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M3
Unit
2
We 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
科学家认为亿万年以前地球上已有生命存在了。然而，
have not found life on any other
planets yet.
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
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。
on them. The sun and its planets are
called the solar system, and our solar system is a
small part of a
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
much
larger group of stars and planets, called the
Galaxy or the Milky Way. There are billions of
更大的星群里的一小部分，这个星系叫银河系，或者银河系。
在银河系中有数十亿计的恒星，
stars in the
Galaxy, and our sun is only one of them.
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
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，
their light has to travel for many
years to reach us. So how large is the universe?
It is impossible to
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。
imagine.
Scientists have sent spaceships to the
plant of Mars to take photos. They even have sent
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
spaceships
to travel outside the solar system. However, no
spaceship has travelled far enough to
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
reach near other stars in our
Galaxy.
我们银河系中的其他恒星。
Scientists have always asked the
question: With so many stars in the universe, are
we alone, or
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，
is there
life out there in space? Have there been visitors
to the earth from other planets? Why has no
还是在远方的太空中还有生命存在呢？有其他行星到地球的来
客吗？