Alternate Planets, Suns, Stars, and Solar Systems Thread

It is different. Any object that is gravitationally round by it's own gravity is considered a planet in this timeline (think Pluto, Ceres, Eris, Sedna, etc, added to the planet list) I have always very much disagreed with the IAU standards. I will not accept the idea of Dwarf Planets not being considered planets. Personal bias and subjectivity, I know, but, it's the way I see these things.
Fair enough :)
Similar in Mass. If you are saying that Nox is the only one that would be large enough to be visible, I do not know about that. I do not know anything about orbital mechanics or how visible these sorts of things would be in space, that's for someone with more talent and knowledge than I to figure out.
I mean that, as the smallest of them, Nox would still just be visible; the Pluto- and Eris-sized moons, being bigger than Nox, could probably also be detected, and Prosperina would be even easier. However, I think it's right to not include any moons smaller than Nox, because below that threshold they would become harder to detect.
Yes they do. I imagine Minthe's main moons are similar in composition to that of Kuiper belt objects. All icy, with the major ones being cryovolcanic like Enceladus. I am also imagining that because of Minthe's lack of a substantial atmosphere, the rings themselves extend fairly close down to the planet. I would imagine that the record of "Moon with a shortest orbital period around a planet" would be one of Minthe's inner minor moons, likely a small moon similar to that of the moons of Saturn that inhabit it's rings. Going around the planet in mere hours.
Sounds reasonable. A minor moon orbiting within a few hundred km of the surface (working with my estimate of 2.6 R⊕) would have an orbital period of 2 hours 21 minutes.
There should have been 5, but you are right, I only listed four. The main 4 I listed are the names that matter. Perhaps I can think of more planets for Nemesis, but I am not sure what else to add to it. What magnitude would Nemesis be?
Given the luminosity value, WolframAlpha says it would have absolute bolometric magnitude (i.e. the entire spectrum, including infrared) of 12.7, and apparent bolometric magnitude of around -0.4. To convert this to visual magnitude, I'll assume, as you said, Nemesis is similar to Teegarden's star - its visual magnitude is about 5 more than bolometric magnitude. This gives an apparent visual magnitude (i.e. visible light) of +4.6, which makes it a naked-eye star, although not a very prominent one, and I think it would be reasonable to not have it be discovered until the early modern era.
Perhaps they would. We can say that the orbital plane of the Nemesis system is tilted so transits are able to be observed like around other stars.
Ok. So we observe Lethe transiting every 21 years? When did we discover it? Is that why we know so little, because we've had so few opportunities to observe it? When was Lethe's most recent transit?
Do all 4 (5?) of the planets transit?
My intentions for the first planet is for it to be a hothouse world similar to Venus. Intense atmospheric pressure. Intense heat. Thick atmosphere. Likely hotter than Venus, possibly.
It sounds a bit like some theories for TRAPPIST-1b, although maybe not quite as hot. I like it!
I would propose that this planet is in a 2:3 resonance, so for every 2 times it orbits, it rotates 3 times. But I do not know if this is possible. If it's not, then we'd have a terminator line where liquid water is present.
Oh, like Mercury! Interesting idea. I don't know much about Mercury's spin-orbit resonance, but I think that it has to do with its high orbital eccentricity. I don't know if this planet would be very stable with high eccentricity; my instinct is that a planet this close to its star should have almost zero eccentricity, but GJ667C has a planet nearly as close as this with eccentricity around 0.1. Apparently, it's not unreasonable to have a 3:2 spin : orbit resonance there.
This would be possible, it seems, but if you do want the 3:2 resonance, then the orbit should be eccentric, which probably means Cocytus should have an eccentric orbit as well for stability. If that's too complicated or not what you're going for, you could either (a) ignore the lack of realism and do whatever you want (always a fun choice, and you're making this up anyway) or (b) just make it tidally locked.
I simply put that "21 years" in since I didn't know what else to put.
21 years is perfectly reasonable.
I wanted to have a resonance, but I do not have the manners to calculate that.
We have our innermost two planets with 1.2 and 5.7-day orbits. This is very close to 5:1; if we change the outermost orbit to 6.0 days, then 6.0/1.2 = 5, then we get a 5:1 mean motion resonance. This adjustment to the orbital period of Acheron also happens to set up another 5:1 resonance with Cocytus. Then, the overall resonance would be 25:5:1.
Generally, mean motion resonances are as simple as that; just deciding on a ratio of small whole numbers, and then adjusting your orbital periods to match.
 
