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I'm trying to create a planet which environment is not immediately lethal to humans, (reasonable oxygen level) however after closer inspection turns out not to be specially Earth-like.

One issues is its hydrosphere. Over 90% of planet surface is water (or ice). It's rather cold tidally locked planet, so to have a reasonable excuse why the water is still liquid, I'd prefer to beef up salinity.

Which is acceptable salinity level in which carbon based life as we know it could realistically thrive? By thrive I don't mean local equivalent of toughest archaea and tiny brine shrimps, but existence of bigger organisms, which weight exceeds at least kilogram)

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  • $\begingroup$ A rather cold tidally locked planet seems unlikely, if you mean locked to its star. A tidally locked planet in the habitable zone of a its star would probably be too hot on the light side and too cold on the dark side. Or the world might be a giant, Earth sized moon tidally locked to a giant planet. $\endgroup$ – M. A. Golding Jan 13 at 17:36
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    $\begingroup$ That's the point of having huge oceans that are hard to freeze - effective heat transfer. $\endgroup$ – Shadow1024 Jan 13 at 18:01
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It depends on how complex life you want to have. For simplicity, I will refer to what we can observe on Earth.

The body of water with the highest salinity on Earth is Gaet'ale Pond

The water of Gaet'ale Pond has a salinity of 43%, making it the saltiest water body on Earth. The waters of the lake originate from a thermal spring located beneath it. For this reason its temperature (50–55 °C) is hotter than the environment.

No life forms are mentioned to be living there, while in the Don Juan Pond in Antartica, studies are equivocal.

Salinity varies over time from 200 to 474 g/L, dominated by calcium chloride, and is over 18 times the ocean's salinity and 1.3 times that of the Dead Sea. It is the only Antarctic hypersaline lake that almost never freezes with a salinity of 338 ‰ = 33.8%. It has been described as a groundwater discharge zone.

Lake Rebta, the third in the ranking, hosts complex life forms like fishes:

The lake is known for its high salt content, up to 40% in some areas, [...] Fish in the lake have adapted to its high salt content by evolving ways to pump out extra salt and keep their water levels balanced. The fish are approximately four times smaller than those living in a normal environment, as a result of salt water fish dwarfism.

Based on the above it seems reasonable that you can have some life forms at around 35% salinity.

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  • $\begingroup$ Thanks for finding info on those tiny fishes, through my own googling I only managed to get invertebrates. But you haven't managed to find anything bigger, have you? (with possibly lower salinity) $\endgroup$ – Shadow1024 Jan 13 at 15:44
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    $\begingroup$ @Shadow1024, to support bigger animals one needs also a bigger food chain. Considered that normally such extreme environments are rather limited in size, that is hardly possible. Maybe for a whole planet it would be another story. At the end we live on a planet full of a toxic gas (Oxygen) $\endgroup$ – L.Dutch Jan 13 at 16:25
  • $\begingroup$ @L Dutch While issue of such food chains is a valid point, a normal small lake in my country usually have a few species of fish that as adults reach a few kilograms. $\endgroup$ – Shadow1024 Jan 13 at 19:07
  • $\begingroup$ @Shadow1024, but a lake takes nutrients from a much broader territory. A hypersaline pond probably not (in the examples I gave only the last one has exchanges with the ocean, and indeed hosts fishes) $\endgroup$ – L.Dutch Jan 13 at 19:37
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Well look at what you have on earth; salinity is usually measured in parts per thousand (ppt or ). The average ocean salinity is 35ppt and the average river water salinity is 0.5ppt or less. This means that in every kilogram (1000 grams) of seawater, 35 grams is salt.

According to Cotruvo, humans can tolerate limited amounts of salts in water “for reasons of health and palatability,” but cannot tolerate excess salt intake. He notes in the article that there are also indications of positive benefits of minerals like magnesium and calcium in drinking water.

"When it comes to treating salt in water, “The universal treatment for salinity is membrane treatment related to desalination. Nanofiltration membranes are capable of removing multivalent ions more efficiently than monovalent ions, so they can be somewhat effective especially in non-seawaters,” says Cotruvo in the article. “Reverse osmosis achieves removal in the [98 to 99] percent range in thousands of high-pressure seawater desalination applications that can operate at rates as high as multimillion gallons per day.”

for more information check this out: https://www.watertechonline.com/understanding-salinity-in-drinking-water/

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    $\begingroup$ OP is not asking about humans, but about carbon based life $\endgroup$ – L.Dutch Jan 13 at 19:36
  • $\begingroup$ I'd argue that humans are carbon based life, it may not be the most all encompassing answer, but I think it does provide an answer, especially considering the OP specifically mentions humans in the question. $\endgroup$ – James Jan 21 at 15:20

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