Thanks to NASA's Kepler spacecraft, we've discovered over a 1,000 new exoplanets, or planets outside our solar system. Other missions have discovered even more. Because there are so many, though, sometimes we forget about all the discoveries made.

Now, however, NASA reminds us again of these amazing alien worlds by releasing some gorgeous travel posters for a few of the more interesting exoplanets discovered.

Kepler 186-f is one of those alien worlds that may sustain life. It's about the same size as Earth and orbits its star within the habitable zone, or that distance from the sun that allows for liquid water to exist on a planet's surface. However, it's star is red, which means that the colors on 186-f aren't like the blues and greens of Earth.

Planet HD 40307g is probably not that habitable, although it's possible that its surface is rocky, like that of Earth. However, this planet is not only twice the size of earth, but it also has eight times more mass, which means it has a lot more gravity, making it more like Star Trek's Vulcan and less like Earth.

Kepler-16b orbits around two stars, making it part of a binary system. Although NASA's poster shows it as more Earth-like, it's possible that 16-b is actually more like Saturn, and mostly composed of gas. It's unlikely that life exists on Kepler 16-b as NASA describes its "temperature similar to that of dry ice." However, Kepler16-b captures imaginations because of its binary stars, which makes us think of Star Wars' Tatooine.

NASA's Kepler mission finds planets by studying how they transit, or pass in front of, their stars. When a planet moves in front of its star, it causes the star's brightness to dim. Kepler measures this change in brightness and confirms if the phenomenon occurs due to a planet orbiting in front of the star.

So far, Kepler has found a total of 1,004 exoplanets, and is still searching. This is extraordinary considering that Kepler's mission nearly came to an end last year after its positioning wheels failed. However, scientists figured out a way to use the radiation pressure of the sun to position the telescope, and its mission continued.

Scientists hope that some of these planets are more like Earth and capable of sustaining life, and it's possible that several exoplanets already discovered by the mission might.

"With each new discovery of these small, possibly rocky worlds, our confidence strengthens in the determination of the true frequency of planets like Earth," says Doug Caldwell, SETI Institute Kepler scientist at NASA's Ames Research Center. "The day is on the horizon when we'll know how common temperate, rocky planets like Earth are."

[Photo Credit: NASA]

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