousands.
The fact that we've detected no confirmed signal despite 60 years of searching is called the Fermi Paradox. "Where is everybody?" Maybe civilizations don't last long. Maybe they don't broadcast. Maybe we're truly alone in a quiet galaxy.
A formula estimating the number of detectable intelligent civilizations in the Milky Way. It multiplies factors like star formation rate, habitable planets, and civilization lifetime.
No. Estimates range from 1 to millions. The Fermi Paradox asks: if there are so many, where is everybody?
The range of distances from a star where liquid water can exist on a planet's surface. Earth is in the Sun's habitable zone.