There have only been around 600 people who have traveled to space. Of the currently-active astronauts, most are scientists and the rest are highly educated. Do these folks, who have touched the vastness of outer space, believe in life on other planets? Their answers are open-minded, because while they know a lot compared to most of us, they are very aware of how little we actually know about space.
"We've basically proven that every star has planets," said Canadian astronaut Chris Hadfield, who has spent 4,000 hours in space. "Then you start doing the math."
The math isn't easy. How many stars are in the universe? Well, that depends on the size of the universe. We're able to observe the cosmic microwave background (CMB), radiation formed around 400,000 years after the Big Bang. It tells us the observable universe goes back around 14 billion years. But there could be something beyond the CMB, or even other universes contained in a massive "multiverse."
Within the constraints of the observable universe, there could be 1,000,000,000,000,000,000,000,000 (or septillion) stars, according to astronomer David Kornreich. (He conceded to Space.com that the number could be a gross underestimate.)
If every one of those stars has at least one planet, then, well, it seems inconceivable that life wouldn't exist elsewhere.
Astronauts also know that "life" doesn't equate to the aliens we see in movies. Read more from Hadfield and astronauts Mae Jemison and Jeff Hoffman on the subject of extraterrestrial life at Mashable.