天文学的近期进展使得探测银河系内乃至其他星系中的行星成为可能。
Recent developments in astronomy have made it possible to detect planets in our own Milky Way and in other galaxies.
这是一项重大成就，因为相对而言，行星体积很小，且不发光。
This is a major achievement because, in relative terms, planets are very small and do not emit light.
发现行星本已证明难度不小，但在行星上找到生命则将被证明是无比困难的事。
Finding planets is proving hard enough, but finding life on them will prove infinitely more difficult.
首先要回答的问题是：一颗行星究竟能否维持生命的存在。
The first question to answer is whether a planet can actually support life.
以我们自己的太阳系为例，金星远远太热，火星又远远太冷，都无法维持生命。
In our own solar system, for example, Venus is far too hot and Mars is far too cold to support life.
只有地球提供了理想的条件，而即便如此，动植物生命的演化也花费了超过四十亿年的时间。
Only the Earth provides ideal conditions, and even here it has taken more than four billion years for plant and animal life to evolve.
一颗行星是否能维持生命，取决于其恒星（也就是它的"太阳"）的大小和亮度。
Whether a planet can support life depends on the size and brightness of its star, that is its 'sun'.
想象一颗体积可达我们太阳二十倍、亮度更高、温度更高的恒星。
Imagine a star up to twenty times larger, brighter and hotter than our own sun.
一颗行星必须距离那颗恒星非常遥远，才有可能维持生命。
A planet would have to be a very long way from it to be capable of supporting life.
另一方面，如果恒星体积较小，那么能维持生命的行星就必须有一条近距离绕其运行的轨道，同时还要为生命形式的发展提供完美的条件。
Alternatively, if the star were small, the life-supporting planet would have to have a close orbit round it and also provide the perfect conditions for life forms to develop.
但我们究竟要怎样才能找到这样一颗行星呢？
But how would we find such a planet?
目前，现存的望远镜中没有一架能够探测到生命存在的迹象。
At present, there is no telescope in existence that is capable of detecting the presence of life.
研发这样一架望远镜，将成为二十一世纪最重大的天文学项目之一。
The development of such a telescope will be one of the great astronomical projects of the 21st century.
用陆基望远镜在另一颗星球上寻找生命是不可能的。
It is impossible to look for life on another planet using earth-based telescopes.
地球自身的温暖大气层以及望远镜产生的热量，将使探测行星这样细小的天体成为不可能。
Our own warm atmosphere and the heat generated by the telescope would make it impossible to detect objects as small as planets.
甚至一架绕地球运行的望远镜，例如极为成功的哈勃望远镜，也因为太阳系中的尘埃微粒而无法胜任。
Even a telescope in orbit round the earth, like the very successful Hubble telescope, would not be suitable because of the dust particles in our solar system.
一架望远镜必须远至木星轨道处才能在外太空寻找生命，因为越向太阳系的外缘行进，尘埃就越稀薄。
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.
一旦我们探测到一颗行星，就必须设法遮住其恒星的光，以便能够真正"看见"这颗行星，并分析其大气成分。
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 analyse its atmosphere.
首先，我们寻找的将是植物生命，而不是"小绿人"。
In the first instance, we would be looking for plant life, rather than 'little green men'.
在一颗行星上最有可能发展出来的生命形式，将会是细菌。
The life forms most likely to develop on a planet would be bacteria.
正是细菌产生了我们在地球上呼吸的氧气。
It is bacteria that have generated the oxygen we breathe on earth.
在地球历史的大部分时间里，它们一直是我们星球上唯一的生命形式。
For most of the earth's history they have been the only form of life on our planet.
作为地球人，我们总是怀抱着这样的希望：我们将被小绿人造访，并能够与他们沟通。
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.
但这种希望始终停留在科幻小说的范畴之内。
But this hope is always in the realms of science fiction.
如果我们能够在另一颗星球上发现像细菌这样的低级生命形式，这将彻底改变我们对自身的看法。
If we were able to discover lowly forms of life like bacteria on another planet, it would completely change our view of ourselves.
正如美国宇航局的丹尼尔·戈尔丁所观察到的那样，"在别处发现生命将会改变一切。
As Daniel Goldin of NASA observed, 'Finding life elsewhere would change everything.
没有任何人类的事业或思想不会被它所改变。"
No human endeavour or thought would be unchanged by it.'