初二上学期英语课文

Scientists think that there has been life on Earth for millions of years.However,we haven't found life on other planets yet.Why not?
The Earth is a plane and it goes around the Sun.Seven other planets also go around the Sun. None of them has an environment with air,so people and things cannot grow, The Sun and its planets are called the solar system.Thesolar system is a small part of our galaxy.
The stars we see at night are the suns in other solar systems. There are more than 200 billion stars in our galaxy,called the Milky Way,and our Sun is only one of them.
But scientists have discovered many other galaxies in the universe.They are along way away and their light has travelled for many years to reach us.It is hard to underdtand how large the univers is.
Sinentists have sent lost of spacecraft to look at other planets in our solar system,and some spacecraft have gone beyond the solar system. Howener, no one has discovered any life in space yet.
But why hans no one from ohter planets sent us a message? Have they tried to send information to us?With so many atars in the universe,are we alone,or is there life out there in space? We don't know...yet.