Specifically, the gas giant Epsilon Eridani b, and a moon that has a mass of ≥0.25 Me. If there's no feasible way the moon could have oceans at a distance of 3.52 AU from a star that emits 0.34 times the amount of light that the sun does.


4 Answers 4


The other answers are incorrect. There are two possible ways a moon of Epsilon Eridani b could have oceans.

One) Subsurface oceans beneath a thick shell of ice.

According to this list

Largest lakes and seas in the solar system

6 bodies in the outer solar system have global subsurface oceans of liquid water, and another 11 are suspected to. And I think there are probably more subsurface oceans in many larger Trans Neptunian Objects.

The question asked for surface oceans of liquid water, but for some types of stories, subsurface oceans might be good.

Two) Tidal interactions between the moon and its planet can cause sufficient tidal heating for liquid water oceans on the surface.

Theoretical studies of the possibiity of habitable exomoons of giant exoplanets show that the extent of tidal heating on those exomoons depends on their distance from their planets, among other factors. As a rule the closer to the planet, the greater the tidal heating on the moon.

If the moon orbits too close to the planet, the tidal heating will be enough to initate a runaway greenhouse effect, turning all the surface water into atmospheric water vapor. If the moon orbits even closer, the tidal heating will cause excesse vulcanism on the moon, making it a volcanic hell like Io.

Thus the phrase "habitable edge" for the inner limit of how close a moon can orbit to its planet while avoiding excessive tidal heating.

And it should be obvious that if the planet and moon orbit too far from their star for stellar radiation alone to warm the moon enough for liquid water, the moon could still be warm enough if it orbits outside the tidal edge but close enough for enough tidal heating for liquid water oceans.

  • $\begingroup$ Ooh dang... I forgot about tidal heating... well, I've already started my worldbuilding based on possibility one, but I'll probably use this at a later point. Is there a way to calculate the tidal heating given a goal temperature? $\endgroup$
    – isdi0
    Nov 19, 2022 at 20:30
  • $\begingroup$ Is there experimental evidence for liquid water under the surface of these moons? Or is it all theoretical calculations based on tidal heating? $\endgroup$
    – Daron
    Nov 19, 2022 at 20:53
  • $\begingroup$ @Daron I do know that there were some probes that used Jupiter's very powerful magnetic field to examine the composition of its moons. Plus, enceladus has jets of water at its south pole. $\endgroup$
    – isdi0
    Nov 19, 2022 at 21:14
  • $\begingroup$ Nice. Now I'm wondering about the extreme solar wind there, not sure how that might affect the situation. Still, it's apparently quite a young system circa a billion years old, so maybe fine for atmosphere to be OK. $\endgroup$ Nov 20, 2022 at 1:07
  • $\begingroup$ As I recall, Io also has some other stuff going on besides tidal heating; there's electromagnetic weirdness involved too, with Jupiter's intense magnetic field. That could also be a source of heating for a moon, though it would also likely just make it even less habitable by creating constant lightning strikes and ionizing radiation. $\endgroup$
    – Hearth
    Nov 20, 2022 at 16:42

Epsilon Eridani B has Mass of roughly 0.63 that of Jupiter (and is therefore not a brown dwarf star due to lack of mass), and a temperature of ~150 K (−123 °C; −190 °F).

This is too cold to have liquid water, and any moons of Epsilon Eridani B would have a similar temperature, and would therefore also not have surface oceans of liquid water.

  • $\begingroup$ You might find my answer interesting. $\endgroup$ Nov 19, 2022 at 20:22

It's too far away from the star and so cold for the surface of any ocean to be liquid.

Taking an example from the moons of Jupiter, Europa in particular, the possibility of a sub-surface ocean might be explored.

Internal heat generated by tidal flexing from the interaction with the orbits of other moons, and perhaps a slightly eccentric orbit might produce sufficient equivalent-to-tectonic stresses within the crust, could enable liquid water to exist.

  • $\begingroup$ Yeah, that could support life! One problem: I asked for surface oceans, because I want any life that develops to become a spacefaring species. $\endgroup$
    – isdi0
    Nov 19, 2022 at 1:29
  • $\begingroup$ It strikes me as a smaller step to drill to the surface of an ice-pack above you than to fly to space. I'm saying it might work for you either way if you chose it. @isdi0 $\endgroup$ Nov 19, 2022 at 1:49
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    $\begingroup$ Well, the problem is: they have no motivation to drill through the ice! From what I know, the radiation on top is intense, and I'm not sure how much radiation they'd be exposed to through all that water and ice, but either way, they'd probably die without protection. Plus, I mean, they don't ever discover fire so they can't become advanced. This is based mostly on human development, because we don't have anything to compare ourselves to, so I could be wrong. Sorry if this comes off as rude or something, I don't mean to be. @Angry $\endgroup$
    – isdi0
    Nov 19, 2022 at 2:00
  • $\begingroup$ You're dead right, their technology would be totally different if the come from below. There's questions about that - possible, but if it doesn't fit with what you're after.... @isdi0 $\endgroup$ Nov 19, 2022 at 2:04
  • $\begingroup$ Alright. So on to my next question, which I've posted separately here. The reason it's separate is because I believe it's against the rules to have multiple questions in one post. @Angry $\endgroup$
    – isdi0
    Nov 19, 2022 at 2:07

Whoops, looking at the numbers, I've just realized it is kinda impossible. So nevermind!
Here's the math (I might've made an error somewhere): $$ f=\frac{L}{4\pi d^{2}} $$ L is the luminosity of Epsilon Eridani, which is 0.34 times that of the sun. d is the distance between Epsilon Eridani and Epsilon Eridani b, which is 3.54 AU
So plugging in the numbers gives us $$ \frac{0.34}{4\pi 3.52^{2}}\approx0.00218365349994 $$ The final value is the fraction of earth's solar flux, which is 1373W/m2, so if we do a simple calculation: $$ 1373\left(0.00218365349994\right)=2.99815625542 $$ The final value we get is 2.99815625542W/m2. Let me know if I got anything wrong!
Anyway, enough math, the final value (2.99815625542W/m2) is very small. Definitely not enough to raise the moon's temperature above 0C (which is the freezing point of water no matter the pressure). So this question answered itself with math!

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    $\begingroup$ Just so as you know, this is technically a hard-science answer to a science-based question - nothing bad in that, you've given equations where just assertions would suffice. I'm not in the position to judge the appropriateness of the math however. $\endgroup$ Nov 19, 2022 at 2:01
  • $\begingroup$ Ah, right. I got a bit carried away in the math... The reason I did that tag instead of hard-science, is because I want to have a wider range of people who can answer the question. $\endgroup$
    – isdi0
    Nov 19, 2022 at 2:03
  • $\begingroup$ My fragile tender eyes are offended by your long decimal expansions. Yuck! Since the exact value is not important, and since you are already using 0.34 AU and not 0.3456801927153 AU, you can save paper and just round 0.00218365349994 to 0.0022. $\endgroup$
    – Daron
    Nov 19, 2022 at 9:27
  • $\begingroup$ @Daron it's probably copy paste from the calculator and I do agree with your complaint about bad layout resulting from that, but do we really need to worry about saving paper on this website ? $\endgroup$
    – Goodies
    Nov 19, 2022 at 10:51
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    $\begingroup$ @Goodies I know. I am being self-deprecating about saving paper. $\endgroup$
    – Daron
    Nov 19, 2022 at 10:52

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