Hydrogen Greenhouse Worlds…

The first planets to form probably attracted a primary atmosphere of H/He from the solar Nebula. In our Solar System these were driven off from the four Inner Planets and retained by the Outer Giants, but in theory smaller planets can retain such a mixture. I’ve speculated about such worlds on these blog pages before and now there’s a new arXiv piece discussing the greenhouse abilities of H/He…

Hydrogen Greenhouse Planets Beyond the Habitable Zone

…the summary conclusion being that 40 bars of H2 can keep the surface at 280 K out to 10 AU around a G type star and 1.5 AU around an M star. Thus planets with oceans of water can exist at Saturn-like orbital distances given enough primary atmosphere. Super-Earths are the most likely to retain their H/He primary atmospheres due to their higher gravity, as young stars put out a LOT of EUV light which energizes the hydrogen and strips it away in a billion years or so, if the planet is too close. Out past ~2 AU for a G-star and that effect isn’t so dramatic, thus a Super-Earth where the Asteroid Belt is today would’ve retained its primary atmosphere and probably be warm & wet.

Such a “habitable planet” is only barely defineable as habitable because it has liquid water, but is unlikely to remain warm/wet habitable if the hydrogen is exploited/depleted by methanogens making methane out of it with carbon dioxide, nor oxygenic photosynthesisers making O2, via CO2+H2O->CH2O+O2, which then reacts rapidly with hydrogen. Could another kind of photosynthesis evolve to restore the hydrogen lost? Hydrogen makers exist on Earth, so it’s not unknown in biochemical terms, but I wonder what other compound they need to release net hydrogen from methane/sugars/water?

Author: Adam

Nothing much to say. What about you?