Is there a habitable zone for gravity?

Chris Smith answered Stephen's question...

Chris - Well, the thing to think about, Simon, is that obviously just because something is bigger than the Earth, it doesn't mean its gravity is going to be the same amount bigger than the Earth because it depends what it's made of.

Jupiter is some 318 times more massive than the Earth is, but it doesn't mean that you would weigh 318 times more on the surface of Jupiter because the radius of Jupiter is about 60000 kilometres compared with Earth's 6000. And because gravity actually works through a centre of mass, and is proportional to the square of the size of something, then actually you are talking about a mass on the surface of Jupiter actually weighing about 3 times more there than it would on the Earth. 

So you can't assume just because something's bigger that the gravity is going to be that much different at the surface because it would depend what the object is made of and how much its mass is. 

But one thing to bear in mind here is, regardless of what gravity might do to the atmosphere of the surface of a planet, water, as Ginny was saying earlier, means that things can live under enormous pressures.

There are things living 10 kilometres down in the oceans quite happily because they've adapted to do so. And we think water is essential for life anyway so even if you did have a very very big planet with very very powerful gravity, which would mean that you as a human would weigh a huge amount on the surface of the Earth, and incidentally NASA thinks that if we experience gravity of about 4 times what we've got on Earth that would probably render a human inviable so we just couldn't exist in that environment, but if you were a water dweller you would probably have no problem at all, like those fish at the bottom of the ocean.