My Oceanic Super-Earth orbits a K3 Star at 0.5 AU inside the Habitable Zone, the planet is composed of ≈30% Water 51.5% silicate shell, and 18.5% iron Core. That resulted in a deep global ocean.

The planet mass is 4.2 Earth with a radius of 1.7 Earth, ann and atmosphere composite of 93% H2O vapor,4% N2,2.7% CO2, and 0.3% other gases with 1.2 bar pressure (Changeable).

In my world, this is a planet to colonize, so I don't expect life. From Space Engine I got ≈20°C as average temperature, and a greenhouse effect of ≈40°C, so the planet temperature without an atmosphere is ≈ -21°C (I verified it to be sure and I got 252.0164 Kelvin).

Is the atmosphere plausible? With a high percentage of greenhouse gasses what would happen to the temperature? Would it go runaway greenhouse effect? And if the atmosphere isn't plausible, which gases I can use with Steam(H2O Vapor) Atmosphere. And what is the best pressure for such an atmosphere?

  • $\begingroup$ A giant star of very high absolute magnitude and your planet orbits at half the distance of Earth from the sun - and you expect it to be habitable? What am I missing here? $\endgroup$ Jan 27, 2021 at 15:19
  • $\begingroup$ @Tantalus'touch.en.wikipedia.org/wiki/K-type_main-sequence_star → K3 would be approx 0.7x the current mass of the Sun, so it's a smaller star than ours, which makes a Habitable Zone of 0.5AU likely (en.wikipedia.org/wiki/… says 0.1 AU - 0.4 AU and 0.3 AU - 1.3 AU) $\endgroup$
    – Raisus
    Jan 27, 2021 at 15:43
  • $\begingroup$ I think we're looking at the same diagram, just reading it differently. The absolute magnitude is greater than the sun's and the size is bigger. Are you sure you don't mean a type 5 dwarf? $\endgroup$ Jan 27, 2021 at 18:23
  • $\begingroup$ Mass is 0.72, radius ≈ 0.77, and luminosity ≈ 0.268. all the values are compared to the sun. $\endgroup$
    – Khalid
    Jan 27, 2021 at 18:52

1 Answer 1


I've done a few calculations based on your planets criteria.

The surface gravity will be 14.24 $m/s^2$, which is 1.45 $g$ and the escape velocity will be 17.573 $km/s$, compared to 11.18 $km/s$.

Also, based on the chart of Atmospheric Escape, atmospheric water vapor would be retained by your planet.

In terms of greenhouse gas potential, water vapor retains more heat than carbon dioxide.

enter image description here

Two other observations about your plant's atmosphere with water vapor accounting for 93 percent of the atmosphere it will have 100 percent humidity, which is very unpleasant. With no atmospheric oxygen any colonists will need to wear atmospheric suits when outside their accommodation.

With a bit of research, because of the small amount of carbon dioxide, there will be a small amount of oxygen due to the photolysis of carbon dioxide and water vapor by ultraviolet light. However this will not be enough for people to breathe and survive.

  • $\begingroup$ Thank you, but will they need to wear a full suit? or just an oxygen mask? $\endgroup$
    – Khalid
    Jan 27, 2021 at 17:41
  • $\begingroup$ Most likely, but sweating & hydration might be issues, particularly when gravity is 1.45g as the colonist will initially have to expend more energy when they have to counteract gravity, such as walking, jumping, pulling themselves up & over rocks and lifting things. $\endgroup$
    – user81881
    Jan 27, 2021 at 17:57

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .