The earth is the only planet that scientists are certain has life. What does the earth have that the other planets don't? For one thing, the earth has just the right temperature. As third planet from the sun, the earth seems to be just the right distance away.The planets that are closer to the sun are (unusual) hot and their surfaces are baking in the sun. The farthest planets are cold balls.
When the earth developed, it (cover) by many gases. The gases caused the earth to be hot. But something (amaze) happened. The temperature was just right for thick clouds to form. It rained very hard a very long time. This gave the earth its oceans. Water made it possible for plants (grow). Then the plants created oxygen in the atmosphere which is the gas that humans and (animal) breathe. .
Mars is the only planet in the solar system seems to be something like the earth. It is smaller than the earth and it is quite a bit cooler. But it is not too cold for humans. The most unexpected sight on Mars is dried-up river beds. Mars was once much (wet), as some scientists believe, than today. Does this mean there could have been living things on Mars? Scientists are not sure, but there (be) no sign so far.
