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I am making a non-habitable planet (Name: Xoo-akedar) for my solar system. Despite being close to the star (see characteristics below), it has a dense atmosphere that prevents the heat from the star from reaching it. I need the planet to be atmospherically dense but very cold.

The planet is larger than the Earth and has more atmospheres, so I believe there are not as many storms, but I don't know if that's reasonable. What would be the characteristics of this planet's atmosphere?

Stellar characteristics:

  • Star type: G0-1V
  • Mass: 2.09×1030 Kg (1.05 Msol)
  • Radius: 716,571 Km (1.03 Rsol)
  • Density: $1.33 \fac{kg}{m^3} (0.96Dsol)
  • Luminosity: 4.71×1016 (1.23 Lsol)
  • Temperature: 5599 K (5326 °C)
  • Habitable zone: 1.58×108–2.82×108 Km (1.06–1.52 AU)

Planetary characteristics:

  • Cultural name: Xoo-akedar
  • UCSN (Universal common scientific name): P(Wa)-PLR-173°.23.°.43° (III)
  • Mass: 9.73×1023 Kg (1.67 MT)
  • Density: $4.25 \frac{g}{cm^3}$ (0.77 DT)
  • Radius: 7454 Km (1.17 RT)
  • Atmospheric pressure: 89.4 atm
  • Atmospheric density: $705.4 \frac{Kg}{m^3}$
  • Nucleus fraction: 24%
  • Gravity: 1.22 g
  • Escape velocity: $13.32 \frac{Km}{s}$ (1.22 VT)
  • Axial inclination: 53.7°
  • Tropics: 0-53.7°
  • Polar circles: 62.6°-90°
  • Albedo: 0.99
  • Greenhouse effect: 0.1
  • Observer height: 1.72 m
  • Horizon distance: 5.02 km
  • Semi major axis (distance from star): 3.33×108 Km (2.22 AU)
  • Orbital eccentricity: 0.041
  • Perihelion: 3.2×108 Km (2.13 AU)
  • Aphelion: 3.47×108 Km (2.31 AU)
  • Axis: 8°
  • Orbit period: 498 days (16 months)
  • Rotation period: 29 hours (1 day)
  • Terrestrial composition: Fe 53%, Ar 32%, Cl 13%, RE (Remaining elements) 2%
  • Atmospheric composition: N 64%, O 27%, H 8%, ER 1%
  • Surface temperature: -208°C (68 K)
  • Does it have a natural satellite?: Yes

Xoo-akedar satellite characteristics

  • Cultural name: Utfar
  • UCSN: P(Wa)-sPR [III]-173°.23°.43°
  • Mass: 9.32×1022 Kg (1.27 ML)
  • Density: $3.51 \frac{g}{cm^3}$
  • Radius: 2031 Km (1.17 RL)
  • Gravity: 0.204 g
  • Exhaust velocity: $2.86 \frac{km}{s}$ (1.2 VL)
  • Albedo: 0.235
  • Semi major axis: 4.086×105 Km (1.06 AUL)
  • Orbital eccentricity: 0.02
  • Perihelion: 4.004×105 Km (1.04 AUL)
  • Aphelion: 4.17×106 Km (1.02 AUL)
  • Orbital period: 24 days
  • Rotating period: 24 days
  • Maximum high tide: 0.922 m
  • Minimum high tide: 0.866 m
  • Terrestrial compound: O 48%, Si 37%, Mg 12%, ER 4%
  • Does it have life?: No
  • Tidally locked?: No
  • Is it round?: Yes
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Not Possible

At this solar distance, star luminosity, and atmospheric composition, the planet should be hot, not cold. While you got your Mass/Solar Distance/Bond Albedo/Greenhouse calculations technically correct, your Bond Albedo and Greenhouse effect you've listed are WAY off based on what your planet is made out of. With that atmosphere, the Greenhouse Effect should be about 30, not 0.1. As for your Bond Albedo, no planet or exoplanet has ever been measured with higher albedo than 0.8. Given your planet's composition though, I would expect and albedo closer to 0.3. Therefore, your planet's average surface temperature should be closer to 82°C, not -208°C.

The only way to get it that cold without changing the planet itself is to move it farther away from the sun.

