# What’s the maximum gravity we could survive?

Finding a planet with a gravitational field that humans can survive and thrive in will be critical if we want to colonize another world. If the gravity is too strong, our blood will be drawn down into our legs, our bones may break, and we may be pinned helplessly to the ground.

Finding the gravitational limit of the human body would be preferable before landing on a massive new planet. Three physicists claim in a paper published on the pre-print server arXiv that the maximum gravitational field humans could survive long-term is four-and-a-half times the gravity on Earth.

Human Limits

Nikola Poljak of Croatia’s University of Zagreb and his colleagues first calculated the compressive strength of a human bone to determine the maximum gravitational force a human could function in. Based on an average mammal bone, they calculated that a human skeleton could withstand a gravitational force greater than 90 times that of Earth. But this is its strength when it is stationary. When we start running, the stress on our bones – as they flex and bend – multiplies by ten. This means that we could run on a planet with a gravitational field ten times that of Earth before our bones cracked.

For the maximum gravity at which we could take a step, the team turned to Hafþór Júlíus Björnsson, an Icelandic strongman who once walked five steps with a 1430 pound log on his back, smashing a 1,000-year-old record.

According to Poljak, Björnsson’s feat is a good comparison because the load on your legs and core muscles in a strong gravitational field feels similar to carrying a heavy weight on your shoulders.

Based on Björnsson’s weight – and the weight of that monster log – Poljak believes the strongman would be able to take a few steps on an exoplanet with a gravitational field 4.6 times ours.

Don’t Hold Us Down

According to Poljak and his colleagues, aiming for an exoplanet with 3 to 4 times the gravity of Earth would be more realistic for the average person – though they would still need rigorous training to achieve the muscle strength of an elite athlete.

Poljak hopes that his study will help us narrow down our search for a habitable exoplanet. “Now we know that there is no point in hoping to settle planets with high g-values,” he says.

Many of the rocky exoplanets discovered so far are much larger than our own planet. Astronomers refer to them as super-Earths. It’s difficult to know for sure what the gravity on another world is without going there, because density varies between worlds, but it doesn’t take much to start adding the pounds. Because volume grows as a cube and surface area grows as a square, so even a slightly larger planet would have much stronger gravity.

There are currently 3605 confirmed exoplanets, 594 of which have the known radii and masses required to calculate their gravity. According to Poljak’s calculations, 422 of these have gravitational fields that are equal to or less than 3.5 times that of Earth.

As for ‘strongman’ Björnsson, there are around another 35 exoplanets he could take a few steps on.

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