Humans are one step closer to finding out if we are alone in the galaxy. After four years of observing the sky and three years sifting and analyzing data, NASA announced that the Kepler space telescope discovered 1,284 planets, nine of which could potentially host Earth-like habitable conditions.

According to NASA, Kepler launched in March 2009 and is the first NASA mission to find potentially habitable Earth-size and super-Earth sized planets. The telescope monitored 150,000 stars in a specific patch of sky for four years to measure "tiny, telltale dip in the brightness of a star that can be produced by a transiting planet," NASA writes on its website

In the cluster of 1,284 planets, 550 of them seem to be rocky planets similar to Earth. Nine of these rocky planets orbit a sun in a "habitable zone," which means the distance from the star allows the planets' temperatures to "allow liquid water to pool," NASA says. These nine planets bring the total number of potentially habitable exo-planets to 21.

"This announcement more than doubles the number of confirmed planets from Kepler," Ellen Stofan, chief scientist at NASA Headquarters in Washington, says in NASA's press release. "This gives us hope that somewhere out there, around a star much like ours, we can eventually discover another Earth."

The method used to find the planets, monitoring the twinkling of stars, is a labor-intensive process, NASA says. Each potential planet is verified one-by-one, but Timothy Morton, associate research scholar at Princeton University in New Jersey, employed the first automated computation technique, which assigned each potential planet a probability percentage and used statistical analysis to confirm a candidate planet's "planet-hood" status, NASA explained.

NASA also discovered during this mission that there might be more planets than stars, according to Paul Hertz, astrophysics division director at NASA.

"This knowledge informs the future missions that are needed to take us ever-closer to finding out whether we are alone in the universe," Hertz said.