WASHINGTON, April 17 (Xinhua) -- Astronomers said Thursday they have for the first time found a planet roughly Earth's size that may have liquid water on its surface and thus, theoretically, be habitable.
The discovery proves the existence of worlds that might be similar to our own and could also shape future investigations of exoplanets that could have terrestrial surface environments, the researchers reported in the U.S. journal Science.
"This is the first definitive Earth-sized planet found in the habitable zone around another star," lead author Elisa Quintana of the SETI Institute and NASA Ames Research Center said in a statement.
The new world, designated Kepler-186f, is the outmost of five planets orbiting Kepler-186, a red dwarf star some 500 light years from Earth.
Of the nearly 1,800 confirmed exoplanets found in the past two decades, about 20 orbit their host star in the habitable zone, a range of orbital distances at which surface water on a planet with an atmosphere would neither freeze nor boil.
However, all of these previously discovered worlds are larger than Earth, and consequently their true nature -- rocky or gaseous- - is unknown.
Kepler-186f was discovered using NASA's Kepler space telescope, which was launched in March 2009 to search for Earth-sized planets orbiting other stars.
On the basis of the observed dimming of starlight from Kepler- 186, the researchers estimated that this newly discovered planet is close to Earth's size -- just 10 percent larger in diameter -- and goes around its star once every 130 days.
"Theoretical models of how planets form suggest that those with diameters less than 1.5 times that of Earth are unlikely to be swathed in atmospheres of hydrogen and helium, the fate that's befallen the gas giants of our own solar system," said Thomas Barclay, a staff scientist for the Kepler mission affiliated with both NASA and the Bay Area Environmental Research Institute.
"Consequently, Kepler-186f is likely a rocky world, and in that sense similar to Venus, Earth and Mars."
But the researchers would not like to call it Earth 2.0. Because Kepler-186f orbits an M-dwarf, which is much smaller and cooler than the Sun, "we consider this planet more of an Earth cousin than an Earth twin," Quintana told Science.
More than 70 percent of the hundreds of billions of stars in our galaxy are M-dwarfs, so the finding could open a wide new hunting ground for extraterrestrial life. "Our galaxy is probably littered with cousins of Kepler-186f," Quintana said.
Traditionally, planets orbiting red dwarf stars were considered to be poor candidates for life. The objection was that star- hugging planets in the habitable zone would become tidally locked, and suffer a synchronous or pseudo-synchronous rotation that could make climate on these planets untenable.
However, more recent modeling studies suggested that such worlds are not necessarily inhospitable, since atmospheric winds or ocean currents could even out extreme temperature variations.
In this case, Kepler-186f is far enough away from its host star that it is unlikely to be locked. This greater distance also reduces the danger to any potential life forms posed by stellar flares, which are more common for dwarf stars, the researchers said.
In addition, M-dwarf stars have other features that make them promising places to look for life.
"For example, small stars live a lot longer than larger stars," said Stephen Kane, a co-author and assistant professor of physics and astronomy at San Francisco State University, "and so that means there is a much longer period of time for biological evolution and biochemical reactions on the surface to take place."
According to the researchers, Kepler-186f is too far away for follow-up surveys to probe its atmosphere, even with next- generation telescopes.
"However, our research tells us that we should be able to find planets around bright stars that will be ideal targets to observe" with NASA's James Webb telescope, said Quintana. The space-based telescope, now under construction, will be able to directly image planets around nearby dwarf stars, and use spectral analysis to characterize their atmospheres.
Finding Kepler-186f is a first, but "it's not a record we wish to keep," Quintana said. "We want to find more of these."