# Background

This planet, located somewhere in the Andromeda galaxy, orbits a M5V star about 0.682764 AU away from its star. The planet is volcanically active due to the gravitational field of the star, kind of like the deal with Io and Jupiter. The planet also has liquid water at the poles in underground pools and frozen and liquid overground pools, where the life is supposed to evolve on the planet. At the equator, there is also a valley that wraps around the entire planet, which forms from chemical weathering by acidic rain. The idea behind the acid rain is that the frequent volcanic eruptions create the sulfur, which combines with oxygen, creating sulfur dioxide, this then rises up into the upper atmosphere, which reacts with other chemicals that then precipitate down as acid rain.

# Question

One of my questions is would water be able to form underground on this planet? Considering the distance to the star and would the volcanic activity make sense, considering an M5V type star is pretty cool and might not have the right composition to form a gravitational field of such magnitude that the planet becomes volcanically active. And my final question, are the chemical processes described accurate? I'm pretty fresh to this whole "worldbuilding" thing (and chemistry, to a fair extent)

• "M5V star about 0.682764 AU away" vs "Io and Jupiter" - the former setup won't affect planet's interior nearly as strong as the latter. Mar 18 '20 at 4:15
• What do you mean by "erosion by acidic rain"? Where does the removed material go to? Mar 18 '20 at 4:25
• @Gimelist all the Hollywood documentaries teach us the superacids will dissolve everything into nothing and a whiff of smoke, letting a jagged hole behind. Mar 18 '20 at 9:58
• Not an answer - but the volcanism piece is totally plausible. If you want your little star, just have the plate tectonics mechanism for volcanic activity like on earth. You don't really need the tidal Io style mechanism. Mar 18 '20 at 13:37

Unpossible.

orbits a M5V star about 0.682764 AU ... The planet is volcanically active due to the gravitational field of the star

Typical characteristics of M dwarfs put the mass of a M5V star at 14% of M.

With a distance of 0.68AU, your planet will be gravitationally less attracted by its star than Earth is attracted by the Sun - more precisely, the intensity of the gravitational field of that star will be about 20% of the gravitational intensity the Earth experiences from the Sun.

Since there's no tidal volcanism on the Earth caused by the Sun, the chances of volcanism on your planet caused by the tides are infinitesimal. The planet should have a humongous size for the gradient in the gravitational intensity between the closest point to the star and its farthest one to create the mantle/crust displacements so large that the tides will cause significant volcanism.

If you look into that 'red dwarf characteristics' table, you'll see the largest M-class red dwarf is 60% of solar mass. With a 0.68 AU, the attraction from its star will be slightly lower than Sun's attraction on Earth. Still no dice.

Red dwarves are explosives and will wipe any atmosphere in their near planets. Far planets will need be a lot massive to sustain one atmosphere. They also could have a non tidal locked rotation and, maybe, a magnetic shield. Gravity of star will not cause significative reactions in planet mantle, need other forces in action.

If planet is big enough, lets say, Earth-like or more, it's geological active and all those reactions will happen. The activity can explain the equatorial valley and liquid water near the poles.

Your planet looks like a Venus that toke less solar radiation (Proxima Centauri, the most common red dwarf, emit 0,17% of Sun radiation)

• Since the current given gravitational interactions aren't enough to generate such volcanic activity, could there be a nearby gas giant that could generate this effect? And how far away could said gas giant have to be to produce this effect. Could I possibly change characteristics of the star to make this feasible (such as spectral type and mass). Another idea is that there could be a moon massive enough to generate this effect, though I'd presume that moon would be massive in relation to its primary. Or could I have multiple gravitational forces acting on the object to make the effects stack? Mar 18 '20 at 4:30
• @Covision, a giant moon of a gas giant, like Titan, would make sense. Planets stabilish their orbits after a razoable distance make those reactions no happen, unless orbit is unstable and then life will have low chances. You no need gravitational forces interacting with planet interior to make it active too, unless this play a role in your work. Mar 18 '20 at 4:45
• Red dwarves are explosives most of them, yes, but we don't have enough knowledge to say this is an inviolable law Whether this is a peculiarity of the star under examination or a feature of the entire class remains to be determined.` Mar 18 '20 at 9:35
• @AdrianColomitchi indeed, a few ones arent. :) or we dont have enough measures of them Mar 18 '20 at 12:49

While your chemistry sounds plausible, there's a couple of major problems with the tidal heating side of it, both relating to how you've set the star system as a whole up.

Firstly, the tidal heating. Most of the tidal heating of Jupiter's moons doesn't actually come from Jupiter's gravity itself. Tidal heating is caused by an object being stretched and relaxed by a varying gravitational pull, but Jupiter is always at pretty much the same distance from the moons and the same side of the moons always face Jupiter*, so the pull and resulting distortion is almost constant meaning that (relatively) little heat is generated this way.

So how do we get tidal heating? The great moons of Jupiter have one method - each other. While their gravitational pull on each other is obviously vastly weaker than that of Jupiter itself, it comes from various directions as the moons pass each other out in their orbits, causing the moon's shape to be pulled around just slightly, but in a varying way, generating heat.

Another method is if your planet's orbit is not circular - when it's closer to the parent body it gets stretched more, and when it's further out less. Again, the change in shape generates internal heating. Finally, the planet's rotation can help - even if the size of the bulge created by the parent's gravity is constant, that bulge will "move" relative to the rotating surface of the planet so long as the planet's revolution time is anything other than one full orbit.

The thing is I don't see any of them working for the setup you described. For the case of another orbiting body affecting it, it would have to be close - Earth is nowhere near getting cooked by Venus or Mars. At the distances in our solar system, even if our closest passing planet (Venus) was as massive as Jupiter, the effect on us would still not be much stronger than the tides caused by our own moon (though we'd have a hard time hanging onto said moon).

If you want to have planets very close to each other in your solar system, that changes things. Gravity falls off with the square of distance, so ten times closer gets you a hundred times more pull. We have seen planetary systems with very close planets (such as the famous Trappist-1 system) so it is definitely possible, but in all of those cases these systems' planets are very close to the star. You can have tidal interactions from other planets, but you'll need to get much closer to your star.

If you want to heat your planet using tidal interactions from your planet's rotation and the star's gravity, you'll also need to get much closer. Put it this way - Earth spins. Earth orbits about 1.5 times from its star as your planet does, but Earth's Sun is something like 8 times as massive as the kind of star you are talking about. Earth gets to keep spinning without appreciable tidal heating, and so will your planet.

Finally, the wonky orbit option. If your planets orbit is very eccentric (meaning that its distance from the star varies a lot) it will gain plenty of tidal heating. You will still need to get closer to the star at some point, but the planet will spend most of its time farther out (which might be a good thing, considering that many M - dwarfs are in the habit of spewing out massive stellar flares and coronal mass ejections).

TL,DR: YES you can have a planet heated by tidal interactions from one source or another. NO, you can't do in anything like the orbit you described. Either be close in and have there be other close in planets, or be close enough that the star starts to rob energy from your rotation (will eventually stop the planet rotating) or have an orbit that alternates between near and far.

One more alternative - if your real interest here is in having a lot of volcanic activity on a planet with a cold surface, consider upping the radioactives. Over 90% of Earth's internal heat comes from the decay of radioactive material, and depending on your planet's age and the nebula it formed from, other planets could have much more.

*I'm talking specifically about the large moons here