For thousands of years， humans 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 looking？

First of all， astronomers look for a star． That's because our own Earth moves around a star（the Sun）． More importantly， it is the correct distance（ 距离） from 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 position， astronomers look for three key things： water， air and rock．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 scrutinizing．astronomers have found a planet that is similar to the Earth. It's Gliese 581g and it's near a star. The astronomers think it has water and rock and the average(平均的) temperature is between-310C and -120C. That's cold，but not colder than Antarctica or Arctic Circle, for example, Gliese 581g is bigger than the Earth. A year on Gliese 581g is only 37 Earth days instead of 365. But astronomers do not think there are big differences and some them thing Gliese 581g will be a new Earth. However, Gliese 581g is twenty light years from the Earth.

（1）What do astronomers have to find first before starting to look for a new place for humans to live？ 　　

A．	new star．
B．	A new planet．
C．	Some heat．
D．	Some light．

（2）What does the rock on a planet tell us？ 　　

A．	The star is in a right position．
B．	We may discover water under it．
C．	The planet is too near to the Sun．
D．	There can be air around the rock．

（3）What does the underlined word " scrutinizing" mean in the last paragraph？ 　　

A．	discussing．
B．	waiting．
C．	looking．
D．	traveling．

（4）What's the main idea of the text？ 　　

A．	Gliese 581g is already a new Earth．
B．	There are planets similar to our Earth．
C．	Water， air and rock are important．
D．	Astronomers are looking for a new Earth．