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I am envisioning a world around a red dwarf star. The planet has violent and accelerated (due to higher geological activity) plate tectonics and has a magnetic field about 75% the strength of the Earth's. The planet (we'll call it GF10) is roughly 40% the size of the Earth. The planet used to be nearly entirely covered by oceans but they eventually evaporated.

The planet had a relatively flat surface and 80% of that surface was covered by 20% of the water found on Earth. Most of said water evaporated, making a dense atmosphere that traps heat. There is only a sea the size of South Africa running horizontally along the planetary equator.

The planet has two moons roughly the size of Phobos. Orbiting the planet elliptically. The planet is gravitationally locked and orbits widely. During the summer weeks it is in the habitable zone, but its orbit takes it dangerously close to a gas giant (as well as to the edge of the habitable zone) to the point that it is viewable from the surface.

My question is this:

How might animal life (assuming life can be relatively easily divided into animal/plant categories) evolve to live on such a world?

I might not have described the orbit correctly so here is a map.

Map of orbit (not drawn to scale):

enter image description here

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    $\begingroup$ Related: What would most likely be the color of leaves on plants on a planet orbiting a red dwarf $\endgroup$
    – Jax
    Apr 20, 2015 at 15:56
  • $\begingroup$ What's this? No methane? You feeling alright? $\endgroup$
    – Frostfyre
    Apr 20, 2015 at 16:13
  • $\begingroup$ Are you talking about how existing life might be able to adapt once the star becomes a red dwarf, or are you asking how life might initially develop on a planet orbiting a star that already is a red dwarf? Also, I'm struggling to picture the planet's orbit around the star; could you clarify that part? Even a hand-drawn sketch-type diagram would probably help this question quite a bit. $\endgroup$
    – user
    Apr 20, 2015 at 17:00
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    $\begingroup$ @HDE226868 I am not really a numbers person and don't have a specified orbital radius. I simply want the orbit to [very briefly] exit the habitable zone on the cold side, interact with the gas giant and renter the habitable zone. $\endgroup$
    – Jax
    Apr 20, 2015 at 18:14
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    $\begingroup$ If the planet is completely within the orbit of the gas giant, it's unlikely to have the same length of year. What would be possible, however would be to have the planets in resonant orbits. A resonant 2:1 orbit, for example, would mean that the gas giant had a year exactly twice the length of the planet's year, and the planet would approach the gas giant at aphelion every other year. $\endgroup$
    – ckersch
    Apr 20, 2015 at 19:38

2 Answers 2

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Since a Red Dwarf is a very dim star, any planet in the habitable zone will be quite close and tidally locked to the primary. The planet will have multiple ecosystem bands based on illumination, with the "hot pole" having the primary directly overhead all the way to the "cold pole" on the opposite side of the planet.

Atmospheric circulation will be fairly intense, with a permanent low pressure cell based on the hot pole and a high pressure cell at the cold pole. Since the planet does not rotate, there is no Coriolis effect to deflect the winds.

Animal life will be adapted to living in a low light environment, probably with high winds near ground level rushing towards the hot pole and equally high winds in the upper atmosphere rushing towards the cold pole. There are no day/night cycles like we have, so the creatures will probably not evolve anything like a circadian rhythm. They may end up with a brain structure similar to a dolphin, being able to "go to sleep" with one half of the brain at a time so they do not become vulnerable to predation or natural disaster.

Living in a permanent twilight, they may develop large eyes, but other forms of sensory apparatus like electrical fields, echolocation or even blind creatures using smell and touch can all be envisioned (there are probably ecological niches for every kind of creature). Flying will be very challenging, since being caught in the wind with no viable means of directing your path means being swept towards the hot or cold poles, and likely being swept out of your own ecological "band". If the winds are strong enough, the ecosystem might resemble a corral reef, with creatures anchored to the ground and extending tendrils into the air to snag pollen, sperm from male creatures or edible matter (even dust carrying minerals might be important for their metabolism.

Most of these assumptions assume a similar biochemistry to that of Earth, but adapted to more extreme conditions.

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  • $\begingroup$ Is it really permanent twilight? Being so much closer to the star to be in the habitable zone would result in an increase of brightness. Of course the star is dimmer, but maybe these effects cancel each other out. It also could be that the brightest wavelength is not in the visible spectrum, but infrared. It would still carry energy, which could be used by plants, which could be eaten by animals (which would also see best in infrared). $\endgroup$
    – JFBM
    Apr 21, 2015 at 13:48
  • $\begingroup$ Most plants and animals would "see" in infrared, but as you get further from the hot pole, you will still have the issue of being farther from the primary and having the primary closer and closer to the horizon, until you pass over the "edge of the world" and into permanent night. So outside of the hot pole, the planet would effectively be in a permanent twilight. $\endgroup$
    – Thucydides
    Apr 21, 2015 at 23:12
  • $\begingroup$ The planet absolute does rotate, and there absolutely is Coriolis effect deflecting the winds! It rotates exactly as fast as it revolves. $\endgroup$
    – Logan R. Kearsley
    Feb 1, 2018 at 0:17
  • $\begingroup$ @Logan R. Kearsley But that is at the length of a "year". But assuming the planet will evolve around the star in more than a few earth-days, this rotation will be much slower than on Earth. I guess that means the effect will be smaller than on earth, right? $\endgroup$
    – Tobias Reich
    Jun 21, 2022 at 13:20
  • $\begingroup$ @TobiasReich That depends on exactly how big of a red dwarf you have. Orbital periods could be as little as a couple of days, up to a few weeks. Yes, the effect is not as strong as it is on Earth... but Venus has obvious circulation cells controlled by the planet's rotation, and it rotates only 1/116th as fast as Earth does! So a rotation period of a few weeks is still going to have significant effects--which are born out in detailed climate simulations of tidally-locked planets. $\endgroup$
    – Logan R. Kearsley
    Jun 21, 2022 at 16:43
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Most likely, if animal life would exists on Terra-like planets orbiting red dwarfs, they would need protection from the frequent solar flares of their red sun. To evolve, they would require protective shells. For mammals, possibly a tough layer of dead skin. They would crawl in caves or hide in the mud until the sky clears itself. The animals who couldn't find adequate shelter or lose theirs at the expense of others would be ''cooked'' and then devoured by the surviving predators after the flares would be gone.

Instead of seeing the blue reflection of the oceans, the sky would be transparent due to infra red light. The vegetation would appear in shades of dark green or even black.

Just guessing, but it's fun

Richard

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