The idea is in a world with 2x the gravity (which follows from 2x the size), gravity would have less an impact in the oceans.

Does that follow?

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    $\begingroup$ Do you mean 2 times the radius (which I think you're implying), or 2 times the volume? A planet with a radius twice that of Earth would only just barely be terrestrial; it might have retained a gaseous envelope of hydrogen and helium. $\endgroup$ – HDE 226868 Feb 6 '18 at 17:34
  • $\begingroup$ You don't consider dolphins sentient? $\endgroup$ – Renan Feb 6 '18 at 17:38
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    $\begingroup$ "In a world with 2x the gravity, gravity would have less an impact in the oceans." And why does this lead you to believe that sentient aquatic life would be more probable? I'm not seeing the correlation between gravity and sentience. $\endgroup$ – F1Krazy Feb 6 '18 at 17:42
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    $\begingroup$ As Renan implies, there is already a vast variety of sentient aquatic creatures here on earth: squids, octopuses, sharks, crabs, fish, dolphins, whales, etc, etc. Do you perhaps mean sapient? $\endgroup$ – AngelPray Feb 6 '18 at 18:13
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    $\begingroup$ Gravity doesn't scale 1:1 with radius. $\endgroup$ – rek Feb 6 '18 at 18:13

The long-standing (and still dominant, as far as I can tell) model of Super-Earths – terrestrial planets two or more times larger than Earth – with oceans predicts they are more likely to be water worlds rather than continental, like Earth:

...if the component rock contributes to water as it does on Earth, a planet with [twice the radius, and] 15 times the mass but only 4 times the area will have a 3.75 times deeper hydrosphere, assuming everything equal. That means 16 km deep oceans.... Both Double-Earths are waterworlds, but one is deep. Neither has any land.
io9, "What a Habitable Planet Twice the Size of Earth Would Be Like" drawing on Sotin et al. in Sotin, C., Grasset, O., Mocquet, A. 2007. Mass-radius curve for extrasolar Earth-like planets and ocean planets. Icarus 191, 337-351

But it is less likely that larger ocean planets will contain any complex life, let alone sentient life:

Simulations of a hypothetical ocean world covered by 5 Earth oceans' worth of water indicate the water would not contain enough phosphorus and other nutrients for Earth like oxygen-producing ocean organisms such as plankton to evolve. On Earth, phosphorus is washed into the oceans by rainwater hitting rocks on exposed land so the mechanism would not work on an ocean world. Simulations of ocean planets with 50 Earth oceans' worth of water indicate the pressure on the sea floor would be so immense that the planet's interior would not sustain plate tectonics, volcanism to provide the right chemical environment for terrestrial life.
Wikipedia, citing Nature.

  • $\begingroup$ That's fascinating. So it just has to be less the 50 earth oceans worth water :-) Or a mechanism could be devised to give the effect of volcanism without plate tectonics. You just need nutrients from the core to upwell into the ocean. $\endgroup$ – Todd Hoff Feb 6 '18 at 19:00
  • $\begingroup$ @ToddHoff Reread the first half: 5 is enough to deprive the oceans of nutrients. Between 5 and 50 you'd get decreasing amounts from vulcanism, but still not enough. $\endgroup$ – rek Feb 6 '18 at 19:50
  • $\begingroup$ As I understand this argument it refers to more water, same gravity, whereas the OP could be dealing with a similar depth and relative abundance of water to land area, so the phosphorus argument doesn't seem a major issue here. $\endgroup$ – StephenG Feb 6 '18 at 20:47
  • $\begingroup$ @StephenG Whoops, looks like I skipped a step when forming my reply. I've editted it to include more of my rationale. $\endgroup$ – rek Feb 7 '18 at 3:04
  • $\begingroup$ It seems like they are assuming that ocean depth will scale as planet radius, which is not necessarily the case. In practical terms surely the topological deviations will be more limited in size by larger gravity, and I don't see why a world with large gravity would support larger variations in topology (height), and surely not 3.75 times larger. $\endgroup$ – StephenG Feb 7 '18 at 11:13

Not exactly

Gravity does not have less of an impact in the ocean, rather, buoyancy offsets gravity. Buoyancy has much less to do with gravity than you imply, rather, it has to do with the weight of the water displaced by an object. If an object displaced nothing (water/air/etc.), but was submerged in water, it would experience gravity as if it was not submerged in water.

This means that the displaced weight of the water would be almost 2x throughout, and the force of gravity would also be almost (due to surface gravity as @notstoreboughtdirt pointed out) 2x, causing a net change of near 0, but increasing the pressure on the body. This means the creature would either have to live in shallower waters (which may or may not be a detriment to developing intelligence), or would have to devote more resources during the infant through adolescent stages to protective structures for internal organs, which would leave less for developing things like the brain, decreasing the likelihood of intelligent life developing.

Source for the cause of buoyancy

Additionally, 2x Gravity implies 2x the mass, not particularly 2x the size. While size does correspond to mass, it only does so with respect to the density of the material. Also, as an object in 3 dimensional space increases in size, it's mass increases disproportionally quickly, for a spheroid (such as our planet) the ratio is 4/3*PI*r^3 where r is the radius (size). That would mean that doubling the size of the planet would octuple (8x) the gravity, so long as the average density of the planet didn't increase.

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    $\begingroup$ I think surface gravity doesn't quite scale with mass, because the surface moves away from the center. $\endgroup$ – user25818 Feb 6 '18 at 18:20
  • $\begingroup$ @notstoreboughtdirt Yeah I had not considered that. you're right in that a larger radius would imply less gravity at a point on the surface of a sphere (g=m/r^2). As the radius increases, the gravity decreases at the inverse square rate assuming equivalent density. $\endgroup$ – GOATNine Feb 6 '18 at 18:24
  • $\begingroup$ Yes, that was the idea. > Gravity does not have less of an impact in the ocean, rather, buoyancy offsets gravity. $\endgroup$ – Todd Hoff Feb 6 '18 at 18:47

I think that sentience would be neither more nor less likely. It would all depend on how much thinking is needed to get food and to avoid being food. Dolphins are pretty smart and sharks are pretty dumb but sharks have been around a lot longer than dolphins because they are so good at getting food and not being food that they don't have to be smart.

Technology (at least as we see it) is unlikely for aquatic life. Where do they learn chemistry by burning things? Where do they smelt and work metals? Where do they melt sand into glass?


More probable? There's no way to know. We might be the weird ones for waiting until we crawled out of the water to develop sapience.

As for using tools? That's a doozy. How would you smelt iron under water, without a fire?

So much of our energy manipulation (combustion technology) doesn't work underwater, so its hard to conceptualize a technologically sophisticated species that lived entirely underwater.


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