LOS ANGELES, April 17 -- Astronomers using the Kepler Space Telescope have discovered the first planet that is about the same size of Earth and orbits a star within the "habitable" zone, NASA announced Thursday.

The new discovered planet, Kepler-186f, "can be thought of as an Earth-cousin rather than an Earth-twin," it has many properties that resemble Earth, according to Thomas Barclay, research scientist at the Bay Area Environmental Research Institute at NASA's Ames Research Center in Moffett Field.

"The discovery of Kepler-186f is a significant step toward finding worlds like our planet Earth," said Paul Hertz, director of NASA's Astrophysics Division.

Kepler-186f resides in the Kepler-186 system, about 500 light years from Earth in the constellation Cygnus. The system is also home to four companion planets, which orbit a star half the size and mass of our sun. The star is classified as an M dwarf, or red dwarf, a class of stars that makes up 70 percent of the stars in the Milky Way galaxy.

The planet is on the "outer edge" of the habitable zone. It orbits its star once every 130-days and receives one-third the energy from its star that Earth gets from the sun, placing it nearer the outer edge of the habitable zone. On the surface of Kepler-186f, the brightness of its star at high noon is only as bright as our sun appears to us about an hour before sunset, according to NASA.

A planet is considered in the "habitable zone" if its distance from a star could allow the pooling of water on its surface. According to NASA, other planets have been found in the "habitable" zone of stars, but they were all at least 40 percent larger than Earth. And none of them are as close in size to Earth as Kepler-186f, whose diameter is only 10 percent larger.

Scientists said, size is a critically important factor. If a planet is about 50 percent wider than Earth, and it' s packing a lot of mass, its gravity could attract a hydrogen-helium envelope, shrouding the surface in a gassy atmosphere that' s too thick for Earth-like life.

Although the size of Kepler-186f is known, its mass and composition are not. Previous research, however, suggests that a planet the size of Kepler-186f is likely to be rocky.

"Being in the habitable zone does not mean we know this planet is habitable," Barclay said. "The temperature on the planet is strongly dependent on what kind of atmosphere the planet has."

"Future NASA missions, like the Transiting Exoplanet Survey Satellite and the James Webb Space Telescope, will discover the nearest rocky exoplanets and determine their composition and atmospheric conditions, continuing humankind's quest to find truly Earth-like worlds," Hertz said.

The Kepler Space Telescope, which simultaneously and continuously measured the brightness of more than 150,000 stars, is NASA's first mission capable of detecting Earth-size planets around stars like our sun.