【推荐1】Scientists think that     1     has been life on earth for     2     (million) of years. However, we haven’t found life on other planets yet. The earth is a planet and     3     goes around the sun. And there are seven other planets that also go around the sun. The sun and its planets are     4     (call) the solar system. The solar system is     5     small part of the universe.

Scientists have launched many spaceships to     6     [ɪkˈsplɔː(r)] other planets in the solar system. Some spaceships have gone     7     [bɪˈjɒnd] the solar system. However, no one     8     discovered any life in space yet.

Why has no one from other planets     9     ( send) us a message? Have they tried to send     10       (information) to us? With so many stars in space, are we alone, or is there life on other planets in space?

Scientists think that there has been life     1     the earth for hundreds of millions of years. However, we have not found life on any other planets yet.

The earth is a planet and it     2     (go) around the sun. Seven other planets also go around the sun. None of them has an environment     3     that of the earth, so scientists do not think they will find life on     4     (they). The sun and its planets are called the solar system, and our solar system is a small part of a much     5     (large) group of stars and planets, called the Galaxy or the Milky Way. There are     6     (billion) of stars in the Galaxy, and our sun is only one of them.

Scientists have also     7     (discover) many other galaxies in the universe. They are very far away and their light has to travel     8     many years to reach us. So how large is the universe? It is     9     (possible) to imagine.

Scientists have sent spaceships to the planet Mars to take     10     (photo). They have even sent spaceships to travel outside the solar system. However, no spaceship has traveled far enough to reach other stars in our Galaxy.