Hycean worlds are planets twice as big as earth and covered in water oceans and with atmosphere full of hydrogen. A dark hycean world is one such world tidally locked to a star with one side forever facing the sun. I don't think the exact type was specified. Would this make much of a difference in terms of life evolving on this planet? I assume the dark side would be covered in ice and an eternal storm would be in the center of the light side.

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    $\begingroup$ This is a hard question to answer, mostly because we only have one datapoint for life evolving, and we don't really understand that very well either. It would be helpful if you went into a bit more detail. Do you mean, "could some kind of organic replicator form?" or "will something that can invent an internet to be insufferably pedantic on evolve?" or something else? Though to be honest the former probably ends up as "maybe" and the latter as "probably not"... $\endgroup$ Commented Jan 9, 2022 at 20:56

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Would your Hycean be called a Hycean ?

The only scenario I can think of to accomodate life on a Hycean (rogue, ocean) planet is to have an enormous blue giant in the vicinity. Such a star can provide heat to sustain life, it may do that from a light year away. You'd either have a giant orbit (millions of years), or no orbit at all.

But keep in mind when you are tidally locked, you DO have an orbit, by definition. The rotation time of your planet is equal to the orbit duration, not zero. With a true Hycean, the rotation would become zero. The star would be seen at a point in the sky that depends on the orbit. Your Hycean day/night cycle could last a million years.



The climate will be much stabler than Earth's climate, there is no day cycle. But there would be an enormous variation place to place. Rather than seasons, you'd have regions, or bands. It will have storms and convection currents in the center indeed, but you'll see that on the edges as well, as a result of the enormous temperature differences with the cold side. If you need an agreeable temperature.. choose your solar distance with care. It can't be too far away !

Evolution age and sequence of events

Whether life can adjust, will also depend on how old your life is: when evolution would predate the escape of your Hycean planet from the solar system, the required adjustments would have enormous impact. Every species depending on a biological clock would die out and evolution probably started all over again.

Where is Life

Forget the dark side. No life will exist there. On the dark side, you'd have temperatures below 50K when the planet is far away from a sun, like a Hycean is. You could park James Webb telescope on the dark side of your planet, and it would work.

There will be species profiting from the edge region, where the temperature gradient exists. That zone will have the full spectrum of life possible on your planet, biodiversity will thrive there.

Life could have to migrate very slowly during evolution, around your planet

When your planet is truely Hycean, or near Hycean, its own rotation has completely stopped, there isn't even a tidal lock with the sun left. A day/night cycle may return, corresponding to the "orbit" which can now take millions of years to complete. As a consequence, your evolution will involve migration of species, around the planet, in the same tempo. Keep that blue giant above the horizon, go west or you'll die !


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