The main source of free energy that allowed life on Earth to develop was our sun. It bombards the planet’s surface with photons of high energy, which provide activation energy for reactions necessary for photosynthesis. Photosynthesis allows plants to grow, and their high-energy organic matter can be consumed by heterotrophs, which can then consume each other as well, etc.

QUESTION: Suppose we have a planet that starts off as a soup of inorganic compounds, as did primordial Earth, but that is quite far away from the nearest star. What might be some other sources/mechanisms by which energy could be harvested and life could form?

Some ideas:

  • Even if the nearest star is far away, some of its light will still hit the planet, and the occasional high-energy photon might strike the surface. So maybe we might still see photosynthesizing organisms (or something like them) spring up, but with much slower metabolisms and longer lifespans (since they have to wait a long time for less frequent high-energy photons).
  • The core of the planet would still be quite hot, meaning that volcanoes and underwater vents could occasionally release hot (and therefore high-energy) bursts of liquid/gas into the environment. Perhaps this could be harnessed, somehow...?
  • I think I remember reading that primordial Earth had a lot of electrical storms. Maybe some sort of mechanism could evolve by which organisms harvest electrical energy?

Can anyone provide any other ideas, or flesh out (or even refute) any of my preliminary ideas?

  • $\begingroup$ Well, if there is a soup of something this means that there is liquid water, which means that the planet is well warmed by its sun... $\endgroup$
    – AlexP
    Dec 5 '19 at 0:56
  • $\begingroup$ @AlexP not necessarily; I do agree that it would be the most likely reason but there could also be the impact of tidal forces, like on Io or Europa generating enough geothermal activity to keep the water liquid. Whether or not this is possible with a planet orbiting a single star is of course an interesting question but in the case of a planet in a binary star system this could be a possibility. $\endgroup$
    – Tim B II
    Dec 5 '19 at 1:03
  • $\begingroup$ @TimBII you don't need tidal forces. The earth generates enough geothermal energy that even if the sun cooled and the oceans froze, there would still be liquid water around volcanic vents at the bottom of the ocean, an living creatures feeding off the energy coming out of them. $\endgroup$ Dec 5 '19 at 1:06
  • $\begingroup$ @MorrisTheCat that is true, but isn't that largely down to radioactive decay within the core? I would expect most planets that form through accretion to have a lot of heavy radioactive elements in their cores as opposed to their crust, but that assumes they're born in an environment rich in such elements in the first place. You are correct of course that tidal forces are not the ONLY source of geothermal energy though and I should have pointed out in my comment that tidal forces are just one example of alternative energy sources. $\endgroup$
    – Tim B II
    Dec 5 '19 at 1:15
  • $\begingroup$ @TimBII The heat in the earth's core is mostly just left over from the formation of the planet, not caused by radioactivity. I mean, SOME heat comes from that, but it's not the primary source. Any planet made of the same stuff Earth is would have a similar heat engine going on. Mars for example had the same kind of tectonic motion going on, but it's smaller than the Earth is and its core is proportionally smaller and has less iron than ours, so it cooled off faster. $\endgroup$ Dec 5 '19 at 1:27

Welcome to the stack! Great question!

The core of the planet would still be quite hot, meaning that volcanoes and underwater vents could occasionally release hot (and therefore high-energy) bursts of liquid/gas into the environment. Perhaps this could be harnessed, somehow...?

This is the most likely answer, and it's occurring right here on Earth as we speak. Riftia Pachyptila are the best known example. These are a species of tube worms that live exclusively on the deep ocean floor around volcanic vents, and they survive entirely upon energy drawn from the vents, completely independent of any energy from the sun. There are a number of other extremophiles and other species that have evolved in this way.

There have been enough examples of life of this kind found here on earth that scientists are considering it more and more likely that life like this could exist in other places in our solar system. Europa is considered the most likely candidate even though its surface is frozen solid, because the tidal forces from Jupiter drive sufficient geothermal activity that vents just like the ones on Earth could be present on the bottom of Europan seas as well and, if so, there might be life there utilizing the same energy sources.

With regard to your other possibilities, the main thing to consider is that there has to be SOME kind of energy source for life to occur. Radiant energy from a star isn't the only possibility, but it is the easiest. Tidal forces creating tectonic energy like you get from the Galilean moons is another. One way or another though, you have to have heat to drive the chemical processes required for life.

I would note that in your third example, those electrical storms found on ancient earth were ALSO driven by energy from the sun, but here again, a nearby gas giant could provide the necessary juice via its own colossal magnetic field. Jupiter has massive electrical storms in its own atmosphere, and it's believed it might be driving similar storms on Io and/or Callisto as well.


The nucleous of the planet could be pretty hot and provide enough energy for a stustainable life.

If the nucleous of the planet contains a enough radioactive materials, it could continue producing heating for billions of years, enough to allow the formation of a complex life.

The main problem is that there could be no photosynthesis, so no oxygen, or oxygen must be produced in another unknow way.

Life is possible in a Reducing atmosphere, on earth there are many extremophiles bacteria able to live in these conditions thanks anaerobic metabolism.

The problem is that all anaerobic metabolisms that we know are all too inefficient to sustain complex life.


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