So I've been thinking about colonists on a huge planet.
If a jupiter-sized planet had a breathable atmosphere, we would still be unable to live on it due to excessive gravity, which would crush us. How big could a planet get without killing or (preferably) immobilizing humans.
If you mean human life I am a lot less optimistic than liljoshu. The problem is long term. There would be nowhere to go to escape the gravity.
One's head is around 50 cm above one's heart. Double the gravity and to maintain normal brain blood pressure while standing you add another 50 cm of water pressure or about 37 mmHg. That takes a normal person to the threshold of hypertension just to stay conscious. Meanwhile the added pressure on his lower leg veins is about three times that. So in 2G the best you can hope for is a reduced life expectancy and huge problems with varicose veins.
Then you have to consider pregnancy.
Then you have to consider stumbling and the greatly increased force with which your upper body and head would hit the ground if you were human normal height.
If you mean life evolved on such a planet, it will be appropriately adapted. The Giraffe and big dinosaurs show that Earth type life can deal with the blood pressure problem. Dwarfism shows that humans of normal intelligence do not need to be more than three feet tall. Bipeds on such a planet will be short and very stocky so that stumbling is not often fatal.
The centaur body plan might work better but that may not be an evolutionary choice. The body plan of all vertebrates is that of a fish. If life always evolves in the oceans first, then the number of limbs is probably decided before adaptation to life on dry land starts.
Edit: one other evolutionary thought. Here on Earth it was life in trees that created grasping hands with opposable thumbs. On a high G planet are trees possible and could anything bigger than a squirrel live in them? And if not, could intelligent technological life ever arise without those hands?
Assuming humans specifically, depends on how in shape a person. The maximum human dead lift is about 1,000 pounds, this was done by a 350 pound guy (most of it muscle). Beyond that weight, movement becomes almost impossible without dropping down. Most people can carry about half their weight distributed like a backpack up to half their weight. Old people with bone problems have a hard enough problem in our gravity.
So you're looking at roughly .8x to 3x surface gravity depending on the person. Average is probably around 1.5x surface gravity.
Different planets with different compositions can have different surface gravities. If you had a much smaller planet made of lead, it could have much higher than a 3x surface gravity.
Soldiers today run,swim,climb with 45 kilograms on their backs 18 kilograms for the body armor and other 5 kilograms of weapons, that almost like carrying another person with them. they can resist 7-9 days carrying this weight before having painful signs of extreme fatigue or before being paralyzed by pain.
Jupiter has 3 times the gravity of earth, a person on a planet like Jupiter will feel like carrying two more people and could resist maybe 2-3 days without adrenaline or particular drugs.
This only means a person born on earth would find it difficult to live on a planet with higher gravity.
But an organism born here could develop stronger muscles capable to live perfectly on two legs, but digitigrade legs would be impossible unless an animal has at least 4 of them since this kind of legs are made to be fast not strong.