If you move the planet out to a Solar distance of ~65AU. Then a planet with this composition would average about -208°C ... but at that point it would be even farther away from the star than Pluto; so, not really an ideal solution.

To get it closer, we need to look at the other physical properties of the planet.

How to get a 0.1 Greenhouse Effect

Earth's atmosphere is very close to optimal for keeping a planet cool. Most gasses are greenhouse gasses like CO2, H2O, etc. which absorb and are heated up by infrared light. There is no gas that negatively effects the planet's solar heat absorption; so, the best gases for keeping a planet cool are to use the ones that do not absorb IR light at all. These are your diatomic gases like H2, N2, and O2. Earth is 97% diatomic air, but your planet has 99% diatomic air; which is very high, but still perhaps believable. So, that means that your planet can have 3x as much air as Earth to achieve the same greenhouse gas factor. So, if you make the atmospheric pressure about 1/3 that of Earth's with its listed composition, you could achieve your Greenhouse effect of 0.1. This would drop your average surface temperature to -88°C.

Maximizing your Albedo

If your planet had liquid water, we could cover your planet in a light snow getting your albedo up to about 0.8, but since it does not, we need to settle on something else. Sand typically has a higher albedo than solid rocks, and the highest albedo type of sand commonly found on Earth is Silicon Dioxide which will give you an albedo of 0.48. Some other kinds of sand like Titanium Dioxide and Zinc Oxide are more white because they reflect more light in the visible spectrum, but they also absorb more light outside of the visible spectrum; so, I can't find any alternate kinds of sand to increase your albedo higher than Silicon Dioxide. That said, no planet is completely homogeneous; so, I would also not expect total Silicon Dioxide coverage either. 0.45 is probably the most you could expect. which combine with the thinner atmosphere would give an average surface temp of -99°C.

If you give your planet a bit of water, the the high temperature variance from day to night caused by your thinner atmosphere will cause frequent snow storms giving you a constant source of fresh powdery snow which could increase your albedo to 0.8 further reducing your average surface temp to -138°C.

Planetary Rings May also help

While an atmosphere hugging the ground will keep a planet warm, something high above the planet that blocks the sun can help keep it cool. If your planet has some kind of a ring system, then those rings could help block the sun and redirect most of its energy back out to space... especially near the equator where most of your heat normally comes from. There is no hard science rules for measuring ring shadows, but if we assume it blocks 1/3 of the total daylight, then your plant's average temp will further drop to -152°C. This is already really freaking cold, so, maybe this is a good place to stop, but if -208°C is an important target for plot or other world building reasons, you'll still need to move it out to 7.6AU (somewhere between Jupiter and Saturn's orbital distance). It's not exactly close to the sun, but it is WAY closer than with your original specs.

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  • $\begingroup$ What about sulfur oxides? I've read they reflect the light away. $\endgroup$
    – alamar
    Commented Apr 17 at 15:01
  • $\begingroup$ Thanks for answering yes I know that a planet near the sun is not the best "idea" but I wanted to make a cold planet near the sun (Not that close but it is in the area that would be known as the habitable zone) since I wanted it to be very different with other planets like Mars and Venus (especially Venus, which is what I based myself on along with the snowball earth hypothesis) $\endgroup$
    – Idon'tknow
    Commented Apr 17 at 17:18
  • $\begingroup$ Although well that's why I took those gases into account to do it, although the issue of greenhouse gases greenhouse, I thought that unlike the earth it does not have as much volcanic activity and it does not manage to produce other greenhouse gases or not enough to maintain them $\endgroup$
    – Idon'tknow
    Commented Apr 17 at 17:18
  • $\begingroup$ @alamar They don't actually reflect the light away, they seed rain clouds which removes water from the air. Less water means a weaker greenhouse effect, but anything you can do to put less H2O into the atmosphere to begin with will have a stronger cooling effect. $\endgroup$
    – Nosajimiki
    Commented Apr 17 at 17:47
  • $\begingroup$ @Idon'tknow The big problem is not how many volcanoes you have, it's the saturation point of the air. If you have 90 times as much atmosphere, then it will take 90 times as much water in the air to trigger precipitation, and so you will get 90x as strong of a Greenhouse effect. So, even if the sources of water vapor are few and far between, it will still slowly accumulate until it reaches saturation. $\endgroup$
    – Nosajimiki
    Commented Apr 17 at 17:56

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