Are planets like the "To'ul'hian Worlds" from Orion's Arm plausible?

Question

The Orion's Arm Universe Project includes a description of what they call "To'ul'hian Worlds", planets that are (in the simplest of terms) a cross between Earth and Venus.

I'm working on designing a similar planet for an unconnected story, and this OAUP page is the closest thing to a reference for my setting that I've found so far.

@galactic_analyzer suggests that the OAUP world's CO₂ atmosphere is problematic, and I'd prefer an Earth-like nitrox atmosphere anyway, so for this question let's make that replacement.

Could a planet like this exist in real life? If not, what specifically causes problems with the concept?

The key points are:

• surface pressure of 10 to 100 bars
• surface temperature of 100 to 200 degrees Celsius
  • Note: at 10 bar, maximum temperature is 180°C; at 200°C, minimum pressure is 16 bar
• liquid water at the planet's surface
• an active carbon cycle
• develops oxygen-producing airborne life
• atmosphere includes thermosphere and ozone layer
• atmosphere consists primarily of carbon dioxide nitrox

Answer 1

Quite simply, it would not be possible. First of all, the only reason why the Earth does not have so much carbon dioxide is because of oxygen-producing life. If your planet has developed oxygen-producing life, it should already have converted most of the CO₂ into oxygen. While oxygen-producing life may have evolved only recently on your planet, other aspects of your hypothetical planet prevent each other from forming.

The early Earth, although rich in CO₂ with higher surface temperatures, did not have temperatures or pressures as high as Venus does today. Like Zxyrra said earlier, Venus’s current atmosphere was formed when greenhouse gases accumulated in its atmosphere, heating rocks to a higher temperature to form more greenhouse gases. This accumulated over time into its current atmosphere. The reason why this did not occur on Earth is because oxygen-producing life acted as a “brake” to halt the process. If oxygen-producing life evolved on your world, the planet would not have temperatures or pressures as high as they are mentioned in your question (10-100 bars, 100-200°C).

There is a great amount of difference between 10 and 100 bars of atmospheric pressure and 100 and 200 degrees Celsius. In one interpretation of your atmospheric pressure and temperature, liquid water cannot exist. A different interpretation would allow it to exist, but I do not know the exact values of your hypothetical planet's atmospheric pressure and temperature. A carbon cycle also could not exist on this world because oxygen-producing life is incompatible with higher surface temperatures and pressures. A thermosphere would also not exist on a planet with such high pressures; an ozone layer would similarly not exist because oxygen originating from photosynthetic organisms is a prerequisite for the formation of one.

In summary, your idea is an interesting thought experiment. However, there is either the toxic, hellish “Venus” type planet or the terrestrial, Earth-like planet with no “in-between.” So, your planet would not exist.

UPDATE: @Lawton edited his/her question to a nitrox (nitrogen-oxygen atmosphere similar to Earth) atmosphere. This is an entirely different scenario. If possible, I will answer his/her new scenario later when I have time.

Answer 2

Life organisms

Earth examples and liquid water

First of all within the ranges given, at temperatures lower than 150 °C and pressures greater than 10 bar water is liquid. Which also means that there is no runaway greenhouse effect under those conditions. Among the living organisms that live and reproduce in those conditions there are the following:

• Methanopyrus kandleri lives optimally at 105 °C (up to 122°C) and it was found also underwater at 200 bar. It can consume CO2 and H2 to produce methane(CH4).
• Pyrobaculum islandicum lives best at 100 °C (up to 103°C). It can survive with only elemental sulfur, CO2 and H2 while acting as the producer of organic matter that the other living being may need.
• Pyrolobus fumari lives best at 106°C (up to 113°C) and was also found underwater at 370 bar. Among the many ways, it can live by consuming O2 and H2.
• Geogemma barossii aka. Strain 121 lives best at 103°C (up to 130°C) and it was found also underwater at 243 bar. It survives by using iron instead of oxygen.
• Pyrococcus furiosus lives best at 100°C (up to 103°C-105°C). It can generate H2, but O2 is toxic to it. In its presence it tries to convert it into water.

Carbon cycle and ozone

Assuming a way of producing O2 from water (which will be discussed later) the presence of O2 in the air would transform the CH4 produced by the Methanopyrus kandleri in formaldehyde(HCHO). This would react with O2 to produce formic acid, which readily decomposes into H2O + CO in the presence of sulfuric acid, which is present in the upper clouds of Venus.

