The hypothetical creature has evolved on a planet with an atmosphere that's much more pressurised than that of Earth. It's humanoid in appearance, in that it has two arms and two legs as well as a head bearing a mouth and eyes. Its native planet has an atm pressure of, say, approximately three times that of Earth. It usually wears a pressure suit to deal with an atm pressure more akin to what we're used to here on Earth — so if this suit were to be removed or damaged, what would happen to the alien? I'm looking for specific symptoms, here. Describe it as if you were describing what would happen to a human body in the vacuum of space.

Here's some additional anatomical & biological details to help you: It's thin and gangly, with a slim figure and very thin extremities. It stands, on average, slightly smaller than your garden-variety human (5.5 feet, usually). Its bones are quite small and thin, almost like fish bones, and most of them are connected to each other rather loosely. They kind of just 'float around' in the body, supported by the high pressure atmosphere, and are filled with a jelly-like fluid. It has a single lung-like organ — gas is sucked into this organ where it comes into contact with various, inward-pointing 'stems' which have a rich blood supply. It inhales this gas through both its mouth and a secondary hole in its ears, but it has no nose. Its blood is yellow because it contains a lot of sodium. It has four eyes, behind which lie sinuses that are filled with fluid (again, to do with dealing with high pressures). It has a rather thick neck, containing lots and lots of blood vessels and airways, which are both generously coated in rigid chitin to prevent them from collapsing due to the high pressure. Its heart is very thin, but very long, running the length of its torso down to its pelvic region (almost like an insect heart). Needless to say, it has a very high blood pressure. Paradoxically, its native planet has very low gravity, so the pressure suit comes with in-built leg supports to help it deal with a higher gravitational stress than what it's used to.

Thank you! The more detailed and gruesome your answers, the better.

  • $\begingroup$ There have been cases of fish that live very deep in the ocean, under enormous pressure, being brought to the surface by fishermen. Pop! $\endgroup$
    – Smoj
    Dec 24, 2015 at 3:41
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    $\begingroup$ You say the planet has a noticably higher atmospheric pressure than that of Earth, which means more atmosphere, which needs more gravity to hold it in place. Yet you say that the planet has very low gravity (and indeed a low gravity would be a requirement for some of the physiology you describe). I don't think you can have it both ways while staying with science as we know it; and if you aren't bothering much with science, then the answer can be "because the author says so". $\endgroup$
    – user
    Dec 24, 2015 at 20:30
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    $\begingroup$ Also see Worldbuilding Meta discussion: Is asking for “descriptive” explanations an accepted request? $\endgroup$
    – user
    Dec 24, 2015 at 20:30
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    $\begingroup$ The reason for the high atmospheric pressure, despite the low gravity, is because the planet has a whole lot of gas without a whole lot of space in which to store it. Sources from inside the planet are constantly contributing to the gas content, and the unusual way in which it formed during its early years was also a factor. $\endgroup$ Dec 24, 2015 at 21:37

2 Answers 2


If the alien is used to three times greater atmospheric pressure and is exposed to earth's atmosphere, it's not likely to be all that gruesome. This is the difference between the top of Mt Everest, and sea level. The alien would benefit from an oxygen mask, but a pressure suit might be overkill. The alien would be better off letting its body adjust to the difference slowly, like mountain climbers do.

At a three times difference in pressure, there will be some swelling and discomfort. Not fun, but in humans, only deadly if the swelling happens in the brain. That can lead to a coma and death in a couple of days if untreated.

Other serious problems come because there is less oxygen available. This can lead to breathing problems, pain, and general exhaustion. In humans, it can result in the lungs filling with fluid, but this is rare, not well understand, and there is no reason to assume it would happen to an alien with different physiology. There might also be confusion and other mental problems from the low oxygen. But unlike a climber on Everest, the alien won't necessarily be dealing with a need to push itself physically, when a bad choice might kill it.

The problem here is that while high altitude can lead to rare gruesome deaths, the solution is to get oxygen, and these days any hospital will be able to supply it and send the alien home with a portable O2 tank. They may suggest it take up scuba diving, which would give it a way to regulate its oxygen on a dive, and enjoy a few hours of higher pressure and relief from gravity under water.

For comparison, a diver at about 100 ft is exposed to about three times atmospheric pressure. The deepest a diver has ever been is 1000 ft, which is about 30 times atmospheric pressure at sea level. Sperm whales have been recorded diving to 10,500 ft, and routinely dive to 4000 ft to hunt.

Your large number of physiological adaptations for pressure may be overkill, or else you need to do more research on how much pressure is necessary. I'd suggest looking up deep sea fish and sharks for ideas.


A factor of 3 isn't that much. Someone already mentioned mountain tops. Having mechanical pressure against the skin with a tight suit, plus an oxygen mask, is all it takes for us to handle vacuum.

A basic SCUBA diver can experience 4 atmospheres, and all I needed to cope with that is a bit of sudafed.

Under real pressure, chemestry is affected and creatures living on the abyssal plane will find their enzymes don't function if brought to the surface.

But I think the only issue your alien will have is the partial pressure of oxygen and what the lung (or whatever) is designed for.

Unless he secrets some slime or has something present in the environment that is sensitive to this range: water is just fine, but something around him might change phase or evaporate too fast?


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