The right size
Gravity must hold the atmosphere.
A smaller Earth
Imagine a planet much smaller than Earth.
What would happen to its atmosphere?
What this lesson covers
The idea
Earth is the right size for life: a much smaller planet would have gravity too weak to hold its atmosphere, which would escape into space, while a much larger one would have gravity strong enough to crush us.
A smaller Earth
Imagine a planet much smaller than Earth.
What would happen to its atmosphere?
- It would escape into space
- It would get thicker
- Nothing would change
Too small, right, too large
Open each tab.
Just right
The planet is also the right size to support an atmosphere. If Earth were much smaller, gravity would be too weak to hold the gases of the atmosphere. If the planet were too large, gravity would pull us down so hard that our bones could be crushed.
Earth is the right size. A much smaller planet could not hold its atmosphere (gravity too weak). A much larger planet would have gravity strong enough to crush us.
Notes
Earth's size is right: it holds an atmosphere without crushing us.
Check yourself
A much smaller planet would not hold an atmosphere because its gravity would be …
If a planet were much larger than Earth, its gravity could …
Which planet has no atmosphere at all?
On Mars the atmosphere is about …
- too strong. Too strong gravity would crush us.
- zero. Gravity would be weak, not zero.
- too weak — correct. Yes!
- crush our bones — correct. Yes!
- let the atmosphere escape. That would happen on a much smaller planet.
- make us float. A much larger planet pulls harder.
- Mars. Mars has a thin atmosphere.
- Mercury — correct. Yes!
- Earth. Earth has an atmosphere.
- the same as on Earth. It is much thinner.
- 100 times thicker than on Earth. It is 100 times thinner.
- 100 times thinner than on Earth — correct. Yes!