For the ozone you mostly need only O2 due to the ozone-oxygen cycle.

Oxygen cycle

As per the missing production O2, this would be what plants usually do. This wikipedia paragraph shows how increasing the temperature is either indifferent or improves the photosynthesis, but this may not apply to temperatures a bit over 100°C.

There are some like the Chloroflexus aurantiacus that are able to do photosynthesis using bacteriochlorophyll instead of chlorophyll and grow at 70°C, but they don't produce O2 (this due to using bacteriochlorophyll). Others like Cab. thermophilum are able to use chlorophyll at 66°C, but they consumes O2 instead of producing it.

Even if I didn't found any O2 producing organism that lives at over 100°C, it's important to notice how such an environment is rather scarce on earth, which makes the few known cases have a rather low statistical relevance. There could be an alternative and possible evolution path where those exist, but it just didn't happen. From the data riported the existence of such a being seems plausible. On the other hand if there isn't such a being then the requested planet can't exist (no oxygen-producing airborne life at those temperatures).

Environment planet-wise

Required differences from Venus

First of all that planet should have a magnetic field like earth to reduce the loss of oxygen and hydrogen due to the solar wind, as they are both needed for life. Having a thermosphere is not a problem as both Earth and Venus have it.

Additionally a day duration more similar to the one on earth would allow for a more even temperature which helps (together with the CO2 that on the surface is a supercritical fluid with a good heat conduction) the organisms have the temperatures more near the mean of 100°C (131°C and they all die). This would have the effect of changing the wind circulation into one more earth-like.

Consequences on sulfur

In a planet with an atmosphere composition like the one of Venus, the surface pressure would be around 90 bar, which is perfectly within range. As per the temperature, it'd surely be higher than the one of a planet like earth, but that would still depend on its distance from the sun. Just put it much further away and you'd get the desired surface temperature. This has also the effect of preventing the formation of the clouds as the sulfuric acid cycle needs a surface temperature of at least 300°C (which is not there) to regenerate the clouds from the acid rain like on Venus.

The result would make all the sulfuric acid stay mostly on the surface and a big reduction in SO2 content in the atmosphere, with clouds being created through evaporation like on earth. It's also worth notice how the surface temperature of 100°C is at 33% between the melting and boiling point of sulfuric acid, while the earth average of the surface sea is 16.1°C, roughly at 16% between the melting and boiling point of water. Being closer to the boiling point would create more clouds than on earth (it limits the photosynthesis), but still way less than the current situation on Venus.

Life related atmospheric composition.

It's also important to consider that there would be a higher concentration of CH4 in the air due to the presence of the Methanopyrus kandleri. Additionally there would be two ways to consume O2: the atmospheric transformation of CH4 into CO2 and the Pyrolobus fumari that consumes H2 and O2. If the amount of CH4 produced is not enough to make the atmosphere fully consume the O2, the Pyrolobus fumari would help consuming the rest. This would result in an atmosphere with mostly CO2, and only in minor part of O2, CH4 and H2.

The low production of CH4 and consumption of CO2 could be attained by carefully choosing the surface temperature to control how fast each species reproduce. The amount of O2 present would probably be enough to kill the Pyrococcus furiosus needed to generate the H2, but I can't see why there couldn't exist a variation able to withstand a concentration of O2 a bit higher than that one.

This difference from Venus would help increasing the greenhouse effect due to the presence of CH4 instead of CO2, with the former having a greater global warming potential. Additionally it would reduce the pressure On the surface and depending on the variation, it may reduce the ability of the superfluid CO2 to conduct heat and keep the temperature uniform. This may complicate a bit the situation, but it wouldn't be a deal breaker

other online resources used:

• https://www.newscientist.com/article/dn14208-the-most-extreme-life-forms-in-the-universe/
• https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4148896/#T1
• https://engineeringunits.com/pressure-at-depth-calculator/
• https://en.wikipedia.org/wiki/Atmosphere_of_Venus

Answer 3

With such temperatures you'd have another problem. At least, if your lifeforms are going to be anything like familiar biology.

Complex proteines break down at those temperature similar to how the proteine in meat changes its properties when subjected to high temperature, giving it its brown rather than pink or red coloration. The only thing coming remotely close to the kinds of lifeforms that could live in your environment are thermophiles with highly specialized enzymes. But that also has its limits, it doesn't allow for very complex and big proteine chains required for complex life. You'd have to deviate from carbon based since carbon based proteines simply won't hold up at such temperatures.