Scientists have for the first time discovered an Earth-sized alien planet in the Goldilocks zone that may have liquid water and right conditions for life.
Using NASA's Kepler Space Telescope, astronomers discovered the planet orbiting a star in the "habitable zone" - the range of distance from a star where liquid water might pool on the surface of an orbiting planet.
The discovery of Kepler-186f confirms that planets the size of Earth exist in the habitable zone of stars other than our Sun, NASA said.
While planets have previously been found in the habitable zone, they are all at least 40 per cent larger in size than Earth, and understanding their makeup is challenging. Kepler-186f is more reminiscent of Earth, researchers said.
"The discovery of Kepler-186f is a significant step towards finding worlds like our planet Earth," said Paul Hertz, NASA's Astrophysics Division director at the agency's headquarters in Washington.
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.
"Finding a habitable zone planet comparable to Earth in size is a major step forward," said Elisa Quintana, research scientist at the SETI Institute at NASA's Ames Research Centre in Moffett Field, California, and lead author of the research.
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 per cent of the stars in the Milky Way galaxy.
"M dwarfs are the most numerous stars. The first signs of other life in the galaxy may well come from planets orbiting an M dwarf," said Quintana.
Kepler-186f 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.
"Being in the habitable zone does not mean we know this planet is habitable. The temperature on the planet is strongly dependent on what kind of atmosphere the planet has,"