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@Epsilon Tauri

I mean that, as the smallest of them, Nox would still just be visible; the Pluto- and Eris-sized moons, being bigger than Nox, could probably also be detected, and Prosperina would be even easier. However, I think it's right to not include any moons smaller than Nox, because below that threshold they would become harder to detect.
Yes, this makes sense. I do not know the orbital periods of these moons. I am guessing that they orbit in such a way that Persephone - Mors exist all together in a space about the size of Earth-Moon radii. This seems like a large enough area of space for bodies of that mass to settle comfortably within. Prosperina would be likely much further out, perhaps 9 times the distance of the Earth and Moon. This would mean that Prosperina would not be tidally locked to Minthe. Due to Minthe's huge gravitational sphere of influence, it may have hundreds of captured moons ranging in sizes from mere mountains (~1km or so), all the way up to Ceres mass moons orbiting on distant, eccentric orbits.

I wouldn't put it past the chance of possibility that Prosperina could even have it's own captured Moons, despite being a moon. The vast distance from Minthe would make it possible. I would suggest for Prosperina to have a relatively fast rotation period (perhaps 9-10 hours), being indicative of an ancient collision in the past, with many of Minthe's outer moons being material that once belonged to Prosperina and whatever large impactor hit it, and around Minthe, some of Prosperina's orbital moons could be co-orbital bodies, much like Jupiter Trojans.

Sounds reasonable. A minor moon orbiting within a few hundred km of the surface (working with my estimate of 2.6 R⊕) would have an orbital period of 2 hours 21 minutes.
That would make sense. There could be multiple small minor moons orbiting within the period of 3 hours to 10 hours, for extremely close moons. Very small moons though, hundreds of meters, to less than a KM in size. With various small rings of material between them, as the inner shepards they are.

Given the luminosity value, WolframAlpha says it would have absolute bolometric magnitude (i.e. the entire spectrum, including infrared) of 12.7, and apparent bolometric magnitude of around -0.4. To convert this to visual magnitude, I'll assume, as you said, Nemesis is similar to Teegarden's star - its visual magnitude is about 5 more than bolometric magnitude. This gives an apparent visual magnitude (i.e. visible light) of +4.6, which makes it a naked-eye star, although not a very prominent one, and I think it would be reasonable to not have it be discovered until the early modern era.
This makes sense. Nemesis, being a naked-eye star would be an "Average" star in the sky. Nobody would really think anything of it until more modern observations. I would propose that Nemesis has a similar mass to Teegarden's star as well. I am curious about how the Oort cloud would be. Would the Oort Cloud exist around both the Sun and Nemesis (since it is around 1ly out, and Nemesis is at 5,000 AU?) Would Sedna and other such bodies exist, with Nemesis also being there?

Ok. So we observe Lethe transiting every 21 years? When did we discover it? Is that why we know so little, because we've had so few opportunities to observe it? When was Lethe's most recent transit?
Do all 4 (5?) of the planets transit?
I imagine Lethe would have been the last planet discovered. If it takes 21 years, I think one transit wouldn't have been enough to "confirm", so we would need multiple to say for certain it was there.

Oh, like Mercury! Interesting idea. I don't know much about Mercury's spin-orbit resonance, but I think that it has to do with its high orbital eccentricity. I don't know if this planet would be very stable with high eccentricity; my instinct is that a planet this close to its star should have almost zero eccentricity, but GJ667C has a planet nearly as close as this with eccentricity around 0.1. Apparently, it's not unreasonable to have a 3:2 spin : orbit resonance there.
This would be possible, it seems, but if you do want the 3:2 resonance, then the orbit should be eccentric, which probably means Cocytus should have an eccentric orbit as well for stability. If that's too complicated or not what you're going for, you could either (a) ignore the lack of realism and do whatever you want (always a fun choice, and you're making this up anyway) or (b) just make it tidally locked.

I am thinking it makes sense for it to be tidally locked. Since these planets are so close to the star, they would have close to zero eccentricity, indeed. My idea is that Acheron is the habitable, Earth like planet of the system. It may have life, but, I am not sure what kind of life would be there.

