Plausibility of helium dominated atmospheres on super-Earths

Question

I've been using this helpful diagram a lot for worldbuilding.

Now I came to think about the fact that helium is more massive than hydrogen, thus a scenario where hydrogen escapes, but helium is retained is conceivable and suggested by the diagram. Since this line of helium retention but hydrogen escape lies between the ice-giants and terrestrial planets, super-Earths come to mind as plausible candidates for such atmospheres. After all, a super-Earth might be a mini-ice-giant or a huge, terrestrial planet. Thus I designed the two following scenarios and would like you to critique them concerning their plausibility. If you have any additional thoughts about these worlds I would love to hear them as well. This question and this answer espacially already provided some interesting information about the conditions on a planet with a helium atmosphere.

Wet

As a low-mass-ice-giant/mini-Neptune/thick-envelope-super-earth (ca. 8.5 Me) Wet formed outside the frost-line and migrated inwards where it got hot enough for its hydrogen envelope to escape (ca. 230 K). According to my back of the envelope calculations, a 95%+ helium envelope of about 0.2 Me should be retained. This setup sounds quite similar to Sol's ice-giants, so a similar internal structure should be expected; a stereopalagic ocean-world with an HO2, CH4, and NH3 ocean, which solidifies deep down into exotic ices.

Dry

Dry is a rocky super-Earth which formed inside the frost limit with a substantial iron core (ca. 2 Me, ca. 1 Re). Like Earth, it accumulated a primordial hydrogen and helium envelope, yet unlike Earth it only lost the hydrogen and a small fraction of the helium as its parent-star ignited. I assume that the plausible pressure range of such a planet might go up to 1000 atm of helium (I'm pulling this out of a hat, what would be a plausible figure?), yet Dry is a moderate chase since the 400 K hot planet lost a significant fraction of the helium during a major impact. Thus the atmosphere of Dry contains 5 atm He and 1.6 atm N2. It seems surprisingly earthlike apart from the helium.

Again, are these scenarios of helium atmospheres plausible or are there issues I missed?

EDIT1: It has been pointed out that the existence of a primordial hydrogen-helium atmosphere on earth is unproven. DRY is possibly impossible.

