[BRIEF NOTE] A note on habitable moons
Jul. 16th, 2012 04:06 pm![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
rfmcdonaldSupernova Condensate's post on habitable exomoons inspired me to make a brief followup.
Wikipedia's summation of the case for natural satellites being capable of supporting life is sound. Certainly I've blogged here enough times about the prospect of moons like Europa (around Jupiter) or Enceladus and Titan (around Saturn) possibly supporting life.
Having Earth-like worlds, however, is more problematic. There is a fairly constant 1:10000 ratio between the mass of gas giants and the mass of their satellite systems, that is, the satellite systems of gas giants amount to one ten-thousandth of the mass of the gas giants. Were this relationship to hold true, to have an Earth-mass moon of a gas giant the gas giant would actually have to be a much larger brown dwarf, a body that--unlike a gas giant, but like a star--actually experienced nuclear fusion. Smaller moons could orbit smaller gas giants, bodies the mass of the Moon (an eightieth the mass of Earth) or Mars (a tenth the mass of Earth), but these bodies are so small that it's unlikely they could remain habitable for very long.
The only way to get Earth-sized moons of a gas giant would be to have the gas giant capture an Earth-sized moons into orbit. This actually isn't implausible, since current theory on planetary formation has nascent planets spiral inwards over time. The capture scenario for Earth-sized moons does suggest, however, that Earth-like habitable exomoons will be rare.
Wikipedia's summation of the case for natural satellites being capable of supporting life is sound. Certainly I've blogged here enough times about the prospect of moons like Europa (around Jupiter) or Enceladus and Titan (around Saturn) possibly supporting life.
Having Earth-like worlds, however, is more problematic. There is a fairly constant 1:10000 ratio between the mass of gas giants and the mass of their satellite systems, that is, the satellite systems of gas giants amount to one ten-thousandth of the mass of the gas giants. Were this relationship to hold true, to have an Earth-mass moon of a gas giant the gas giant would actually have to be a much larger brown dwarf, a body that--unlike a gas giant, but like a star--actually experienced nuclear fusion. Smaller moons could orbit smaller gas giants, bodies the mass of the Moon (an eightieth the mass of Earth) or Mars (a tenth the mass of Earth), but these bodies are so small that it's unlikely they could remain habitable for very long.
The only way to get Earth-sized moons of a gas giant would be to have the gas giant capture an Earth-sized moons into orbit. This actually isn't implausible, since current theory on planetary formation has nascent planets spiral inwards over time. The capture scenario for Earth-sized moons does suggest, however, that Earth-like habitable exomoons will be rare.