We have our innermost two planets with 1.2 and 5.7-day orbits. This is very close to 5:1; if we change the outermost orbit to 6.0 days, then 6.0/1.2 = 5, then we get a 5:1 mean motion resonance. This adjustment to the orbital period of Acheron also happens to set up another 5:1 resonance with Cocytus. Then, the overall resonance would be 25:5:1.
Generally, mean motion resonances are as simple as that; just deciding on a ratio of small whole numbers, and then adjusting your orbital periods to match.

Adjusting the planets to that configuration is also possible. Is it possible that more planets could exist within this system? I could think of some more additions for this system. I would propose them here, and let's see what you think, while listing already existing ones:

Phlegethon is the first planet around Nemesis. It is 0.81 Earth Masses. Observations of the planet have confirmed that it likely has a thick Venus like atmosphere, making the planet similar to Venus. Due to close proximity to Nemesis, it is likely much hotter than Venus is. It orbits Nemesis every 1.2 Earth days. It is tidally locked. The planet is not within the habitable zone as its expected irradiation is more than 4 times that of Earth.

Acheron is the second planet around Nemesis. It is 1.04 Earth Masses. Observations of the planet confirm that it is in the habitable zone around Nemesis and is host to liquid water. It orbits Nemesis every 6.0 days. Compared to other exoplanets around other stars, Acheron is the most Earth-Like exoplanet currently known by mankind. It is not currently known if the planet is host to life by humans. The planet is also tidally locked.

-- These 3 planets could possibly exist between Acheron and Cocytus--

Cerberus is the third planet around Nemesis. It is 0.203 Earth Masses. Poorer in iron, richer in water ice than in our Mars. Cerberus looks like a combination of Mars and Europa: feathery cirrus clouds float above rusty cracked ice-covered seas. Likely an atmosphere of 0.4 Earth Atmospheres. More than Mars, but less than Earth. Enough to form thin wispy clouds in it's reddish skies.

Minos is the fourth planet around Nemesis. It is 0.126 Earth Masses. Similar to Cerberus, this world is host to a Martian-thickness atmosphere and is quite cold. More icey than Cerberus is, though. It is very similar to our Mars, although it is more of an iceworld than Mars is.

Rhadamanthus is the fifth planet around Nemesis. At 2.9 Earth Masses, despite Rhadamanthus's rather Jovian appearance, it's a water world. It's dense atmosphere is rich in ammonia and water vapor that hides deep worldwide oceans, down to where atmospheric pressures are high enough to trap heat. It may very well have life down in those dark seas. Complex life may be sparse - if not entirely absent. Possibly gaining energy from the flashes from the constant lightning in it's inner depths?

--

Cocytus is a low density world as big as Venus but just 0.5 Earth mass and 5/8 Earth's gravity. It's like a much bigger Titan on a balmy interglacial day: warmer and wetter, 'wetter'. If you could call gasoline wet, that is. Air pressure at sea level is 4 atmospheres. It's atmosphere is mostly hazy with hydrocarbons evaporating off of ethane seas. Rain won't be a rarity, as on Titan in OTL. If life is found to exist here, it may exist in similar forms to Titan's life. Life here would likely still be evolving very slowly, thanks to how cold the planet is. Cocytus goes around Nemesis every 30 days.

Hecate is an 2.1 Earthmass Ice World, existing at an orbital distance of around 150(?) days. A frozen, bleak world marked by it's airless nature - it would be otherwise unusual. If not, thanks to it's high rate of rotation. Rotating every 9 hours or so, the planet has had no major moons to slow it's day length down, and it exists outside of the tidal-locking zone of Nemesis. Perhaps it's rotation rate may have been caused by an impactor in it's past, if the large population of asteroids just past it's orbit is any indication. None of them stuck around in Hecate's sphere of influence to create any moons, though.

The final planet of Nemesis discovered to date is Lethe. A large mini-Neptune about 5.2 Earth masses. Not much is known about it other than that it exists, and takes 21 years to go around Nemesis. It exists within darkness with both the Sun and Nemesis respectively as the brightest objects in the skies of those planets.
 
