enter image description hereenter image description hereI was wondering if the parameters for this world will work. It doesn’t need to be pleasant for humans but it will have at least multicellular life.

  • Object name: Adii-1
  • Mass: 0.33334x Earth’s
  • Mean Diameter: 0.70403x Earth’s
  • Mean Gravity: 0.67253g
  • Density: 5.267g/cm^3
  • Escape velocity: 7.697km/s
  • Average surface temperature should be around 42°c
  • Bond Albedo: 0.54
  • Greenhouse effect should be 34.5°c (1.5°c more than Earth’s)
  • Atmosphere pressure: 4.76 atm
  • Atmospheric content: 63% Nitrogen, 20.3% Argon, 10.5% Oxygen, ~4-6% water vapour, 0.19% Carbon Dioxide, 0.01% non-harmful trace gases

Here are my calculated partial pressures:

  • Oxygen: 0.4998 atm (Think nearly 50% Oxygen on Earth instead of 21%, a lot but it should still be breathable. Will lead to severe fires and possibly giant insects though! :P )
  • Nitrogen: 2.9988 atm (Think nearly 4x the Nitrogen we normally breathe. This is similar to levels that divers have to deal with.)
  • Carbon Dioxide: ~0.009 atm (Around 9000ppm. May give humans headaches but wouldn’t kill you in the short-term.)
  • Argon: <1.25 atm (reason this is less precise is because I used 4-6% of the Argon, which was initially 26.3%, to replace it with water vapour instead) (Again, very high Argon but I’m pretty sure some divers tolerate these levels in Nitrogen/Argon mixes.)

(NOTE: this doesn’t have to be pleasant for humans to breathe, although I would like if it could be breathable by some form of bipedal carbon-based lifeforms similar to humans)

  • Air density: 5.821kg/m^2
  • 41.4% surface water (oceans, lakes etc, mostly concentrated in the northern hemisphere)

Adii-1’s internal structure consists of 75.4% silicate mantle and 24.6% metallic core.

Adii-1 is a moon that orbits a super-Jupiter gas giant called Adii every 12 hours, 5 minutes and 14 seconds (giving a solar day of 12 hours, 8 minutes and 39 seconds because Adii-1 is tidally-locked to Adii). Adii-1 has 0 eccentricity and obliquity in relation to its parent, Adii. However, Adii has an obliquity of 86° in relation to the star (also giving Adii-1 about that obliquity in relation to the star). Adii’s orbit around the star has an eccentricity of 0.023.

Adii’s parameters:

  • Mass: 8.501x Jupiter’s mass
  • Mean Diameter: 11.548x Earth’s diameter
  • Mean Gravity: 20.262g
  • Rotational period: 8 hours, 24 minutes and 54 seconds
  • Atmospheric content: 92.1% Hydrogen, 7.49% Helium, 0.351% Methane, 0.059% other suitable trace gases.

The star is called Ahknaral. Adii orbits Ahknaral every 107.476 days (Earth days). Adii’s semi-major axis is 0.4106 AU.

Ahknaral is a K0.9 type main sequence star with a luminosity 0.40064x that of our sun and a temperature of 5097 Kelvin. It has a mass of 0.79139x that of our sun.

What would the climate/weather roughly be like on Adii-1? Do these parameters work? I know the stellar flux on Adii-1 is probably quite high (around 2.2 at least), but I’ve heard worlds with a high albedo and less surface water may get around the classic habitable zone limits a bit.

I have added a black and white map of Adii-1’s surface. Black = land and white = water. I have also added a rough coloured map for showing mountainous areas etc. White-covered area in the coloured pic always sees gas giant Adii in the sky.

If anyone is wondering, I am basing Adii-1 on a moon I found in Space Engine. All I have changed is the atmospheric content to hopefully make it breathable for some kind of bipedal lifeforms similar to humans.


In case some of my size and distance parameters are off, I will provide a reference to the object in Space Engine that I’m basing Adii-1 on: RS 0-8-7493430-135-21-7-1479547-324 A3.1. I have also been more precise with some of the data provided. I have also provided some more data for Adii.


I actually tested the distance parameters for a world with Adii-1’s parameters on a star with Ahknaral’s parameters in universe sandbox 2 and surprisingly, it works and has my desired temperature. However, on other calculators it doesn’t work, perhaps due to albedo not being taken into account.

UPDATE 3 Modified gravity and diameter again slightly and added internal structure details and escape velocity for Adii-1.

