For thousands of years, humans(人) have explored (探索) the Earth. Nowadays, we are exploring space. Astronomers (天文学家) are the modern-day explorers. Now, many astronomers are looking for new planets and new places for humans to live in the future. But where do astronomers start exploring?
First of all, astronomers look for a star. That's because our own Earth moves around a star— the Sun. More importantly, it is neither too far from nor too near to the Sun for heat and light. So when astronomers have found the star, they look at the planets around it. In recent years, astronomers have found nearly 400 new planets with stars. However, many of these planets are either too near to the star or too far away.
However, if the planet is in a good place, astronomers look for three key things: water, air and rock (岩石). Water is important because all life needs water. Humans can drink it and they can also grow plants with water. And plants produce air for humans to breathe and food to eat. Rock on a planet is also important. That's because there is often water under the rock.
After many years of exploring, astronomers have found a planet like the Earth. It's Gliese 581g and it's near a star. The astronomers thinkithas water and rock and the temperature is between -31℃ and -12℃. That's cold, but not colder than Antarctica or the Arctic Circle, for example, Gliese 581g is bigger than the Earth. A year on Gliese 581g is only 37 Earth days, not 365 Earth days. But astronomers do not think these are big differences and some of them think Gliese 581g will be a new Earth. However, Gliese 581g is twenty light years from the Earth.
