A new estimate from NASA suggests that there are a billion Earths in our galaxy, an extraordinary number that suggests the search for alien life has limitless possibilities.

By Earths, NASA means rocky planets that are about the same size as our blue ball, and orbits similar sized stars to our own in a the “habitable zone” where its temperature is relatively stable and where liquid water could exist on the surface, according to a Washington Post report.

That means that just in our Milky Way galaxy — let alone the countless galaxies in our universe — there are a billion planets that could host life, or perhaps even creatures as advanced or more advanced than us.

But how did scientists come up with this number? They used NASA’s Kepler Space Telescope, which was able to discover some new planets, including Kepler 452b, which has been making the most headlines lately. Kepler 452b is the most Earth-like planet ever found outside of our solar system, and even though it’s 60 percent bigger in radius than the Earth, it is probably a rocket planet and it is in the habitable zone of a parent star that is a G-type “yellow dwarf” star. The only problem is, it’s tough to know for sure the relative sizes of everything: Kepler 452b is 1,400 light years away and it is impossible to take a direct image of it.

Kepler is the only instrument that has been able to detect the planet, as it is too far away and dim to be detected by other instruments. Therefore there’s no guarantee it’s rocky, or that is has an atmosphere, or any water on its surface. Right now, all scientists have seen is the dimming of starlight from the host star, allowing for some rough estimates.

Scientists have taken some of these planetary discoveries over a small “cone” of view in our galaxy and used it to extrapolate a rough estimate of just how many of these types of planets we are likely to find in the Milky Way. And that’s where they got their “1 billion” figure.