请教老师：

It is impossible to look for life on another planet using earth-based telescopes. Our own warm atmosphere and the heat generated by the telescope would make it impossible to detect objects as small as planets. Even a telescope in orbit round the earth, like the very successful Hubble telescope, would not be suitable because of the dust particles iron solar system. A telescope would have to be as far away as the planet Jupiter to look for life in outer space, because the dust becomes thinner the further we travel towards the outer edges of our own solar system. Once we detected a planet, we would have to find a way of blotting out the light from its star, so that we would be able to 'see' the planet properly and analyze its atmosphere. In the first instance, we would be looking for plant life, rather than 'little green men'. The life forms most likely to develop on a planet would be bacteria. It is bacteria that have generated the oxygen we breathe on earth. For most of the earth's history they have been the only form of life on our planet. As Earth-dwellers, we always cherish the hope that we will be visited by little green men and that we will be able to communicate with them. But this hope is always in the realms of science fiction. If we were able to discover lowly forms of life like bacteria on another planet, it would completely change our view of ourselves. As Daniel Goldin of NASA observed, 'Finding life elsewhere would change everything. No human endeavor or thought would be unchanged by it.'

1）为什么用by the telescope 特指？书上翻译成“不可能找到比行星更小的物体” - 应属错误，应该是“不可能找到和行星一样大的物体”吧？

2）为何不是the thinner the dust becomes， the further we travel towards...一致句型呢？

3）For most of the earth's history they have been the only form of life on our planet. - 不理解为何使用现在完成时？原因是：到现在，细菌早已不是唯一的生命形式了。细菌是唯一的生命形式是在地球的早期，所以应该是用过去完成时！？