When planets are formed do only gas and ice giants accrete hydrogen and helium? Does this accretion have to be beyond the "snow line"?

In other words, would all hydrogen or helium in the atmosphere of a super-earth come from outgassing (radioactive decay, etc)?

  • $\begingroup$ I guess they would accrete hydrogen as part of molecules (where the outgassed hydrogen comes from), but I am asking about a primary, as opposed to secondary, atmosphere. $\endgroup$ – Jack R. Woods Jun 25 '17 at 4:10

The snow line is the inner radius from the star where the water condenses onto dust and larger particles as ice (ie snowballs). The hydrogen and helium never freezes out. Planets start forming by accreting these particles (with or without ice but definitely not with H or He), but when a growing planet reaches a sufficiently high mass, it starts to pull in H and He (and more importantly, can hold onto them as they have low atomic mass and thus tend to escape quickly unless there is enough gravity). Exactly what mass that occurs at depends on a lot of factors (temperature, pressure, etc), but it happens below Neptune's mass and in part of the super-earth mass class. The upper range in mass of the super-earth are also called mini-Neptunes or gas dwarf planets and from their typically low densities it is clear that they are mostly composed of H and He.

However, beyond the snow line it is more likely for a planet to grow to a large mass since more of the surroundings has condensed out of the gas into small particles, therefore accretion is more efficient. So, it may be that mini-Neptunes are usually forming beyond the snow line and then some of them migrate closer to its star. But, I don't see any reason to doubt that some of the many mini-Neptunes with H and He found by Kepler/K2 inside the snow-line were formed in situ. I am not an expert in this field, and perhaps the experts would disagree.

  • $\begingroup$ I guess I'm trying to figure out if there is a difference between a mini-Neptune and a rocky super-earth that is large enough (and with the right effective temperature) to accrete hydrogen and/or helium from the primordial disc. The latter would have a much higher density. I am not sure whether being large enough to retain H and/or He is the same as being able to accrete it. I guess, bottom line, I am curious as to whether a large rocky world with plate tectonics,etc could have an upper envelope of helium in its atmosphere. $\endgroup$ – Jack R. Woods Jul 1 '17 at 14:34
  • $\begingroup$ All gas giants are thought to have a "rocky" core presumably with plate tectonics under their gaseous outer layers. That is, until/unless the pressures and temperatures get too high for solids. All planets begin by accreting solids and later accumulate different amounts of gas. So, I am not sure what you are asking. $\endgroup$ – eshaya Jul 2 '17 at 21:18
  • $\begingroup$ @ eshaya Sorry, it's hard to verbalize. I look at this plot abyss.uoregon.edu/~js/ast121/lectures/lec14.html and see that if you made Earth a little colder and/or larger you would have a planet that would retain helium in its atmosphere. I'm trying to imagine a planet we could walk on that would still have helium in its atmosphere. $\endgroup$ – Jack R. Woods Jul 8 '17 at 23:41
  • $\begingroup$ Indeed, a planet with roughly the same temperature but 3 or 4 times more mass could have water and a Helium atmosphere. Not so good for humans because the air pressure and gravity would be too high and it would be nearly impossible to ever get the oxygen fraction to a breathable level. $\endgroup$ – eshaya Jul 10 '17 at 16:39

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