After spending years searching for a planet with life, scientists may have found one. In September 2010, a team of US scientists discovered an Earth-sized planet called Gliese 581g. It has a surface temperature neither too hot nor too cold for liquid（液态）water. Scientists think there is a chance that the planet could have life.
The planet Gliese 581g goes around a star called Gliese 581, which is about 20 light-years from Earth. That is about 194 trillion（万亿）kilometres －500 million times farther away from us than the moon.
A spacecraft traveling at a one-tenth of the speed of light would reach Gliese 581g within about 220 years. A human can’t live that long, but robotic explorers could make the job easier. However, our fastest spacecraft don’t come anywhere near that one-tenth light-speed mark.
At present, scientists don’t know what the surface might be like on Gliese 581g. What they know is that the planet is at the sight distance from its star to have liquid water. It’s also at the right distance to have an atmosphere that could protect the water if it is on the surface.
Although spacecraft won’t be getting there anytime soon, one way to look for life on Gliese 581g is to turn our radio telescopes（望远镜）towards the planet and search for radiation（放射）. By studying the radiation, scientists would find out about the chemicals（化学物质）around the planet and discover it Gliese 581g has an atmosphere. If it has one, studying the atmosphere would give us a good idea if the planet has life or not. But it’ll probably be many years before we can do this properly.
People have been arguing about whether there is alien life on Gliese 581g. Any discussion about alien life is just guessing at this point, according to its discoverers.