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I'm no scientist and my poor brain box is on its last legs; I was wondering if any of you smart beans out there would be able to help me figure this out? Here are the specs of my system;

Kereiol is a 2 billion year old quiescent carbon-rich M star with a temperature of 3,100 kelvins. If it flares at all they are tiny. It has four orbiting planets, only one of which, Liskuel, is within the goldilocks zone. Liskuel is 0.15 AU from Kereiol and has an orbital period of 58 days.

Liskuel is a tidally locked (0% obliquity) wet, rocky carbon-born planet with an active core. The surface is 70% deep water and 30% dry land. It is a similar size and mass to earth, with a strong magnetosphere and thick atmosphere. Liskuel is tidally locked in a synchronous rotation with its sun.

One of the biggest obstacles to life evolving on a red dwarf planet is the short distance at which the planet must orbit its sun in order to remain in the goldilocks zone. This close orbit almost always leads to tidal locking, which creates extremes of temperature at either pole. A strong atmosphere would be needed to regulate this and render the planet habitable.

Unfortunately, even the older Red Dwarves are prone to volatile solar activity such as flares, which produce torrents of charged particles capable of stripping off sizable portions of the planet's atmosphere, as well as starspots which dim the stars emitted light by up to 40% for months at a time.

In addition to providing a barrier against solar activity, increased atmospheric pressure would also decrease the thermal contrast between the polesby virtue of retaining more gases, meaning smaller forcing and lower wind speed. (which would be great.)

In summation, temperature regulation is an issue, one that would be helped by a dense atmosphere. But, as I have learned, just increasing the size of the planet wouldn't help, sooo....

My question is, is it possible to have a planet with earth-like mass and size but double the atmospheric pressure? If so, how? (Without reference to terraforming or advanced technology).

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    $\begingroup$ Atmospheric pressure depends directly on how much atmosphere there is. Earth has some $5\times10^{18}$ kg of atmosphere; if it had double the amount then pressure would be double. $\endgroup$
    – AlexP
    Commented Jul 11, 2019 at 12:58
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    $\begingroup$ Have you considered using heavier gases in your atmosphere, like chlorine, sulfur, sulfur dioxide, ozone, maybe even some rare gases like xenon? $\endgroup$
    – aadv
    Commented Jul 11, 2019 at 15:34
  • $\begingroup$ Thanks guys! That's fantastic to know. @aadv that's exactly what I was looking for, thank you! Would you recommend any particular one? I know it'll have a big effect on the world so I don't want to just pick one at random. $\endgroup$
    – MissMermaid
    Commented Jul 11, 2019 at 22:42
  • $\begingroup$ Some bacteria use light energy to produce chemical energy like plants, but instead of taking in carbon dioxide and water and putting out free oxygen they like to play around with sulfur compounds; they could be the primary producers in an alien ecology. I'm reminded that sulfur is usually a solid, so animals couldn't reduce sulfur for energy like some bacteria do - maybe some other sulfur compound? Noble gases could be used if you want less weirdness. Life doesn't use them much (or at all). Light debris would float above ground if there was enough xenon which would be funny. $\endgroup$
    – aadv
    Commented Jul 12, 2019 at 13:23

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Trivial real world case: Venus is about 98% of Earth's mass, and has roundly 100 times the Earth's atmosphere. This extreme atmosphere is mostly carbon dioxide, with the famous clouds composed mainly of sulfuric acid droplets.

Interestingly, at altitudes just above the cloud tops, the environment is the most Earthlike to be found anywhere in the solar system off Earth -- pressure and temperature are very similar to Earth at sea level, though there's no free oxygen -- but it's probably the only place in the system where a human could survive with just a breathing mask and air bottle.

The mechanism that gave Venus such a deep, massive atmosphere was probably (unless planetologists have changed their minds again) thermal outgassing of surface rocks as the runaway greenhouse took over a couple billion years ago.

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