Have you seen this? : https://www.nasa.gov/jpl/spitzer/pia19345/how-to-make-a-helium-atmosphere/ — Morris The Cat, Aug 06 '19 at 14:22
Have you come across the phrase "gas dwarf" before? See also Kepler 138d for a planet that appears to have a surprisingly low density given its size. — Starfish Prime, Aug 06 '19 at 14:23
@StarfishPrime I have but doesn't refer to a few earth mass sized planet almost purely made of hydrogen found at the outer fringes of a solar system? — TheDyingOfLight, Aug 06 '19 at 14:57
@MorrisTheCat Quite interesting and relatively in line with my thoughts. It sounds like a scenario for WET if it were to migrate further inwards. The details about the development of the atmosphere are exactly what I'm looking for. — TheDyingOfLight, Aug 06 '19 at 15:00
No, merely a low density world between the mass of Earth and Neptune (give or take a bit, see kepler 138d, etc). Orbital radius is not specified. — Starfish Prime, Aug 06 '19 at 15:02
I'm having a hard time finding Good Maths on this topic, but my FEELING is that your Dry scenario is going to require more mass than that though. EDIT: Oh hey, this might help : https://astronomy.stackexchange.com/questions/21211/helium-in-terrestrial-planet-atmospheres — Morris The Cat, Aug 06 '19 at 15:05
@MorrisTheCat The link talks about radioactive decay creating a helium atmosphere. That's not what I´m looking for. It is undisputed that even Earth used to have an H2/He atmosphere. You might be right about the mass though. — TheDyingOfLight, Aug 06 '19 at 15:17
@TheDyingOfLight "undisputed that even Earth used to have an H2/He Atmosphere" ??? Are you sure about that? Where Earth formed there was almost no helium and what little hydrogen there was, was in the form of frozen ices. I don't think that's an accurate claim. — userLTK, Aug 06 '19 at 15:44
@userLTK Now here's a funny story. Wikipedia claims that the first atmosphere consisted of solar nebular material as well as all the other websites dealing with the evolution of the Earths atmosphere. But when I checked out the paper Wikipedia cites it says there is no evidence for such an early atmosphere... you are right. — TheDyingOfLight, Aug 06 '19 at 16:15
Clearly if all the other gases freeze out (or at least liquify), that's the "easy" way to have an atmosphere that's primarily He. — Carl Witthoft, Aug 06 '19 at 17:22
Retaining He is one thing, but actually capturing it in the first place is entirely another. I suggest the following: https://arxiv.org/abs/1401.2765 http://adsabs.harvard.edu/abs/2018A%26ARv..26....2L — Arkenstein XII, Aug 07 '19 at 01:00
I'm not an expert but couldn't going through a gas cloud / nebula allow an Earth-like world to capture a lot of Helium as an atmosphere? Perhaps even after it was stripped of its own atmosphere. The issue here I guess is that going through a gas cloud like this would feed the star and make it burn hotter so might blow away more of the atmosphere.
The question is, though: do you want your world to have life develop on it? — Nierninwa, Nov 05 '19 at 17:44
Are there going to be other components to the atmosphere? What about average planetary temperature? Composition and density play a large role in this. — Michael H., Dec 13 '19 at 14:54
My question is why can't this just happen to form very rarely, like why can't it be that a gas giant, one of a trillion quintillion just so happen to have a helium dominated atmosphere? — skout, Dec 14 '19 at 22:19
@MichaelH. These things can be however they might play out. This question isn't about the specific conditions on a planet with a helium dominated atmosphere, it is about the plausability of a pathway in planetary evolution leading to such a setup. Since I pointed out that super-earths are decent candidates for a helium setup, their diversity will create a large number of different conditions. — TheDyingOfLight, Dec 14 '19 at 22:49
@skout Well, certainly. If you propose a nearly infinite number of attempts, eventually even the most unlikely outcome will occur. Never mind that these helium gas giants will become more common/likely as the universe ages. But I'm looking for the likelihood of a distinct class of helium atmosphere super-earths. — TheDyingOfLight, Dec 14 '19 at 22:53

score 3 · Answer 1 · edited Jun 16 '20 at 11:03

Digging in the mare magnum of Internet I have stumbled upon helium planets

A helium planet is a planet with a helium-dominated atmosphere. This contrasts with ordinary gas giants such as Jupiter and Saturn, whose atmospheres consist primarily of hydrogen, with helium as a secondary component only. Helium planets might form in a variety of ways. Gliese 436 b is a possible helium planet.

Helium planets are expected to be distinguishable from regular hydrogen-dominated planets by strong evidence of carbon monoxide and carbon dioxide in the atmosphere. Due to hydrogen depletion, the expected methane in the atmosphere cannot form because there is no hydrogen for the carbon to combine with, and hence carbon combines with oxygen instead, forming CO and CO2.

A possible helium planet can be Gliese 436 b

the planet's surface temperature is estimated from measurements taken as it passes behind the star to be 712 K (439 °C; 822 °F). This temperature is significantly higher than would be expected if the planet were only heated by radiation from its star, which was prior to this measurement, estimated at 520 K. Whatever energy tidal effects deliver to the planet, it does not affect its temperature significantly. A greenhouse effect could raise the temperature to much higher degrees than the predicted 520–620 K.

However, when the radius became better known, ice alone was not enough to account for it. An outer layer of hydrogen and helium up to ten percent in mass would be needed on top of the ice to account for the observed planetary radius. This obviates the need for an ice core. Alternatively, the planet may be a super-earth.

Wrapping up, if the helium planet forms by losing hydrogen, the wet scenario is less plausible: there would too few hydrogen to form large amounts of water.

Plausibility of helium dominated atmospheres on super-Earths

Wet

Dry

1 Answers1