How does a water world stay wet when the Sun is too far away? We’ve looked at hydrogen/helium greenhouse effects – too much of a ‘good thing’ means the ocean is of super-critical steam not liquid water. But a planet without a Sun is a perfect candidate for a deep, dense atmosphere to keep its water wet. As James Kasting has pointed out, if it’s too cold then the hydrogen will condense and the atmosphere will collapse, so there has to be a decent heat-flow to keep the hydrogen gaseous, above about 33 K at the top of the atmosphere. Coincidentally that’s roughly Earth’s equilibrium temperature if we took away the Sun. But things were warmer in the past – the radioactive ‘glow’ of uranium, thorium & potassium-40 were about 6-10 times higher in the first half billion years of Earth’s youth. A planet unshackled from its Sun might keep such early warmth in and retain liquid water oceans, even in deep space, to this day.
But are there other heat sources? A low mass Brown Dwarf might burn deuterium for ~50 million years, then settle in to a slow decline as its heat is trickled away into space. Planets would form around such an object, but with a twist. Since the brown dwarf has a much lower mass & luminosity than the Sun its corresponding protoplanetary disk is smaller, denser and colder. Its planets form much closer to the parent body at Galilean moon style distances. This means inter-moon interactions & tidal forces become important, like the Galilean moons. Tidal energy powers extreme volcanism on Io and (probably) keeps an ocean warm beneath the ice of Europa – but the mutual interactions between the moons mean that their orbital eccentricity keeps getting “pumped up”, rather than dissipated away as heat once for all time.
With a heat source can Life form and persist on such worlds? We really don’t know, but some limitations do arise on what that Life might do in energetic terms. Animal life, as we know it, requires free oxygen at least at some point in the food-chain, but such doesn’t have to come from photosynthesis as we know it. Europa, for example, is an ocean encrusted in ice that is continually bombarded by high energy protons. This causes the dissociation of the ice and a slow build-up of oxygen trapped in it. With some kind of motion between surface and ocean, even via the slow burial of the ice by meteorite produced “regolith”, then there’s a supply of oxygen to the sea. Could it be sufficient to sustain animals? We won’t know until we go…
At the other end of the Solar System, where the Sun threatens runaway greenhouse effects, there are also unexpected prospects for Life. Present day Venus is very, very dry, but the deuterium-to-protium ratio in its clouds is much, much higher than the solar average, indicating the lighter hydrogen was lost to space. To achieve its present ratio there must have been a couple of hundred metres worth of water over the whole planet’s surface. Potentially more. As it was broken down by the Sun, oxygen would have accumulated before reacting with the surface rocks, potentially building up to high levels. Could hot, oxygenated oceans exist around planets of other stars to this day? With more water it’s possible that “ocean planets” could remain in a “wet greenhouse” state indefinitely, losing hydrogen to space, but with sufficient to remain wet for aeons. Such planets would be unimaginably hot from a terrestrial lifeforms point-of-view, but the potential of biology to adapt hasn’t been disproven. Some solvents, like foramide, have much higher temperature stability ranges than water, and thus may allow an alternative biochemistry.
Such wonders will remain theoretical to us if we remain in our star system. But do we need to launch probes to discover more, or is there an easier way? …Part IV