Yes, this makes sense. I do not know the orbital periods of these moons. I am guessing that they orbit in such a way that Persephone - Mors exist all together in a space about the size of Earth-Moon radii. This seems like a large enough area of space for bodies of that mass to settle comfortably within.
Makes sense. If the inner moons are all orbiting within that distance, I'd expect them all to be tidally locked - after all, the Moon is tidally locked to Earth, and Minthe is bigger than Earth. With orbits this close in, I think a mean motion resonance would also be likely.
We want Persephone to be outside the ring system; around 50 000 km is a good minimum, and this gives us a minimum orbital period for Persephone of a little over 12 hours. An object orbiting Minthe at the Earth-Moon distance would have an orbital period of just under 11 days. Based on this, then, any resonance between the inner moons should have no more than around a 20:1 ratio between the orbits of Persephone and Mors.
There are a lot of options for what orbital resonance to set up. You could pretty much pick anything and it would probably be reasonable; the Wikipedia page for orbital resonance has a bunch of good examples of what's possible. You could just do a 1:2 resonance chain (i.e. 1:2:4:8), a bit like the Galilean moons might eventually be in the future, or maybe a 2:3 resonance chain, 1:2:3:6 or 1:2:4:6. If you're feeling creative, though, feel free to choose something more complicated.
Prosperina would be likely much further out, perhaps 9 times the distance of the Earth and Moon. This would mean that Prosperina would not be tidally locked to Minthe.
That would be an orbit around 3 500 000 km. Interestingly, this would actually make Prosperina the only round moon in the solar system not to be tidally locked to its planet. This would make sense here.
Due to Minthe's huge gravitational sphere of influence, it may have hundreds of captured moons ranging in sizes from mere mountains (~1km or so), all the way up to Ceres mass moons orbiting on distant, eccentric orbits.
Up to Ceres mass - does that mean Sedna might have been captured and be one of the moons orbiting Minthe, or have humans also observed Sedna ITTL?
I wouldn't put it past the chance of possibility that Prosperina could even have it's own captured Moons, despite being a moon. The vast distance from Minthe would make it possible. I would suggest for Prosperina to have a relatively fast rotation period (perhaps 9-10 hours), being indicative of an ancient collision in the past, with many of Minthe's outer moons being material that once belonged to Prosperina and whatever large impactor hit it, and around Minthe, some of Prosperina's orbital moons could be co-orbital bodies, much like Jupiter Trojans.
Prosperina reminds me a bit of a larger Triton/Earth's Moon blend. The planet:moon mass ratio here is only 60 or so, less even than Earth and the Moon. There might be debate ITTL over officially making Minthe-Prosperina a double planet, especially if Prosperina does have some moons of its own. The barycenter of the Minthe-Prosperina system would be outside Minthe, which shouldn't be too much of a problem for the inner moons, but it might be wise to pull Mors in a bit closer to maintain stability.
Speaking of double planets, with the different definition of a planet, is Pluto-Charon considered a double planet ITTL?
That would make sense. There could be multiple small minor moons orbiting within the period of 3 hours to 10 hours, for extremely close moons. Very small moons though, hundreds of meters, to less than a KM in size. With various small rings of material between them, as the inner shepards they are.
10 hours is reasonable for the outer limit of the rings; that gives us an outer radius of 43-44 000 km. This sounds good!
This makes sense. Nemesis, being a naked-eye star would be an "Average" star in the sky. Nobody would really think anything of it until more modern observations. I would propose that Nemesis has a similar mass to Teegarden's star as well. I am curious about how the Oort cloud would be. Would the Oort Cloud exist around both the Sun and Nemesis (since it is around 1ly out, and Nemesis is at 5,000 AU?)
I don't know how much the Oort cloud would be affected. Its outer limit is around 13 times the distance of Nemesis, like you said..
Nemesis's orbital period would be around 340 000 years.
Would Sedna and other such bodies exist, with Nemesis also being there?
I'm not really sure how the presence of Nemesis and Minthe would affect the Extreme Trans-Neptunian Objects (ETNOs). Some of them would probably have been captured by Minthe - Sedna might end up as one of the Ceres-sized outer moons you mentioned earlier. Minthe might also lead to an alignment of the orbits of ETNOs, similar to real theories of planet nine.
I imagine Lethe would have been the last planet discovered. If it takes 21 years, I think one transit wouldn't have been enough to "confirm", so we would need multiple to say for certain it was there.
Ok, makes sense.
Adjusting the planets to that configuration is also possible. Is it possible that more planets could exist within this system?
Yes, there's plenty of room for more planets here.
-- These 3 planets could possibly exist between Acheron and Cocytus--
It would make sense to set these up so that they have their own resonances.
Cerberus is the third planet around Nemesis. It is 0.203 Earth Masses. Poorer in iron, richer in water ice than in our Mars. Cerberus looks like a combination of Mars and Europa: feathery cirrus clouds float above rusty cracked ice-covered seas. Likely an atmosphere of 0.4 Earth Atmospheres. More than Mars, but less than Earth. Enough to form thin wispy clouds in it's reddish skies.
I assume the wispy clouds are water?
Is this all conjecture, have we somehow managed to actually image this planet in sufficient detail, or do the humans not know this yet?
Minos is the fourth planet around Nemesis. It is 0.126 Earth Masses. Similar to Cerberus, this world is host to a Martian-thickness atmosphere and is quite cold. More icey than Cerberus is, though. It is very similar to our Mars, although it is more of an iceworld than Mars is.
Is there life on Minos or Cerberus?
Rhadamanthus is the fifth planet around Nemesis. At 2.9 Earth Masses, despite Rhadamanthus's rather Jovian appearance, it's a water world. It's dense atmosphere is rich in ammonia and water vapor that hides deep worldwide oceans, down to where atmospheric pressures are high enough to trap heat. It may very well have life down in those dark seas. Complex life may be sparse - if not entirely absent. Possibly gaining energy from the flashes from the constant lightning in it's inner depths?
Oh, that's a cool concept for a planet. I like it!
Hecate is an 2.1 Earthmass Ice World, existing at an orbital distance of around 150(?) days. A frozen, bleak world marked by it's airless nature - it would be otherwise unusual. If not, thanks to it's high rate of rotation. Rotating every 9 hours or so, the planet has had no major moons to slow it's day length down, and it exists outside of the tidal-locking zone of Nemesis. Perhaps it's rotation rate may have been caused by an impactor in it's past, if the large population of asteroids just past it's orbit is any indication. None of them stuck around in Hecate's sphere of influence to create any moons, though.
150 days -> 0.248 AU
I like the resonance you've made with Hecate and Cocytus.
How did we measure the rotation rate for a world so far away? It's far enough from Nemesis that it might be possible to directly image it - have we?
 
