the earth we live on is a planet¹. Are there people living on any other planets²？

　　answer this question， we have to talk out why there are people on the earth. A few billion years ago， the earth was only a sea of fire. There was no life on the earth， not to mention human beings. After a very， very long period of time， the earth gradually cooled down. Oceans and land appeared. Because there is air and water on the earth and the climate is mild， life evolved³.

　　After a long period of evolution， human beings gradually came into being. There are many other planets like the earth. As long as there is water and air on them， and as long as it is not too cold or too hot， there might be life， even human life on them. However， up to now， other planets with people living on them have not been found.