Astronomers have discovered the first Earth-sized planet in the "habitable zone," or the distance from a star that a water can pool on the planet's surface. Nasa Nasa / Reuters / Reuters While other Earth-like planets have been found, they've all been bigger than Earth, making understanding what they're made of a bit more challenging to understand. The new planet, Kepler-186f, resides about 500 light-years away from us in the constellation Cygnus (pictured below). Astrobobo/Astrobobo What scientists know: Its year is 130 days and that it gets one-third the heat energy that we do from our sun. What scientists don't know (yet): Kepler-186f is likely to be very rocky, but researchers can't confirm its mass, density, or composition. Wait, rewind. What is Kepler? Tap to play or pause GIF Tap to play or pause GIF NASA / Via youtube.com Kepler-186 is another system found using NASA's Kepler space telescope. The telescope, launched in March 2009, has discovered thousands of possible exoplanet candidates, with hundreds confirmed. And what's an exoplanet? AP Photo/NASA It's a planet that orbits a star outside our solar system. The new planet is the outermost of five planets in the system. NASA Ames/SETI Institute/JPL-CalTech / Via nasa.gov The other four measure less than 50% the size of Earth and orbit 4, 7, 13, and 22 days, making them way too hot to consider inhabitable. Kepler-186f orbits a smaller, cooler star that's half the size and mass of our sun, meaning it might be more of an Earth cousin than an Earth twin. Harvepino/Harvepino To give you an idea of the star's dimness, the brightness of the red dwarf's light at high noon is only as bright as our sun looks an hour before sunset. Why is this important? View this video on YouTube youtube.com The discovery is a big step toward finding more planets like ours. Kepler-186f proves Earth-sized planets can form in the habitable zone. And while we won't be rocketing toward it anytime soon, it might lead to siblings — and potential other life. The findings appear in a paper published in Science.