Helium atmosphere planets

I note the existence in one of my pregen systems that has a gravity of 1.2g but an atmosphere that is primarily Helium (and oxygen).

How is this possible? If a planet is heavy enough to have a Helium atmosphere, wouldn't it become a gas giant? And if it's not heavy enough, wouldn't the helium escape long before oxygen could be generated by life?


  • Someone asked a similar question on the SFRPG boards that I run recently - basically, the right kind of "superearths" could retain helium without being massive enough to hold hydrogen (and thus really snowball into a gas giant) - it's a pretty thin line and I don't think it's likely to be a very common class of world, but they are theoretically possible.

    If you want to see more indepth discussion, see the thread at http://www.sfrpg-discussion.net/phpBB3/ ... =30&t=2278 (I figure that'd be easier than copying my posts here!)

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