For thousands of years, humans have explored the Earth. Now astronomers are exploring space, looking for new planets or a “new Earth” for humans to live on in the future. But what will a new Earth look like? And how do we find a “new Earth”?

First of all, astronomers look for a star. That’s because our own Earth orbits a star (the Sun). When astronomers have found a star, they look for planets around it. In recent years, astronomers have found thousands of new planets orbiting suns. Once they find a new planet, it’s important to measure the distance between the planet and the sun. In fact, most planets (unlike our Earth) are either too near to the star or too far away.

When they find a planet in a good position, astronomers look for three things: water, air and rock. Water is important because if humans go there, they will need water to drink and to grow plants. If plants can be grown, they can produce air for humans to breathe. Then, astronomers also look for rocks, as water is often under the rocks.

In recent years, astronomers have found a few planets that are very similar to Earth. For example, Gliese 581g is a planet at a safe distance from the nearest star. Astronomers think it has water and rocks. The average temperature is between -310°C and -120°C which is cold, but not colder than Antarctica or the Arctic Circle. Humans can live in those temperatures.

However, there are differences between our Earth and this planet. Gliese 581g is bigger than the Earth and a year on Gliese 581g is only 37 Earth days instead of 365. And the biggest problem is the distance. Gliese 581g is 18 trillion kilometers from the Earth. With current space technology, it will take humans 766,000 years to travel there.

Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions.

The main idea of the passage is .