Alternate Planets, Suns, Stars, and Solar Systems Thread

[Cydonius]

@Cydonius

So if Mars had the same mass and diameter as the Earth (roughly speaking), what would need to change about it's atmosphere to have liquid water and oceans on it's surface? If we assume that Mars has an atmosphere similar to that of Earth's, a magnetosphere and the same rotation period similar to that of Earth - would this Mars be livable by humans? I imagine that humans would see that this Mars is only slightly brighter than ours in the sky due to it being larger and a little closer, but this doesn't strike me as a change that would change much about human history until later on in the modern era for humans.

Sorry, I have no idea how to work that out. I can do the radius vs mass vs density vs surface gravity thanks to some equations I picked up from a Youtube channel called Artifexian, he's an Irish guy doing a long-running series on world-building etc that started from 'create your star' and is refining inwards from there. But those four properties are all related in a way that let me do a spreadsheet to just iterate one at a time and see what the result was. They don't depend on how far out from the star the planet is...

Maybe go have a hunt for resources on terraforming Mars? I bet someone has already worked out what you'd have to do to OTL Mars to get to that point, and if it would work on OTL Mars then it would definitely work on this ATL one.

As to the last point, it would still be dimmer than Venus as Venus has total cloud cover (i.e. it's brighter) and is also closer to the Sun. It wouldn't be visibly orange anymore, not with a hydrological cycle, lots more water and ice and plenty of clouds on top of that, but otherwise it's not going to have any impact on Earth until very recently in history. Unless life evolves there earlier than here and comes to pay a visit...

 

[CarnelianClout]

@Cydonius

With Mars's moons, with Phobos being the mass of Ceres, existing at one half of the Moon's distance, and Deimos also Ceres Mass, almost at the same distance as our moon from Earth - while those moons would be smaller, they might be enough to create some kind of limited tides on Mars. This Mars would likely be an excellent opportunity for humans to colonize, and humans would not need to wear pressure suits, either. Depending on the atmosphere, they might just need breathing masks and a thick coat.

 

[RaptorWolf]

In this hypothetical scenario, there is a binary solar system, in which both stars are G0 yellow dwarf stars, which are 105% as wide, 110% as massive and 126% as bright as our sun each. Using AbbydonX's mathematics, such a binary results in a habitable zone spanning from 1.43 to 2.7 astronomical units. There is only one Earthlike planet within that habitable zone, and that's because it orbits the binary elliptically, from 1.43 to 2.7 AUs.



Now, an elliptical orbit means that seasons are not of even length. So how would an elliptical of 1.27 AUs affect the seasons? Also, would the luminosity of sunlight differ between seasons in this specific detail of orbit?

 

[Dibwys]

In this hypothetical scenario, there is a binary solar system, in which both stars are G0 yellow dwarf stars, which are 105% as wide, 110% as massive and 126% as bright as our sun each. Using AbbydonX's mathematics, such a binary results in a habitable zone spanning from 1.43 to 2.7 astronomical units. There is only one Earthlike planet within that habitable zone, and that's because it orbits the binary elliptically, from 1.43 to 2.7 AUs.
Now, an elliptical orbit means that seasons are not of even length. So how would an elliptical of 1.27 AUs affect the seasons? Also, would the luminosity of sunlight differ between seasons in this specific detail of orbit?

With two G0 stars I expect that the habitable zone would be a bit larger than you mentioned. However. A wide habitable zone in which a planet goes from near the inner edge to near the outer edge is going to have considerable seasonal differences due to the differences in illumination over the course of the year. If the planet has a significant axial tilt one hemisphere will have short warm winters and long cool summers, and the other hemisphere will tend to have short hot summers and long cold winters (and in both cases spring and fall are also stretched out). This generalization will be strongly modified by the distribution of the continents, and probably by other factors. Also, if the binary is - from the point of view of the planet - an eclipsing binary the illumination the planet receives will drop by almost half for much of the duration of the eclipse - probably something like 'an unusually dim/cool few hours' every (few days?). [Note, this effect affects the part of the planet that is illuminated at the time of the eclipse, and only for a few hours.]

 

[RaptorWolf]

If the planet has a significant axial tilt

Would 19.7 to 26.9 degrees over a 122,000-year cycle qualify as "significant"?

