How much stronger would gravity have to be before humans would find it impossible to walk? That question could face humanity if it ever embarks on the colonization of exoplanets – planets orbiting stars other than the Sun. Now, using relatively simple calculations, Nikola Poljak, Dora Klindzic and Mateo Kruljac at the University of Zagreb in Croatia have proposed an upper limit on exoplanet gravity that would allow a trained athlete to live comfortably on a distant world.
Astrobiologists often consider the potential habitability of newly-discovered exoplanets by measuring the properties of their atmospheres. However, as the Apollo Moon missions proved, humans can use technology to survive in a distant hostile atmosphere. Gravity, however, is an environmental factor that humans would find extremely difficult (if not impossible) to control. Indeed, the three researchers argue that gravity should be a crucial parameter when evaluating whether humans could survive on a distant world.
In their study, the physicists considered how the performance of various systems in a trained athlete’s body would change when subjected to gravitational fields stronger than the Earth’s field (gᴇ). They first calculated the maximum stress a typical human skeleton could withstand while running. Inputting various properties of human bone into their equations, they ascertained an upper limit of 10gᴇ before bones begin to break.
Inverted pendulum model
Next, the trio made a similar assessment for muscular strength – calculating that a trained athlete could no longer get up while seated or lying down above a stricter upper limit of 5gᴇ. By studying the ability of a “strongman” to walk while carrying a 650 kg log the researchers placed an upper limit of 4.6gᴇ for locomotion. To do this, they used an ”inverted pendulum” model of the legs to consider the energy the body uses in the walking process.
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The team also showed that in stronger gravitational fields, blood would tend to sink towards the legs, requiring the heart to work harder to pump it up towards the brain. Ultimately, survival would require higher blood volumes – making the body feel weaker initially while blood cells are created. It would also result in higher blood pressures, which pose significant health risks. These factors drive the limit down to 4gᴇ as the maximum gravity an athlete’s circulatory system could withstand in the long term.
These limits may narrow the range of potentially habitable exoplanets, but the physicists showed that through rigorous strength and endurance training, future colonists could broaden their options for a new home. The researchers calculated that by training an average human to the level of a typical athlete, planets with up to 4gᴇ would be a more feasible option. The team now hopes their work will help astronomers to make their search for potential future colony sites more precise – although any future colonization would be contingent on the development of a vast array of new technologies for interstellar space travel.
The calculations are described in a preprint on arXiv.