Many exoplanets, even small ones, have densities so low that it indicates that they have hydrogen atmospheres. Is a hydrogen gas atmosphere of a terrestrial planet somehow detrimental to biology as we know it? Does it for example cause a flow of free protons that prevent the formation of larger molecules. Are such planets discarded from the list of habitable candidates?

Since oxygen is the second most common chemically reactive element in the universe, and with all of that hydrogen around, shouldn't water oceans form, below the surface of which the composition of the atmosphere doesn't matter much?

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    $\begingroup$ What is your definition of "habitable"? $\endgroup$ Jul 5, 2017 at 20:51
  • $\begingroup$ If there is an atmosphere above liquid water, the gases solve in water until an equilibrium is reached at the surface between gas molecules entering and leaving the liquid. Increasing pressure increases the amount of solved gas. Below the surface, the composition of the atmosphere does matter. $\endgroup$
    – Uwe
    Jul 5, 2017 at 21:22
  • $\begingroup$ @RussellBorogove For anything alive on earth today, or known to ever have been.That could reproduce and sustain its process of whatever it is for millions of years. (Would you have any other suggestion :-) $\endgroup$
    – LocalFluff
    Jul 5, 2017 at 22:29

3 Answers 3


We would not find it habitable. There would be very little free oxygen since it would tend to fuse with the hydrogen.

Also, I disagree that low density planets would have a hydrogen atmosphere. It would take a significant gravity well to keep hydrogen from "evaporating off" from the planet. At best, you would end up with a trace atmosphere like the Mars.

A look at the chart in the upper right on this wiki (which I've seen here before) shows that only massive planets can retain a hydrogen atmosphere.


I would worry that your Hydrogen atmosphere would wither away. Simplifying a bit, there are two mechanisms at play.

Jean's escape

The molecules making up the atmosphere have their velocities distributed according to the so-called Mawell-Boltzmann distribution. The lighter the mass of the molecules, the broader is this distribution. By that I mean that the lighter the mass, the more molecule will be found in the tail of velocities higher than the escape velocity. The width of the distribution if proportional to the square root of the mass of the molecule, which brings a factor 4 between Hydrogen and our atmosphere.

Hydrodynamic escape

If the gas in the upper atmosphere is a good absorber of ultra-violet light, then it will be heated up and then this will build up pressure which will push the outmost layers out. Rinse and repeat. If I remember correctly, this effect is actually the dominant one. Unfortunately, I don't have time to double-check, sorry about that.

Just to be clear about the difference with the previous mechanism. Jean's escape considers an atmosphere in thermodynamical equilibrium, i.e. the temperature at any given altitude is constant, and this is a pure effect of the thermal agitation of the molecules. Hydrodynamic escape on the contrary is a dynamic phenomenon.

A nice illustration: HD 209458 b

This is a Jupiter-like planet which is estimated to loose from 100 to 500 million kg of Hydrogen per seconds! The Wikipedia article has more details.


The radiation is by far the main problem. A cold, airless planet actually could be habitable to extremophiles. However practically everywhere in the visible universe besides Earth is bathed in massive radiation, and it is a freak coincidence that the Earth has a sufficiently strong radiation defense- how it works is unknown- for anything to survive at all.

So if there is sufficiently low radiation, great, you can put life there. If it has some oxygen and sufficient gravity to retain it, and is at least -50 degrees c or so warm, then advanced life forms might even survive. You could even find a warm air pocket underground, shielded from radiation. But realistically these conditions are rare to nonexistent outside of Earth.

Furthermore even the earths magnetic field is buffeted all the time and it will not last very long.

  • $\begingroup$ This whole answer is nonsense. $\endgroup$ Jul 6, 2017 at 22:27
  • $\begingroup$ Much like your posting $\endgroup$
    – James
    Jul 7, 2017 at 3:48
  • $\begingroup$ This is a late comment, but it's actually not a mystery about how Earth is protected from radiation: both the magnetosphere and the atmosphere protect against radiation (nasa.gov/analogs/nsrl/why-space-radiation-matters). And from what I've read about the magnetosphere weakening, that seems to be more of an issue for satellites than for life on Earth. $\endgroup$
    – Pitto
    Sep 15, 2020 at 11:19

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