All-Ocean Planet?

January 6, 2025 1:46 PM

That is: a water covered planet with no visible land anywhere. Is anything known about planets that would rule out the possibility?
posted by Lemkin to Science & Nature (7 answers total)

Not only is it not ruled out so far as we know, exoplanet researchers actively suggest that such planets may exist, and there's at least one a handful of known exoplanets that are currently considered candidates to have such conditions. See Hycean Planet on Wikipedia. There may be other types of water-world planets discussed in the scholarly literature, but this is the most likely type I've heard of.
posted by SaltySalticid at 1:54 PM on January 6 [3 favorites]

It's frozen, but all ocean under the ice on Europa. Attempt no landings there, though.
posted by LionIndex at 1:59 PM on January 6 [7 favorites]

The Ocean World article is probably a better overview.
posted by SaltySalticid at 2:02 PM on January 6 [1 favorite]

Earth itself is theorized to have been a so-called "water planet" roughly 3bn years ago, during the Paleoarchean era. Our home planet would be an n=1 sample, but it would be "proof-of-concept" as far as it happening in other solar systems.
posted by They sucked his brains out! at 3:40 PM on January 6 [2 favorites]

It's frozen, but all ocean under the ice on Europa.

Very nit-picky but Europa is a moon, not a planet, but still a pretty wild place.
posted by They sucked his brains out! at 4:53 PM on January 6 [4 favorites]

I thought the question was about liquid water on the surface of planets. If ice covered moons with liquid underneath are of interest, there's also Enceladus.
posted by SaltySalticid at 5:03 PM on January 6 [1 favorite]

Bigger nerds may be able to correct me, but in our case, we have continents because of plate tectonics, or "big chunks of the lithosphere slowly moving around on top of more fluid material beneath." Convection currents in this fluid material create earth's magnetic field, which in turn prevents our atmosphere and water from being blown away into space by the solar wind.

So, at least on Earth, the presence of water and the land that rises above the water are probably inseparable. As a counterexample, Mars no longer has a fluid core, plate tectonics, much of an atmosphere, or liquid water.

Could there be planets with different dynamics, like plate tectonics lifting mountains that never break the surface of a planetary ocean? Sure, we just haven't found them.
posted by pullayup at 6:23 AM on January 7