Some people think aliens visit Earth, but scientistsscientists/ˈsaɪən.tɪsts/L1科学家（研究科学的人）people who learn about the world and how things work say spacespace/speɪs/L1太空（地球大气层以外的区域）the big area beyond the sky where stars and planets are is very big. The nearest star to our Sun is very, very far away from us. A spaceship needs thousands of years to get there, so travel to other stars is hard.
Traveling to Earth needs too much energy, so getting here is very hard. Earth's air can be bad for aliens because it has oxygen and can hurt them. So, aliensaliens/ˈeɪ.li.ənz/L1外星人（来自其他星球的生命）creatures that come from another planet probably do not come here to visit us.
Scientists still look for aliens with big telescopes and computers. They listen for sounds from space, but they do not find anything yet. It is a big mysterymystery/ˈmɪs.tər.i/L1神秘的事物（未知或令人困惑的事情）something that we do not know or understand, but it is fun to think and talk about. Maybe one day we will find them in spacespace/speɪs/L1太空（地球大气层以外的区域）the big area beyond the sky where stars and planets are!