@Epsilon Tauri
Makes sense. If the inner moons are all orbiting within that distance, I'd expect them all to be tidally locked - after all, the Moon is tidally locked to Earth, and Minthe is bigger than Earth. With orbits this close in, I think a mean motion resonance would also be likely.
We want Persephone to be outside the ring system; around 50 000 km is a good minimum, and this gives us a minimum orbital period for Persephone of a little over 12 hours. An object orbiting Minthe at the Earth-Moon distance would have an orbital period of just under 11 days. Based on this, then, any resonance between the inner moons should have no more than around a 20:1 ratio between the orbits of Persephone and Mors.
There are a lot of options for what orbital resonance to set up. You could pretty much pick anything and it would probably be reasonable; the Wikipedia page for orbital resonance has a bunch of good examples of what's possible. You could just do a 1:2 resonance chain (i.e. 1:2:4:8), a bit like the Galilean moons might eventually be in the future, or maybe a 2:3 resonance chain, 1:2:3:6 or 1:2:4:6. If you're feeling creative, though, feel free to choose something more complicated.
This all makes sense to me. Persephone can exist around Minthe, orbiting every 12 hours, and it goes from there, perhaps. Looking through resonances, this is what I would propose for the 4 inner major Moons. how does this sound, for example?

Persephone - 12 hours
Hecae - 24 hours
Nox - 36 hours.
Mors - 72 hours.

That would be an orbit around 3 500 000 km. Interestingly, this would actually make Prosperina the only round moon in the solar system not to be tidally locked to its planet. This would make sense here.
Being 9 times the distance Earth-Moon distance, would mean that Prosperina would be well out of the influence range for the 4 inner moons. I imagine that perhaps Prosperina can have a Ceres Mass Moon that orbits around it every 3 days, as well as perhaps a dozen smaller moon-moons? Tiny ones, under 30km in size. The main Moon would have a roughly equatorial orbit, while the rest of them are likely captured Moons, so they would not have equatorial orbits like Prosperina's major Moon.

