M3U2 Scientists think that there 1. _________(be) life on the earth for hundreds of 2. ___________(million) of years. However, we have not found life on any other 3. __________(planet) yet. The earth is a planet and it goes 4. _____ the sun. Seven other planets also go around the sun. 5. __________(没有一个人) of them has an 6. ____________(环境) like that of the earth, so scientists do not think they will find life on 7. _____(they). The sun and 8. ___(it) planets 9. ____________(call) the solar system, and our solar system is a small part of a much larger 10. ________(群) of stars and planets, called 11. ______ Galaxy or the Milky Way. There are billions of stars in the Galaxy, and our sun is only one of them.Scientists have also 12. ______(discover) many other galaxies in the 13. ___________(宇宙). They are very far away and their 14. _________(光) has to travel for many years 15. ___________(reach) us. So how large is the universe? It is 16. _____________(不可能的) 17. _______________(imagine). Scientists have 18. _____(send) spaceships to the planet Mars 19. ____________(take) photos. They have even sent spaceships to travel outside the solar system. 20. ____________, no spaceship has travelled far enough 21. ___________(reach) other stars in our Galaxy. Scientists have always 22. _____(ask) the questions: with so many stars in the universe, are we alone, or is there life out there 23. _____ space? Have there been 24. ______(visitor) to the earth from other planets? Why has no one 25. ______________(联系) with us? We do not know the answers... yet.