This generalization will be strongly modified by the distribution of the continents, and probably by other factors.

How so?

 

[Dibwys]

Would 19.7 to 26.9 degrees over a 122,000-year cycle qualify as "significant"?


How so?

Continents heat up and cool down differently from oceans, they block/redirect the flow of ocean current that redistribute heat, and mountains modify air flows.

An axial tilt like you describe would count as 'significant', and the variance would lead to significant climatic changes over that time period.

 

[Ten Thousand Red Lakes]

In this hypothetical scenario, there is a binary solar system, in which both stars are G0 yellow dwarf stars, which are 105% as wide, 110% as massive and 126% as bright as our sun each. Using AbbydonX's mathematics, such a binary results in a habitable zone spanning from 1.43 to 2.7 astronomical units. There is only one Earthlike planet within that habitable zone, and that's because it orbits the binary elliptically, from 1.43 to 2.7 AUs.



Now, an elliptical orbit means that seasons are not of even length. So how would an elliptical of 1.27 AUs affect the seasons? Also, would the luminosity of sunlight differ between seasons in this specific detail of orbit?

The habitability of this planet is questionable at best. The semi-major axis is 2.065 AU, and the period is 2.047 years. Assume that the planet has a blackblody temperature of 290K. When it is at periapsis, it receives 2.09 times the normal insolation, and has a blackbody temperature of 350K. At apoapsis, it receives 0.59 times the normal insolation, and has a blackbody temperature of ~250K. This is not a sustainable equilibrium.

The habitability zone there does not refer to a specific planet, but to the set of all potential planets. A habitable planet at the inner edge is small, dry, and highly reflective. A habitable planet at the outer edge is large and in possession of a very thick atmosphere. There's no way for one planet to have both.

There are two potential stable equilibria for this planet, but neither are habitable.

 

[Ten Thousand Red Lakes]

Equilibrium A is a Venus analogue. At periapsis, this planet receives 64% of Venusian insolation, and 18% at apoapsis. It's a bit cooler, to be sure. Assuming the same atmosphere/albedo, it would have an average temperature of 609K at periapsis and 443K at apoapsis. This is still well above the boiling point of water at apoapsis, so this should be a stable equilibrium.

Equilibrium B is a Snowball Earth. A high (.65) albedo is combined with a thin atmosphere without much water vapor. At periapsis, it averages 243K, and maybe 260K at the equator. At apoapsis, it averages 177K. Nitrogen and Oxygen will stay in the atmosphere throughout, but the carbon dioxide will deposit near apoapsis. The equator shouldn't melt even at periapsis.

If Earth was given this orbit, it would have a temperature of 303K at periapsis (86 F) and 220K at apoapsis. The runaway greenhouse effect won't substantially occur at periapsis, and every swing into apoapsis will bring out dramatic expansions of the frozen areas. I don't know where the cutoff point in terms of mass is for this planet to tip over into equilibrium A, but it's probably north of 1.5 earth masses.

 

[Ten Thousand Red Lakes]

With two G0 stars I expect that the habitable zone would be a bit larger than you mentioned. However. A wide habitable zone in which a planet goes from near the inner edge to near the outer edge is going to have considerable seasonal differences due to the differences in illumination over the course of the year. If the planet has a significant axial tilt one hemisphere will have short warm winters and long cool summers, and the other hemisphere will tend to have short hot summers and long cold winters (and in both cases spring and fall are also stretched out). This generalization will be strongly modified by the distribution of the continents, and probably by other factors. Also, if the binary is - from the point of view of the planet - an eclipsing binary the illumination the planet receives will drop by almost half for much of the duration of the eclipse - probably something like 'an unusually dim/cool few hours' every (few days?). [Note, this effect affects the part of the planet that is illuminated at the time of the eclipse, and only for a few hours.]

Considerable seasonal differences is rather understanding it. Insolation increases by 257%! Temperature must increase by 37%. If we set the temperature at apoapsis to a chilly 265K, with a severe winter overtaking the planet down to the tropics, it rises to 365K at periapsis, with the oceans evaporating quite swiftly. If we keep temperature at 325K for periapsis, it freezes down to 235K at apoapsis.