Perhaps we can call Prosperina's Moon Erebus. Erebus would be larger than our Charon is, but not by much.

Up to Ceres mass - does that mean Sedna might have been captured and be one of the moons orbiting Minthe, or have humans also observed Sedna ITTL?
We can say that Sedna is a captured moon of Minthe, orbiting in an eccentric elliptical orbit that takes multiple years to go around Minthe.

Prosperina reminds me a bit of a larger Triton/Earth's Moon blend. The planet:moon mass ratio here is only 60 or so, less even than Earth and the Moon. There might be debate ITTL over officially making Minthe-Prosperina a double planet, especially if Prosperina does have some moons of its own. The barycenter of the Minthe-Prosperina system would be outside Minthe, which shouldn't be too much of a problem for the inner moons, but it might be wise to pull Mors in a bit closer to maintain stability.
Speaking of double planets, with the different definition of a planet, is Pluto-Charon considered a double planet ITTL?
Given that Prosperina and Minthe are so far apart, the barycenter would indeed be outside of Minthe. This would mean that Pluto-Charon, and Prosperina-Minthe would be considered planets. As long as a moon and it's planet's barycenter is outside of the planet itself, both would be considered full planets.

Perhaps Prosperina could have rings, while Minthe has very faint rings - Prosperina's rings could be quite visible in character.

10 hours is reasonable for the outer limit of the rings; that gives us an outer radius of 43-44 000 km. This sounds good!
Yup, glad that makes sense there.

I don't know how much the Oort cloud would be affected. Its outer limit is around 13 times the distance of Nemesis, like you said..
Nemesis's orbital period would be around 340 000 years.

If Nemesis takes 340,000 years to go around the Sun, how long would it take Minthe to go around the Sun? Minthe exists far beyond the Kuiper belt as to not be an influence. Perhaps a similar orbit to that "Planet 9".

I'm not really sure how the presence of Nemesis and Minthe would affect the Extreme Trans-Neptunian Objects (ETNOs). Some of them would probably have been captured by Minthe - Sedna might end up as one of the Ceres-sized outer moons you mentioned earlier. Minthe might also lead to an alignment of the orbits of ETNOs, similar to real theories of planet nine.
I imagine that much of them exist as Moons of Minthe. Minthe would perhaps have the most moons of any planet in the solar system, with such a large sphere of influence. More influence than that of Jupiter, or Saturn, in terms of the area that Minthe influences. In terms of OTL planet definitions, the whole "clearing of the orbit" thing, Minthe would be the most "planet-like" of all the planets, due to how much it dominates it's region of space.

I assume the wispy clouds are water?
Is this all conjecture, have we somehow managed to actually image this planet in sufficient detail, or do the humans not know this yet?

Humans do not know of this yet.

Is there life on Minos or Cerberus?

Probably is, single cellular life hanging out towards cracks in the ice where light and warmth fades in. Not much energy to go around, so it wouldn't be that impressive.

Oh, that's a cool concept for a planet. I like it!

Thanks!

150 days -> 0.248 AU
I like the resonance you've made with Hecate and Cocytus.
How did we measure the rotation rate for a world so far away? It's far enough from Nemesis that it might be possible to directly image it - have we?

It is a property of the planet which is not known by humans yet.
 
Redoing my alternate Venus.
May I present the planet Neptune (OTL Venus). Neptune is the second planet of the Solar System and the closest to Earth, having an orbit of 0.78 AU
Named after the Roman God of the sea duo to it's slight blue appearance in the sky. Neptune is often called the brother planet of Earth.
With Neptune having 81.5% the mass, 95% the radius of the Earth and duo to a similar density also a similar gravity to Earth, it's no wonder that the two worlds are often considered as twins.
Neptune has a 20.3 hour day with a 19.8° axial tilt. Giving it a similar day to Proterozoic Earth.
With a atmosphere only 0.81 percent that of Earth's comprising of 98% Nitrogen, 1,36% Argon and 0,35% Co², the temperature is still a hot 43.5°C.

It's insides are similar to Earth. A iron-nickel core roughly taking 20% of it's mass being around 500°C cooler than the core of Earth, a thick mantle and a thin crust. And a magnetic field strenght of around 0.256 gauss. It's geomagnetic pole having an tilt of around 6 degrees.

