Unit 2We have not found life on any other planets yet.我们还没有在其他行星上发现生命。Scientists think that there has been life on earth for hundreds of millions of years. However, we have not found life on any other planets yet. 科学家认为亿万年以前地球上已有生命存在了。然而，我们还没有在任何其他行星上发现生命。The earth is a planet and it goes around the sun. Seven other planets also go around the sun. 地球是颗行星，它绕着太阳转。其他七颗行星也绕着太阳转。None of them has an environment like that of the earth, so scientists do not think they will find life on them.这些行星没有一颗像地球那样的环境，因此科学家们认为他们不会在它们上面发现生命。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, called the Galaxy or the Milky Way .太阳和它的行星被叫做太阳系，我们的太阳系是一个由恒星和行星组成的更大的星群里的一小部分，这个星系叫银河系，或者银河系。There are billions of stars in the Galaxy, and our sun is only one of them. 在银河系中有数十亿计的恒星，而我们的太阳仅仅是其中之一。Scientists have also discovered many other galaxies in the universe. 科学家们也已经在宇宙中发现了许多其他星系。They are very far away and their light has to travel for many years to reach us. So how large is the universe It is impossible to imagine. 它们很遥远，它们的光必须要经过许多年才能到达我们。因此，宇宙有多大那是难以想象的。Scientists have sent spaceships to the plant of Mars to take photos. They even have sent spaceships to travel outside the solar system.科学家们已经发射了宇宙飞船到火星上去拍摄照片。他们甚至发射了宇宙飞船到太阳系之外进行探究。However, no spaceship has travelled far enough to reach near other stars in our Galaxy. 但是，没有一艘宇宙飞船已经旅行到足够远而到达我们银河系中的其他恒星。Scientists have always asked the questions: With so many stars in the universe, are we alone, or is there life out there in space( 题目 ) 科学家们一直在问这些问题：宇宙中有这么多的星球，我们是孤单的呢，还是在远方的太空中还有生命存在呢Have there been visitors to the earth from other planets有其他行星到地球的来客吗Why has no one communicated with us We do not know the answers…yet.为什么还没有人和我们联系我们不知道答案……
