Whilst astronomy is about things in space, it's important to think about our own planet!
What is our planet?
Our planet is Earth, the 3rd planet from the Sun and the only planet which is known to have life on it. The atmosphere is 100,000 km thick and contains lots of oxygen which is what we breathe. It is also quite warm, unlike Mars which has an average temperature of −55 °C.
View of clouds from the International Space Station
Why is life only on Earth?
Earth has all of the things which life needs on planets. It is warm enough that water does not freeze but cold enough that water does not boil, it has oxygen in the atmosphere, and it has carbon.