Scientists think that there has been life on the earth for hundreds of millions of years. However, we haven’t 【小题1】 life on any other planets yet.

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

Scientists 【小题6】 many other galaxies in the universe already. They are very far away and their light 【小题7】 to travel for many years to reach us. So how large is the universe? It is impossible to imagine.

Scientists have sent spaceships to the planet Mars to take photos. They have even sent spaceships to travel outside the solar system . However, no spaceship has travelled far enough 【小题8】 other stars in our Galaxy. Scientists have always asked the questions: With so many stars in the universe, are we alone or is there life in space? Have there been visitors to the earth from other planets? Why has no one 【小题9】 with us? We 【小题10】 the answers yet.

Our earth gets its warmth and light 【小题1】. The sun is a huge blazing（酷热的）ball, a million times 【小题2】 the earth. And it is millions of miles away. The earth spins（旋转） round once every 24 hours. It turns to face the sun and spins away every 24 hours.

When our part of the earth looks on to the sun, we see sunlight and we 【小题3】. When our part turns away from the light, we have night.

When our part begins to turn 【小题4】 and we see the first sunlight, we call it morning.

When we are facing the sun most directly, it is noon. Then the sun is closest to begin straight（直接地）above us. When we begin to 【小题5】 from the sun, we have afternoon, evening and night.

The universe is very big. But how large is the universe? It is impossible 【小题1】 how large the universe is.

But we know the earth is our home. It is a planet and it 【小题2】 around the sun. The moon goes around the earth. There are seven other planets go around the sun. The sun and its planets are a system 【小题3】 the solar system. So far, some astronauts 【小题4】 to the moon. Some scientists 【小题5】 many spaceships to some other planets. But we haven’t 【小题6】 life on other planets, because none of them 【小题7】 an environment like the earth.

Scientists have also found many other galaxies in the universe. But they are very far away, so no spaceship has travelled far enough 【小题8】 them. Is there any life out of the solar system? Have there been visitors to the earth from other planets? I think there is still a long way 【小题9】 to know that. When I grow up, I 【小题10】 an astronaut and work for China’s space station.