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What is an Ocean Planet or Sauna Planet?

An ocean planet is (currently) a hypothesized world whose surface is entirely covered with water, possibly having great depths of the order of a 100 kilometers (60 miles) or so. A paper by Selsis et al. (2007) suggested (based on theoretical calculations) that it might take as long as 5 billion years for such an ocean planet to lose all of its water, even if the planet-star distance were only 4% of that between the Earth and the Sun. Such a close-in ocean planet would obviously be very steamy so the term “sauna planet” has also been used in this context.

So far ocean planets have not been unambiguously observed. For most exoplanets the only handle we can get on the bulk composition is the density (and not all exoplanets have even a density estimate). The ambiguity between the signatures of an ocean planet and those from a planet that simply has a thick atmosphere have been discussed, for example, by Adams et al. (2008). A paper by Seager et al. (2007) claims that a water world could be identified with only a 5% uncertainty in a planet's mass and radius (this is not currently achievable) if the planet is composed of more than 25% water. However, the same paper states that just oceans that are even 100 km deep would not be discernible even with 2% uncertainties in mass and radius.

Curiously, the Wikipedia page for the exoplanet GJ 1214b says that it could be an ocean planet, but the paper that the article cites as justification for the statement actually says the opposite. (Namely, a paper by Rogers et al. (2010), which says that, “An important conclusion from this investigation is that, under most of the conditions we considered, GJ 1214b would not have liquid water.”) That's Wikipedia for you.

Some interesting theoretical conisderations about ocean planets and waterworlds are discussed in this article at

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File under: Exoplanet types; exoplanet taxonomy; exoplanet classification; What is an ocean planet? What is a sauna planet. Ambiguities in bulk composition of exoplanets. Define ocean planet?

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