[Space] Exotic star system features, and their storytelling role

[Anthony]

Originally Posted by lwcamp

Another is to have (initially) very few non-condensable gases in the air (so very little nitrogen, argon, oxyen, etc.), but to have surface water.

Incidentally, given what we've seen of planet formation, it's probably possible for an outer system object to migrate into the inner system and either collide with something or just orbit, resulting in something with enough water to produce oceans dozens of miles deep (at 1G and 15C, looks like you get Ice-VI at around 50 miles depth, which puts a limit on ocean depth). Probably have a lot of ammonia though.

[AlexanderHowl]

You can have liquid water on the surface of rogue planets as long as you have an hydrogen atmosphere at 1,000 atm. The insulative quality of the hydrogen atmosphere would trap the heat generated by the hydrogen rain and radioactive decay. Of course, the environment would be quite exotic, but you could easily have organic life in the oceans (any oxygen that escaped would quickly combine with the hydrogen in the atmosphere to become water).

[Flyndaran]

Miles of ice could protect hydrothermal vents for hundreds of millions to even billions of years, I believe. So life on rogue planets isn't even that unlikely given a vaguely Earth or Mega-Earth size.
Ironically, they could last longer than many orbiting main sequence stars due to that pesky red giant phase.

[lwcamp]

Originally Posted by Anthony

Incidentally, given what we've seen of planet formation, it's probably possible for an outer system object to migrate into the inner system and either collide with something or just orbit, resulting in something with enough water to produce oceans dozens of miles deep (at 1G and 15C, looks like you get Ice-VI at around 50 miles depth, which puts a limit on ocean depth). Probably have a lot of ammonia though.

It seems that ammonia rapidly (on geological scales, at least) undergoes photolysis when exposed to sunlight and is removed from the atmosphere. This would give you a nitrogen atmosphere (assuming the hydrogen escapes).

Luke

[RyanW]

Just to rain on everyone's parade: Newer research suggests that stars under 0.3 solar masses are extremely unlikely to support life. Even though models suggest atmospheric and oceanic circulation could keep the temperature stable enough despite tide locking, the tidal heating while orbits settle during formation would probably boil off most of the volatiles of any planet in the habitable zone. So most likely no oceans under all those tiny red dwarfs.

[Anthony]

Originally Posted by AlexanderHowl

You can have liquid water on the surface of rogue planets as long as you have an hydrogen atmosphere at 1,000 atm.

Don't need that. Just need enough heat to raise temperature to above 0C and enough gravity so water vapor doesn't escape, once the partial pressure of water vapor above the ocean exceeds the vapor pressure of water at 0C the ocean won't boil or freeze. You'll get gradual loss of water (caused by photodissociation of water resulting in O2+H2, where the H2 escapes) until you have enough shielding, probably formation of an ozone layer.

[Agemegos]

Originally Posted by RyanW

Just to rain on everyone's parade: Newer research suggests that stars under 0.3 solar masses are extremely unlikely to support life. Even though models suggest atmospheric and oceanic circulation could keep the temperature stable enough despite tide locking, the tidal heating while orbits settle during formation would probably boil off most of the volatiles of any planet in the habitable zone. So most likely no oceans under all those tiny red dwarfs.

The Hayashi Track argument?

[RyanW]

Originally Posted by Agemegos

The Hayashi Track argument?

More like, as the luminosity of a star - and thus the radius of orbits where planets could retain liquid water - decreases, the odds that such a planet will have at some point in tidal evolution lost all its volatiles during a sort of "super Io" period increase. When you're that close, tidal heating isn't completely overwhelmed by radiant heating like it is in a more Earth-like system.

There's a paper here.

[Agemegos]

Originally Posted by RyanW

More like, as the luminosity of a star - and thus the radius of orbits where planets could retain liquid water - decreases, the odds that such a planet will have at some point in tidal evolution lost all its volatiles during a sort of "super Io" period increase. When you're that close, tidal heating isn't completely overwhelmed by radiant heating like it is in a more Earth-like system.

There's a paper here.

Thanks. I hadn't seen that.

[AlexanderHowl]

I tend to be skeptical when people present arguments in astronomy why certain aspects of planets are impossible. I am old enough to remember the arguments for why Sol was the only star in the entire universe with planets. Since then, I have seen practically every 'impossible' type of planet that did not violate physics be discovered, and I am at the point right now where I would not be surprised if we discovered a planet made from ice cream. After all, it is not like the elemental components are uncommon in the Universe.