Unit 2 Activity 2 Read the passage and choose the best title for it.

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

科学家们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. None of them has an environment like that of the earth, so scientists do not think 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 Galaxy or the Milky Way. There are billions of stars in the Galaxy ,and our sun is only one of them.

地球是颗行星，它围绕着太阳转。此外还有七颗行星也围绕着太阳转。这些行星没有一颗有像地球那样的环境，因此科学家们认为他们不会在它们上面发现生命。太阳和它的行星被叫做太阳系，我们的太阳系只是一个由恒星和行星组成的星群里的一小部分，这个星系比太阳系大得多，叫银河系，或者银河。在银河系中有数以十亿计的恒星，而我们的太阳仅仅是其中之一。

Scientists have also discovered many other galaxies in the universe. 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.

科学家们也已经在宇宙中发现了许多其他星系。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 planet Mars to take photos. They have even sent spaceships to travel outside the solar system. However, no spaceship has travelled far enough to reach other stars in our Galaxy.

科学家们已经发射了宇宙飞船到火星上去拍摄图片。他们甚至发射了宇宙飞船到太阳系之外进行探索。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? Have there been visitors to the earth from other planets? Why has no one communicated with us? We do not know the answers...yet.

料学家们一直在问这些问题：宇宙中有这么多的星球，我们是孤独的吗，抑或在远方的太空中还有其他生命存在呢？有从其他行星到地球的来客吗？为什么还没有人和我们联系?我们还不知道答案……