With new exoplanets being discovered on a regular basis, it’s only natural to wonder about the possibility of humans actually exploring, and even establishing permanent settlements on them in the future.

But we know that a lot of planets are just not likely to be that hospitable to us. Toxic atmospheres. Crushing gravity. Extreme temperatures. The details of what we’re looking for in the galactic real-estate market are dictated by our own limits of survivability. So, in the grand scheme of things, how robust are humans really?

1. Ionizing Radiation

In space, high energy particles and photons can tear through your cells like microscopic cannon balls, doing damage that can kill individual cells and even induce cancer. But radiation is around us in some form all the time here on Earth in the back ground. Our bodies are quite well adapted to what’s in the background.
According to the International Commission on Radiological Protection Report 103, your lifetime risk for getting cancer or other undesirable genetic effects goes up by about 4 % for every 1.0 Sv of radiation exposure one receives. Unfortunately the probability of developing superpowers is much, much less… even if the exposure comes from the bite of a radioactive spider. Here on Earth, the atmosphere protects us from much of the cosmic radiation in space. Typical background dose rates work out to around 0.003 Sv per year. This comes from those cosmic rays that do reach us, terrestrial sources in the ground, radon gas, and you know… the potassium 14 in bananas. But in Ramsar, Iran, background doses have been recorded up to 0. 260 Sv per year (although looking a little deeper, it seems typical doses are closer to about 0.010 Sv per year), and people have been living there for generations without the kinds of problems one might expect. Some scientists have even argued that these elevated exposures can result in overall beneficial health effects, though this is an ongoing debate. The International Atomic Energy Agency establishes limits for people who regularly work with ionizing radiation at about 0.020 Sv per year, so that’s probably a reasonable upper limit for any would-be astronaut colonists.

The good news is that many forms of radiation can be mitigated. Alpha particles have a tiny range through most matter. Even most photons can be shielded with enough concrete. So if the radiation levels are the only problem, it might be something we can mitigate.

2. Air and the Need to Breathe

Normal air here on Earth is about 78 % nitrogen and almost 21% oxygen. The remaining one point something percent is argon, with trace amounts of carbon dioxide, and a few other elements mixed in, but it’s the oxygen that’s important to us humans. The minimum oxygen concentration necessary for humans to function normally is about 19.5%. Technically humans can “survive” O₂ concentrations down to about 6%, but it wouldn’t be pleasant. Below that 19.5%, threshold, you can have impaired mental function, loss of coordination and exhaustion quickly sets in – getting worse with the less O₂ you have. With too much oxygen humans and other forms of Earth life will generally be okay, but fire and explosion risks go up. Optimal oxygen levels established by the US Occupational Safety and Health Administration are in the range of 19.5% to 23.5%.

And it’s not just a case of concentration either. As anyone who’s ever spent any time at high altitudes can tell you, pressure is important too. At sea level, atmospheric pressure is about 100 kPa. Oxygen partial pressure is about 20 kPa (there’s a slight adjustment for water vapor). Pressure decreases with altitude. At Mt Everest base camp (5500 m above sea level) the pressure is about half of that at sea level–that means half the partial pressure of oxygen. This loss of pressure reduces the efficiency of gas exchange in one’s lungs and this can lead to all the pleasant side effects of altitude sickness.

You also have to worry about toxic effects with other gases. One particular concern with space flight is carbon dioxide toxicity. On average humans produce CO₂ at a rate of about half a liter per minute (and that’s just existing as a human, not including driving or generating electricity by burning fossil fuels). That can accumulate pretty fast. On spacecraft and submarines, carbon dioxide is removed with zeolite scrubbers–chemical sponges that soak it up. While typical air only has trace amounts of carbon dioxide, when it builds up it can become toxic leading to cardiac arrhythmias and impaired consciousness. Above 10% and you’re in serious trouble… convulsions, coma and death. Generally it seems that recommendations are to keep the levels below about 1%.

The chances of finding a planet with a breathable atmosphere are quite low. On the other hand, humans are pretty good at bubbling up and bringing our atmosphere with us. If this is the only problem, it’s another one good engineering can solve.

3. Temperature

This is something else humans have lots of experience regulating. Arguably the hottest inhabited place on earth is Dallol, Ethiopia, with an average temperature of about 31 C, and summer heat pushing 40 C. On the other end of the spectrum is Oymyakon, Russia with a lowest recorded temperature of -66.7 C in 1933. With heating an air conditioning, we can of course go to further extremes. Temperatures outside the International Space Station can reach as high as 121 C on the sunny side and -157 C on the dark side.

Of course, the more extreme the temperatures get, the more costly the endeavor. Ideally we’d want to inhabit a planet where liquid water could exist naturally, at least some of the time. So in this sense, we’d still be looking for interplanetary real-estate in the Goldilocks zone. Too far outside of that and you can probably still go there, but it would be too costly to stay long term.

4. Gravity

I once had an astrophysics professor who was fond of telling us that gravity runs the show.

We know humans can survive over the long term even in the complete absence of gravity. Sort of. Unfortunately without gravity humans experience progressive bone loss due to an absence of stress on their bones, vestibular problems, elevated blood pressure , and muscle atrophy. Of course in the short term… it’s totally awesome! So on that end, there really isn’t a lower limit.

It’s the other end of the scale that really limits us. When gravity gets stronger than what we’re used to on earth, everything gets more challenging. Eventually you reach a point where your bones would be crushed under your own weight. According to this article, which is based on a paper eventually published in The Physics Teacher in 2019, the upper limit of what humans could endure is about four-and-a-half times what we experience here on Earth. Of course, even then you’re limited to only taking a few steps at a time, and you’ve have to be as strong as Hafþór Júlíus Björnsson to do it. (He’s the World’s Strongest Man winner who played Ser Gregor Clegane, the Mountain That Rides, in the Game of Thrones.) For the rest of us, that upper limit is probably closer to three times Earth’s gravity.

Gravity is one of the harder problems to solve from an engineering point of view. When air is a problem, you can build a “bubble” habitat. You can shield people from radiation. But on a given planet, you’re more-or-less stuck with the gravity it comes with. One potential solution might be to use high gravity planets simply as a source of raw materials to build massive space stations where the gravity might be more manageable, but again, it’s going to be expensive.