Aliens

stars in our galaxy, which is about 100,000 light-years across.

potentially habitable rocky planets around Sun-like stars, a conservative count. The nearest is likely about 20 light-years away.

Earth-like planets in the galaxy today.

of Sun-like stars host an Earth-size planet receiving about as much light as Earth does (1–4 times).

communicating civilizations right now (range 4–211), the nearest within about 17,000 light-years. Assumes intelligence takes about 5 billion years to arise, as it did here, and civilizations broadcast for 100 years. Under looser assumptions: at least 928.

If the odds that a habitable planet ever produces a technological species are better than this, we are probably not the first in the galaxy.

stars, about 2.5 million light-years away.

of all stars ever formed in Andromeda may have planets with basic and complex life. The richest zone is a ring 12–14 kiloparsecs from the centre.

kiloparsecs from the centre: where the most stars with habitable planets sit in Andromeda versus the Milky Way. In Andromeda that peak sits twice as far out.

Earth-like planets (100 billion billion). Earth formed after 80% of them.

at least, the chance we are not the only civilization the universe will ever have. More than ten times as many planets are still to form as exist today.

If the odds per habitable planet are better than this, humanity is not the only time technological intelligence has evolved.