In our search for extraterrestrial life, we must first figure out where to look. With so many new planets being discovered every day, we start with determining which ones are likely to have liquid water, that being the thing we believe that makes a planet likely to support life. We also have to look at the planet’s composition (liquid, solid or gas), along with its density and size. Finally, does that planet exist in its star’s habitable zone? All of these factors are important in determining the habitability of a planet.
A group of researchers have come up with a mathematical solution to find out how many planets in our own galaxy could support life. And it’s a lot more than we probably ever imagined. Their computational method suggests that at least 100 million planets in the Milky Way are habitable. And these planets don’t just harbor microbial life, but more complex life forms (although they are probably not intelligent beings like us).
These scientists looked at data for more than 1,000 planets in our galaxy that orbit around their own stars. By considering everything that makes a planet habitable, they created a Biological Complexity Index (BCI). Their calculations found that around 2 percent of those 1,000 planets have a higher rating than Europa, one of Jupiter's moons that may contain life. Factor in the fact that there are over 10 billion stars in our galaxy, their final number of habitable planets comes out to a considerable 100 million.
Of course, we won’t know for sure, at least until we develop something like a warp drive, because even the closest of these planets is really far away: at least 20 light-years. So for now, we’ll just have to take scientists’ word for it. However, it is probably safe to say that that we are definitely not alone in our galaxy.