Im working on a world building Project that is based on a moon surrounding a gas giant. The moon in question is referred to in the simulation as "Melorn" and marked in yellow. There are other moons orbiting the planet which will cause gravitational Forces afflicting said moon. Im assuming that this will provide much needed heat fueling the formation of live since the moon and its central body are located outside the star's habitable zone. Also it will heat its core and cause a magnetic field to be maintained which protects the moons atmosphere.

The Specifics of the System are as follows:

Mass of the Star: 3 Solar Masses

Mass of the Host Planet: 11 Jupiter Masses

Mass of the Moon Melorn: 0.6 Earth Masses

Moon Radius: 5507 Km

Surface Gravity: 7.89 m/s^2

Semi Major Axis of the Host Planet: 7 AU Semi Major Axis of the Moon related towards the Host Planet: 900000 km

Day Length on Melorns Surface: 39.6 Hours.

Neighbouring Moons possess 0.6 and 0.8 Earth Masses and approach Melorn 300000km and 800000 km respectively

The Moon is of course tidally locked towards its host Planet. He orbits the Planet in a nearly circular orbit.

My Question: How exactly should the Atmosphere of Melorn be composed in order to make the surface of the moon as suitable for life as possible?

There should be a similar amount of liquid water on the surface as there is on earth, but a part of it also exists in the form of polar caps or mountain glaciers.

Simulation_Screenshot from Universe Sandbox.

  • $\begingroup$ Unfortunately, nobody has even the faintest idea what atmosphere is necessary (underline necessary) for life as we know it. For example, we know that the composition of the atmosphere of Earth 3.5 billion years ago when life as we know it first appeared was very different from what he have today; but nobody knows exactly (or even roughly) what it was. All we know is that there must have been no oxygen, quite a lot of nitrogen, and at least 20 times as much carbon dioxide as today. Maybe some methane and ammonia too. $\endgroup$
    – AlexP
    Commented Apr 20 at 18:12
  • $\begingroup$ Im assuming we need greenhouse gasses such as Carbon Dioxide or Methane in order to trap solar energy in the world's Atmosphere. $\endgroup$ Commented Apr 20 at 18:15
  • $\begingroup$ Yes, I get it, but you may want to edit the question to ask about something definite, such as for example, to have liquid water on the surface. Because, I repeat, nobody knows what atmosphere Earth had when life first appeared. $\endgroup$
    – AlexP
    Commented Apr 20 at 18:17
  • $\begingroup$ Yes, im looking for a condition that includes a similar amount of liquid water at the surface compared to earth. I will edit it accordingly. $\endgroup$ Commented Apr 20 at 18:20
  • 1
    $\begingroup$ Even so, we still don't know. For example, animals want lots of oxygen and as little carbon dioxide as possible. Plants would be very much happier with half the amount of oxygen we currently have and five or more times as much carbon dioxide. Our current atmosphere is only a snapshot, an instant in the geological evolution, and we know for sure that the amount of oxygen and carbon dioxide have varied a lot in various geological ages. $\endgroup$
    – AlexP
    Commented Apr 20 at 18:33

1 Answer 1


Your stellar system, as described, probably isn't going to be habitable by terrestrial-flavored surface-dwelling life, regardless of the gas mix of the worlds involved.

The problem comes with your choice of star. At 3 solar masses, it is going to be a lot hotter and brighter than the Sun... a real-life example might be Gamma Ursae Majoris (aka Phecda). It has 65 times the luminosity of the Sun, and that means its lifetime is going to be significantly shorter... of the order of 4-5 billion years, which doesn't necessarily bode well for the evolution of life. More importantly, with a surface temperature of 9300 K, there's going to be a lot of short wavelength radiation... the black body spectrum peaks at 300 nm, so nearly 40% of the emitted light is ultraviolet, of which nearly 5% is strongly ionizing vacuum UV. That's very bad news for atmosphere retention... warm, low gravity worlds will find their upper atmospheres subject to photodissociation, and monatomic hydrogen and oxygen radicals will shoot off into space leaving a dry, unbreathable world behind.

cause a magnetic field to be maintained which protects the moons atmosphere.

Magnetic fields can help limit atmospheric stripping due to solar wind effects, which is important for Earthlike worlds around Sunlike stars. But they cannot protect against ionization and Jeans escape, which is important for worlds around Phecda-like stars.

the moon and its central body are located outside the star's habitable zone.

The notion of habitability is fuzzy when that much UV is involved, but from a point of view of received energy I think your world is just inside the inner edge of the habitable zone. Consider that it is orbiting 7 times further out than Earth, but the star involved is more than 72 times more luminous. But for the whole giant-dessicating-germicidal-UV-lamp thing, it would probably be quite warm. You'd want a minimally greenhousy atmosphere to avoid getting cooked, which means you want to keep CO2 and water vapor down. A thin atmosphere might help, but it then exposes the surface to more UV. I don't think there's a winning combination for you here.

I think you need to shrink your parent star right down. Even 2 solar masses is hazardous (Sirius-A is still 25 times more luminous than the Sun) so you'd be much better off limiting yourself to G or K-class stars.

  • $\begingroup$ Thank you for your reply. Sure i could shrink the star. What about an F type star? They are more massive than the sun, but still considered Main Sequence stars. Sirius is an AM star. I'm thinking of maybe 1.5 to 3 times the luminosity of the sun However, I'm opting for a system that's based on the following premise: Bigger star, but this is mitigated by a greater distance and thus less UV-Radiation and heat. Are there any measures i can take in order to reduce the radiation that's incoming and get rid of the jeans escape? I'm also open for scenarios that include artificial measures. $\endgroup$ Commented Apr 21 at 7:57
  • $\begingroup$ Edit: Sorry i failed to find out how to add line breaks $\endgroup$ Commented Apr 21 at 7:59
  • $\begingroup$ @Zadai.Fehbiab no linebreaks in comments, unfortunately. Remember that a physically bigger star will appear smaller in the sky, because the luminosity of a star scales more quickly than its radius! You can build a sunshade and stick it in a lagrange point for a planet, but that's awkward for a moon unless you build an enormous sunshade. For bigger, hotter stars the problem is the proportion of UV... moving further away reduces the total insolation, but not the proportion of ionizing radiation. $\endgroup$ Commented Apr 21 at 10:52
  • $\begingroup$ @Zadai.Fehbiab also, Sirius-A is a main sequence star. Its a binary, sure, but Sirius-A by itself is a perfectly normal hazardously hot and bright star. A cooler F-class star might suit your needs, but so would a slightly larger and brighter G-class. $\endgroup$ Commented Apr 21 at 10:59
  • $\begingroup$ Okay, but could there be an extra thick ozone layer or a similar phenomenon that stops some of the ultra short wave radiation coming from let's say a Sirius like star? $\endgroup$ Commented Apr 21 at 14:16

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