Celestia Forums

I want to know, for example, the boiling point of water and ammonia or other substance in an atmospheric pressure 6x that of Earth. And to do this there may have some formulae.

Another question is: Can a planet as hot as Earth, with a gravity of about 5m/s?? with magnetic field hold an, let's say, 10x Earth's atmospheric pressure? Note: This planet orbit a M dwarf star that has many times lower stellar wind.

For information on boiling points of substances such as water, you need to have the phase diagram for the substance. For water, see here, though there are probably better ones out there.

As for atmospheres, it really depends what the atmosphere is made up of. If you're going for an Earthlike (or Titanlike), nitrogen-dominated mix, ultraviolet radiation from the star drives the exosphere temperature up, which means it's easier for molecules to escape. On the other hand a Venuslike, carbon dioxide-dominated mix will tend to have low exosphere temperatures, making it easier for the planet to hold onto its atmosphere. (NB surface pressure of Venus is much higher than the values you're after).

Now you've stated your example planet is in orbit around an M-dwarf, so prepare for some bad news: in the habitable zone of a red dwarf, you get a lot of ultraviolet radiation, particularly from flares. In addition, planets there are expected to have weaker magnetic fields than Earth because they are tidally-locked and rotate slower. Astrobiology has just published a load of articles about habitable planets around red dwarfs: it is suggested that a habitable planet would be subject to significant atmospheric erosion if the ratio between carbon dioxide and nitrogen in the atmosphere is less than 96%. To make things worse, these calculations do not take into account sputtering by the red dwarf's solar wind.

Probably to retain a habitable environment around a red dwarf star, you need a planet quite a bit more massive than Earth, which would presumably have a stronger magnetic field, and also a smaller atmospheric scale height, which would help keep the atmosphere in.

See the two related articles titled "Coronal Mass Ejection (CME) Activity of Low Mass M Stars as An Important Factor for The Habitability of Terrestrial Exoplanets", available for free download (at the time of writing) here

I forget to say that the atmosphere composition is: N, CN, CH4 and NH3.
This planet is more massive, quite Neptune mass planet, and orbit a M dwarf star out the habitable zone. It's distant enough to the star so that the global temparature of surface is about -56??C, in which ammonia is liquid, even having greenhouse gases with some abundance.
This planet have low concentration of oxigen, so no O2 and very few H20 ice. It is mostly rocky. But it's a carbon planet, so it's mostly silicon carbide and although it's radius is 16737 km, it has a bit lower density than Mars, 3.75 g/cm??. It's lower density is due to it's carbon composition, even though it has a bit big core, the core also have lower density: is made of Fe, Ni, S and C. The planet is mostly carbon with nitrogen, much silicon, some sulphur, phosphorus, big metal content and in great amount.
Gravity is higher: 17.53 km/s??, and having stronger magnetic field, I think it's enough to hold dense air: 7 atm. It only does not have very thick air like Venus because it suffered many impacts in it's geological past with very big debris of space, so it lost most of it's original atmosphere, like what happened with Earth in the past.
And to finish, M stars may not have much hydrogen in it's dust disk of gas to form many gas giants, but may have enough heavy material to form many or at lest some rocky worlds. This M star i'm talking about is bigger: 50% Sun radius and old. Not old of the early Universe, but this red star will form in the far future, when heavier elements will be in greater amounts, along with carbon.

Neptune-mass planets located beyond the habitable zone shouldn't have any problem holding on to atmospheres.