Life on a small Neptunian/big super-Earth planet?

Question

I want to create an alien life for a planet which is at about 6–8 Earth masses. It has a thick atmosphere made of hydrogen, helium, methane and nitrogen. I want that life to evolve slowly and be quite complex, and being structured enough to become intelligent. It should not follow very strictly the natural selection (I mean, no one has to have special advantages to survive to predators or something so) and be very different to earth-like life. The planet has 3 moons and orbits an orange dwarf 5 Gy old, with a luminosity of 0.38 solar luminosities, from a distance of about 1.6 AU.

What kind of biochemistry could it be made of? How could it look like? What kind of ecosphere could it develop?

Answer 1

Since the atmosphere contains hydrogen and methane (and not oxygen and CO₂), the plants probably use hydrogenic photosynthesis. This question has some nice answers about the feasibility.

Since the luminosity is $0.38\,L_☉$, and the distance is 1.6 AU, the planet gets $\frac{0.38}{1.6^2} \approx 0.15$ times the insolation Earth gets from (the) Sun. It will be dim and cold. There must be either an internal heat source (such as some radioactive elements, rather probable with a super Earth) or a thick, heat capturing opaque atmosphere (probably both).

Quite within the possibilities of life, just remember the planet is dim, and the gravity probably high, the plants get however more energy from the photosynthesis, but the animals get less nutrition from eating the plants (compared to Earth). The rest is up to your imagination.

Answer 2

Answering the second part of the question, the ecosphere wold probably resemble the deep oceans of Earth. The upper atmosphere has a zone where enough light energy is captured to power hydrogenic photosynthesis (much like plankton in the oceans of Earth), and the detritus of dead "plant" matter drifts into the depths where deep diving "fish" eat it or prey on each other.

Some species of "fish" might swim to the surface to graze on the "plankton" directly, and so some species of predators might also evolve to live near the surface as well. Since the environment is dim and cold, one could speculate that the creatures might either evolve monster eyes to operate in the dim light, or perhaps not evolve eyes at all and utilize other senses like sonar or detecting electrical fields for long range detection of food, prey or danger.

Creatures operating near the "surface" would need some sort of insulation to retain heat, and since they all exist in a gas giant atmosphere, would probably be "hot air balloons" in principle in order to remain aloft in the environment. This might take the form of being "built" around gigantic "bladders" where the body heat keeps the lifting gasses warm, and a metabolism which concentrates hydrogen in the bladder while expelling the other elements.

It is very unlikely that such an environment will be conducive to intelligence and certainly not for technology (there are not even rocks for a stone age), so the planet will be mostly a novelty for alien astrobiologists to study, or a place to be "mined" for 3He and other elements by industrialists who are not very concerned about the "fish".

Answer 3

This question is essentially the same as my question linked in Radovan's answer. However I will answer your specific questions:

I want that life to evolve slowly and be quite complex, and being structured enough to become intelligent.

It seems that hydrogenic photosynthesis which converts methane and sunlight into hydrogen and biomass is 4-5X as efficient at building biomass as oxygenic photosynthesis but the energy gains from reducing carbohydrates with hydrogen as opposed to oxidising them (the opposite of 'reduction') is 4-5x less.

In my setting, I wish to avoid this effect. However if you are after a slower pace of life then stick with this constraint and simply have your animals need to consume 4-5x the biomass for a terran equivalent energy level or make them have much slower paced metabolisms and lives.

It should not follow very strictly the natural selection (I mean, no one has to have special advantages to survive to predators or something so) and be very different to earth-like life.

Dispensing with natural selection is entirely a separate question to your hydrogen based atmosphere. However, the difficulty your animals have in acquiring energy rich biomass will likely produce very different evolutionary outcomes to that we have seen on earth. Maybe you just want a different evolutionary outcome and pressures? In which case the difference should be sufficient without disposing of natural selection.

The planet has 3 moons and orbits an orange dwarf 5 Gy old, with a luminosity of 0.38 solar luminosities, from a distance of about 1.6 AU.

The hydrogenic photsynthesis process is 4-5 more efficient making biomass than the terran version, so at 0.4 level of insolation you could still end up with twice the amount of biomass we have on earth - but there may be few complex animals. However just because animal diversity might be lower and have slower metabolisms doesn't mean they can't develop complex intelligence - you just need to posit suitable evolutionary pressures to create such intelligence - and overcoming a lack of energy rich food sources as an excellent candidate.

What kind of biochemistry could it be made of? How could it look like? What kind of ecosphere could it develop?

As mentioned in my linked question, a suitable biochemistry for a hydrogen atmosphere is discussed in this paper by Bains et al. He suggests dimethylsulfonium propionate as an alternative to carbohydrate based energy metabolisms but doesn't elaborate on it much further. Please also see my newly filed question Pharbohydrates vs Carbohydrates which has asked exactly this specific question.