发现:适宜生命存活的一类地行星

　　An Earth-size planet has been spotted orbiting a nearby star at a distance that would makes it not too hot and not too cold - comfortable enough for life to exist, researchers announced September 29.

　　If confirmed, the exoplanet, named Gliese 581g, would be the first Earth-like world found residing in a star's habitable zone - a region where a planet's temperature could sustain liquid water on its surface. And the planet's discoverers are optimistic about the prospects for finding life there。

　　"Personally, I would say, my own personal feeling is that the chances of life on this planet are 100 percent," said Steven Vogt, a professor of astronomy and astrophysics at the University of California. "I have almost no doubt about it."

　　Gliese 581g is one of two new worlds the team discovered orbiting the red dwarf star Gliese 581, bumping that nearby star's family of planets to six. The other newfound planet, Gliese 581f, is outside the habitable zone, researchers said。

　　The star is located 20 light-years from Earth in the constellation Libra. Red dwarf stars are about 50 times dimmer than our sun. Since these stars are so much cooler, their planets can orbit much closer to them and still remain in the habitable zone. Estimates suggest Gliese 581g is 0.15 astronomical units (One astronomical unit is the average distance between the Earth and sun) from its star, close enough to its star to be able to complete an orbit in just less than 37 days。

　　Gliese 581g has a mass three to four times Earth's, the researchers estimated. From the mass and estimated size, they said the world is probably a rocky planet with enough gravity to hold onto an atmosphere. Just as Mercury is locked facing the sun, the planet is tidally locked to its star, so that one side basks in perpetual daylight, while the other side remains in darkness. This locked configuration helps to stabilize the planet's surface climate, Vogt said. Between blazing heat on the star-facing side and freezing cold on the dark side, the average surface temperature may range from minus 31 to minus 12 degrees Celsius, the researchers said。