  • $\begingroup$ Please try to condensate all the edits in as few shots as possible. Repeated small edits bump the post up and this causes an unfair exposure with respect to other posts. $\endgroup$
    – L.Dutch
    Jun 7, 2023 at 17:54
  • $\begingroup$ First thing that stands out is complex life surviving in 10% oxygen sound hard. What have you found so far about that happening on Earth? $\endgroup$
    – Daron
    Jun 9, 2023 at 18:11
  • $\begingroup$ @Daron this is 10.5% oxygen composition in an atmosphere that is 4.76x the pressure of Earth’s. The partial pressure of oxygen on Adii-1 is actually more like Earth having 50% oxygen. $\endgroup$ Jun 9, 2023 at 19:08
  • $\begingroup$ @casualworldbuilder Huh? How come the air is so dense when the gravity is less than Earth? $\endgroup$
    – Daron
    Jun 10, 2023 at 8:52
  • 1
    $\begingroup$ @Escapeddentalpatient. I don't know much about Venus's athmospthere. This question is starting to look like Greek Numbers Salad. $\endgroup$
    – Daron
    Jun 10, 2023 at 12:13

2 Answers 2


Some parameters of Adii-1 are:

Object name: Adii-1

Mass: 0.3x Earth’s

Diameter: 0.7x Earth’s

Gravity: 0.65g

Density: 5.267g/cm^3

A density of 5.267 grams per cubic centimeter is 0.9552 that of Earth. Since Adii-1 has a mass of 0.3 Earth, its volume must be 0.3 Earth Volume divided by 0.9952, or 0.314 that of Earth.

Adii-1 has a diameter of 0.7 that of Earth. Thus its volume should be 0.7 cubed times that of Earth. the volume of Adii-1 should thus be 0.343 that of Earth. That is more than volume of 0.314 that of Earth calculated from it's mass and density.

According to this surface gravity calculator:


With a mass of 0.3 Earth and a diameter of 0.7 Earth Adii-1 should have a surface gravity of 0.61 g, which is a little less than given above.

The escape velocity of a world is very important for its ability to retain an atmosphere.

According to this escape velocity calculator:


With a mass of 0.3 Earth and a diameter of 0.7 Earth Adii-1 should have an escape velocity of 7.323 kilometers per second, which is 0.654 that of Earth.

The parameters of Adii-1's atmosphere are:

Atmosphere pressure: 4.76 atm>

Atmospheric content: 63% Nitrogen, 20.3% Argon, 10.5% Oxygen, ~4-6% water vapour, 0.19% Carbon Dioxide, 0.01% non-harmful trace gases

Air density: 5.821kg/m^2

Earth has a sea level pressure of 1 atmosphere or 760.0 millimeters of mercury (mmHg). A pressure of 4.76 atmospheres equals 3,617.6 mmHg.

Assuming that the atmospheric gases have pressures determined by their percentages given above, the atmosphere would contain 2,279.09 mmHg of Nitrogen, 734.3728 mmHg of Argon, 799.848 mmHg of Oxygen, 144.7 to 2174.06 mmHg of water vapor, and 6.87344 mmHg of Carbon Dioxide.

Habitable Planets for Man , Stephen H. Dole, 1964:


has a table 2 on page 16 giving the upper limits of pressure of various gases which humans can tolerate. The upper limit for Nitrogen is 2,330 mmHg, just above the figure for Adii-1, the upper limit for Argon is 1,220 mmHg, far above the pressure of Argon in Adil-1's atmosphere, and the upper limit for Carbon Dioxide is 7 mmHg, a little bit above the amount in Adil-1's atmosphere.

According to table 4 on page 21, the upper limit for water vapor is 25 mmHg, far below the pressure in Adil-1's atmosphere.

What about the safe pressure range for Oxygen? On page 15 the safe pressures for Oxygen are given as 60 mmHg to about 400 mmHg. And 400 mmHg is about half of the pressure of 799.848 mmHg of Oxygen in the atmosphere of Adil-1.

So you need to reduce the partial pressures of water vapor and Oxygen significantly to make the sea level atmosphere of Adil-1 survivable for humans.

Orbits a super-Jupiter gas giant called Adii every 12 hours. Adii-1 has 0 eccentricity and obliquity in relation to its parent, Adii. However, Adii has an obliquity of 85° in relation to the star (also giving Adii-1 that obliquity in relation to the star). Adii has an eccentricity of 0.023.

The star is called Ahknaral. Adii orbits Ahknaral every 107 days (Earth days). Adii’s semi-major axis is 0.4106 AU.

Ahknaral is a K0.9 type main sequence star with a luminosity 0.4x that of our sun and a temperature of 5097 Kelvin. It has a mass of 0.79x that of our sun.

If Ahknaral has a luminosity 0.04 o the Sun, the distance at which Adii and Adii-1 receive as much radiation from Ahknaral as Earth gets from the Sun, which I call the Earth Equivalent Distance or EED, should be the square root of 0.4 times 1 AU, or 0.6325 AU.

Orbiting Ahknaral at 0.4106 AU, which is 0.649 of the EED around the star Ahknaral, is the equivalent of orbiting the Sun at a distance of 0.649 AU.

As you can see from the table here:


there is only one estimate of the inner edge of the Sun's circumstellar habitable zone which less than 0.649 AU, and many which are much farther from the Sun.

I note that planet orbiting a star at 0.649 of the star's EED would receive about 2.374 times as much radiation from its star as Earth gets from the Sun.