 

[Ten Thousand Red Lakes]

I think maybe you could get something of this sort to work if you had a moon of a gas giant around a late M-type dwarf, with a thick atmosphere and a high albedo. A short enough period could keep this roughly habitable. But this is plainly unstable around a G-type star.

 

[Ten Thousand Red Lakes]

I think maybe you could get something of this sort to work if you had a moon of a gas giant around a late M-type dwarf, with a thick atmosphere and a high albedo. A short enough period could keep this roughly habitable. But this is plainly unstable around a G-type star.

Although a gas giant in the habitable zone of a M9 dwarf would tidelock basically immediately, which makes this impossible.

 

[Workable Goblin]

Although a gas giant in the habitable zone of a M9 dwarf would tidelock basically immediately, which makes this impossible.

So? You're not looking for the gas giant to be habitable, but rather the moon. The moon is tidally locked to the gas giant, which is tidally locked to the star; very well, the moon still gets a day-night cycle, though not a 24-hour one.

Or are you saying that the tidal locking will cause any lunar orbits to be unstable because the stellar gravity will pull them off of the gas giant? If so, you should just say that.

 

[Pax_Nihil]

So? You're not looking for the gas giant to be habitable, but rather the moon. The moon is tidally locked to the gas giant, which is tidally locked to the star; very well, the moon still gets a day-night cycle, though not a 24-hour one.

Or are you saying that the tidal locking will cause any lunar orbits to be unstable because the stellar gravity will pull them off of the gas giant? If so, you should just say that.

From what I can understand that is likely the moon would need some crazy orbital speed to avoid the star mucking up its ability to host life.

 

[Dibwys]

The further away from a planet a moon is the less likely that moon is to remain in its orbit about that planet. The closer a planet is to a star the closer a moon has to be to that planet in order to stay in its orbit.

 

[hurax]

The further away from a planet a moon is the less likely that moon is to remain in its orbit about that planet. The closer a planet is to a star the closer a moon has to be to that planet in order to stay in its orbit.

I wanted to have my worlds be moons tidally locked to a gas giant, but orbiting a more sun-like star so I can have them more familiar. They are loosely based on the Galilean moons of Jupiter, but there are three of them in the habitable zone, with tidal heating making the main difference. I have made some calculations, but mostly to have day/month lengths that arent too punishing, hope they aren't too far off.

 

[Ten Thousand Red Lakes]

So? You're not looking for the gas giant to be habitable, but rather the moon. The moon is tidally locked to the gas giant, which is tidally locked to the star; very well, the moon still gets a day-night cycle, though not a 24-hour one.

Or are you saying that the tidal locking will cause any lunar orbits to be unstable because the stellar gravity will pull them off of the gas giant? If so, you should just say that.

It's not impossible for a body under this much tidal force to have a high eccentricity, but it's pretty tough. This orbit has an eccentricity of 0.310. Europa is under ~the same tidal force and has an eccentricity of 0.009.

 

[Ten Thousand Red Lakes]

From what I can understand that is likely the moon would need some crazy orbital speed to avoid the star mucking up its ability to host life.

No.

 

[Workable Goblin]

It's not impossible for a body under this much tidal force to have a high eccentricity, but it's pretty tough. This orbit has an eccentricity of 0.310. Europa is under ~the same tidal force and has an eccentricity of 0.009.

Ah, I think what happened was that I lost track of the plot--I was just considering the habitable moon of a gas giant without any cares about eccentricity or anything.

If you want to stick to the eccentricity idea, maybe a better idea would be a habitable "moon" around a brown dwarf which itself is orbiting a G-class star? You could probably find some arrangement that sticks more to the "chilly" model and avoids "Venus".

 

[Pax_Nihil]

No.

In my defense, I was younger than I am right now and probably should have proofread my post one final time before posting it because it did not come across like I was thinking it in my head lol

 

[Dibwys]

I wanted to have my worlds be moons tidally locked to a gas giant, but orbiting a more sun-like star so I can have them more familiar. They are loosely based on the Galilean moons of Jupiter, but there are three of them in the habitable zone, with tidal heating making the main difference. I have made some calculations, but mostly to have day/month lengths that arent too punishing, hope they aren't too far off.

Most such have pretty long daylight/night cycles, and one side is very well illuminated for much of the night. The larger the primary the more heat the moon will receive from it.