Why does earth have an atmosphere and other planets don't?

The lighter molecules require the least energy to escape. This is why Earth's atmosphere has so much less hydrogen and helium than say Jupiter or Saturn (proportionally). We've lost some of our hydrogen and helium, but held onto (so far) heavier gases like nitrogen and oxygen. read more

Several planets and moons in the Solar System and Universe do indeed have atmospheres. Earth is not the only planet with an atmosphere in the Solar System: Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune all have clear atmospheres that vary in composition and scale. read more

"Many details as to why Earth is the only planet with liquid water in our solar system need to be worked out," said Diana Valencia, a graduate student in Earth and Planetary Sciences at Harvard University. read more