I note that Adii-1 is supposed to have an orbital period around Adii of 12 hours. Io, a moon of Jupiter, has an orbital period around Jupiter of 1.769 Earth days, or 42.456 hours, 3.438 times as long as 12 hours. But Io orbits too close to Jupiter to be habitable, since tidal heating effects cause the constant volcanos on Io. Io orbits within what is the called the "habitable edge" around Jupiter, the inner limit to how close a moon can orbit without suffering too much tidal heating to be habitable.

So Adii-1 should be far within the Habitable edge of Jupiter and should be far too hot for life, even if it didn't receive over 2 times as much energy from its star as Earth gets from the Sun.

I may have more to write on the subject later.

  • $\begingroup$ I will have a look at changing the atmospheric contents and maybe tweak the gravity parameters when I get the chance to. As for the distance Adii-1 is from Adii, I’m hoping the eccentricity being 0 would stop volcanism and tidal heating from toasting Adii-1. As for the stellar influx, couldn’t an albedo of 0.54 prevent Adii-1 from getting too hot? $\endgroup$ Jun 7, 2023 at 8:35
  • $\begingroup$ Great answer. Would like to add that Adii-1 would lose hydrogen over time, like Venus. And what do we know about the magnetism and accompanying radiation belt of the super Jupiter? Would Adii-1 not be blasted with radiation, further contributing to its hydrogen deficiency? $\endgroup$ Jun 7, 2023 at 11:55
  • $\begingroup$ @MennovanLavieren both Adii-1 and Adii (the super Jupiter) have magnetic fields. I may add more detail to the internal composition later if needed. $\endgroup$ Jun 7, 2023 at 13:12

One problem I see is that you have a very high atmospheric pressure on a very small, low-gravity world with a lot of water but very little water vapor in the atmosphere.

Worlds with low gravity and high atmospheric pressure usually have a lot of toxic heavy elements in the air (like sulphur) or molecules like equally toxic Methane. The lighter the elements in your atmosphere, the more your atmosphere will depend on gravity to keep it near the surface.

Which brings me to water vapor. I could be wrong, but 40% is a substantial ocean and yet you have low gravity and low water vapor in the atmosphere despite a very high average temperature (Earth's average temperature is only 15°C). I'd guess your atmospheric water vapor would be a very uncomfortable 20% or more. But, that would improve the chances of a high-pressure atmosphere.

Out of curiosity, what's causing the heat? Your star is colder than Sol. Gravitational sheer from the gas giant, like Io? That would lead to substantial volcanism, which also contributes to your thick atmosphere, but it's also hard to breathe.


I think some of your paramters are a bit off balance. If I try to balance them (as I've described), I think the world would be uninhabitable. I could be wrong. Let's see what others on the Stack have to say.

Note: Let me share a quick story. When I was a teenager, I spent weeks reading a book about the physics of rocketry. I used the mathematics to build my own rocket! Everyone was proud! Proud parents. Proud science teachers. My peers were less impressed, but that's teenagers for you. Anyway, the day came when I launched the rocket. It failed miserably. The point of my story? It wasn't enough to use the mathematical tools I found in the book, I needed to actually understand how they worked together. You're kinda in the same boat. You've learned a lot about the "what" you need to have to create a habitable world, but you're only beginning to gain experience with the "why" and "how" those "whats" work together. No worries! That's why we're here. As you work through this, keep in mind that many of those paramters depend heavily on others. Like air pressure depending on either gravity or heavy elements and/or molecules. Keep track of things like that and your proverbial rocket will fly straight and true. Cheers.

  • $\begingroup$ Heavy and toxic gases are not required for a world with a low surface gravity to have a dense atmosphere. Titan has a surface gravity of only 0.138 g, and an atmosphere of ordinary nitrogen, but it has a a higher surface pressure than that of Earth., because Titan simply produced more atmosphere than Earth did. $\endgroup$ Jun 7, 2023 at 7:52
  • $\begingroup$ The heat is mostly caused by the Ahknaral because Adii and Adii-1 are 0.41 AU away from it, which is normally too close but due to Adii-1’s albedo being 0.54 and the land cover being higher I am hoping for the temperature on Adii-1 to be on average only 42°c. $\endgroup$ Jun 7, 2023 at 8:32
  • $\begingroup$ By the way, very interesting story! That does kind of relate to my situation a bit although I made this more as a casual intellectual hobby. Thank you for the motivation! $\endgroup$ Jun 7, 2023 at 9:20
  • $\begingroup$ @M.A.Golding Titan is a lot colder, almost cryogenic. Very cold bodies can easily hold on to lighter elements, unless they are bombarded with radiation. Adii's properties are more like Venus in this regard. $\endgroup$ Jun 7, 2023 at 11:50
  • $\begingroup$ @JBH I see that OP added water vapor to his question's initial data, maybe you should review your answer. 4-6% of atmosphere in water vapor - I even feel like it's too much for 4.76 atm and 42C mean temp, and I think that the water would get blown out of this planet by solar radiation, together with nitrogen, argon and oxygen too, because of too low gravity vs base temperature. Dense atmosphere with high temp is possible (Venus) but low mass atoms require higher planet gravity to persist in the atmosphere. $\endgroup$
    – Vesper
    Jun 7, 2023 at 13:55

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