Despite it's slight blue atmosphere, it's not a water world like Earth. Instead it could be described as a planet wide muddy desert, with small surface patches of turquoise, iron rich water and a flat surface. Duo to the dryness of Neptune, clouds are few. Concentrating on the few lakes of Neptune and it's more wet poles.
Duo to the lack of a large hydrosphere, plate tectonic as known on Earth couldn't form in Neptune. The weather being mostly sandstorms, thunderstorms and in rare cases rain. Thus the only highest points on Neptune are it's volcanoes.
If one wants to find water on Neptune, one needs to go deep. To the underground lakes, rivers and wet patches if Nepture.
And this is also where one can find the majority of the primitive single celled life of Neptune.
And with primitive, one means it. They are comparable to the life found on Earth before the Great Oxidation Event. Maybe it was because of bad luck, their relative small living area or not having the need for multicelular and oxygen producing evolution. Either way, they have even less time than life on Earth, with it's home world being even closer to the aging sun.

Neptune is by far the closest planet to our home. But still, it is a harsh, unforgiving world. Even to his own offsprings. Let alone the offsprings of Earth who will in the next decade try to land a manned mission on Neptunes surface.
Truly, Neptune is the center of attention in the media and astronomical and astrobiological scene. Having received many rovers on his land, lakes and even floating in in the atmosphere. Not forgetting the countless satellites around him, observing both the surface and underground of Neptune.
There are strong discussions about either preserving the ecosystem of Neptune or to take the opportunity and turn Neptune into a second Earth.

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This all makes sense to me. Persephone can exist around Minthe, orbiting every 12 hours, and it goes from there, perhaps. Looking through resonances, this is what I would propose for the 4 inner major Moons. how does this sound, for example?

Persephone - 12 hours
Hecae - 24 hours
Nox - 36 hours.
Mors - 72 hours.
That sounds great. Maybe give or take a couple of minutes, because Hecae would probably be unlikely to have a period of exactly one Earth day (keep the resonances exactly precise relative to each other, just probably don't have whole numbers of hours).
Being 9 times the distance Earth-Moon distance, would mean that Prosperina would be well out of the influence range for the 4 inner moons. I imagine that perhaps Prosperina can have a Ceres Mass Moon that orbits around it every 3 days, as well as perhaps a dozen smaller moon-moons? Tiny ones, under 30km in size. The main Moon would have a roughly equatorial orbit, while the rest of them are likely captured Moons, so they would not have equatorial orbits like Prosperina's major Moon.

Perhaps we can call Prosperina's Moon Erebus. Erebus would be larger than our Charon is, but not by much.
The radius of Prosperina's hill sphere is 620 000 km - roughly 1.6 Earth-Moon distances - so any moons of Prosperina need to stay within that, or they become moons of Minthe instead.
With a 3-day orbit, Erebus's semi-major axis would be 42 000 km, so well within the hill sphere. Erebus would be tidally locked to Prosperina.
A dozen smaller moons is certainly a lot - Pluto only has a third as many - but it's not necessarily unrealistic, especially considering that Prosperina's hill sphere extends roughly ten times as far as Pluto's furthest moon anyway.
Perhaps Prosperina could have rings, while Minthe has very faint rings - Prosperina's rings could be quite visible in character.
That's a fun idea. Prosperina's radius is about 0.68 Earth radii (4350 km), and, assuming its rings have the same icy composition as it, they should extend out to 1.66 Earth radii (10 620 km) (the Roche limit). This is around a quarter of the way to Erebus's orbit. That means there won't be a Cassini Division-type gap in the rings at a major resonance with Erebus, since Erebus orbits too far out.
A few of Prosperina's subsatellites would probably orbit within the rings and shepherd gaps there.
If Nemesis takes 340,000 years to go around the Sun, how long would it take Minthe to go around the Sun? Minthe exists far beyond the Kuiper belt as to not be an influence. Perhaps a similar orbit to that "Planet 9".
Minthe's orbital period would be about 6500 years.
 
That sounds great. Maybe give or take a couple of minutes, because Hecae would probably be unlikely to have a period of exactly one Earth day (keep the resonances exactly precise relative to each other, just probably don't have whole numbers of hours).
Then it would be easy to just add an hour to the orbital periods of these planets so that it doesn't seem so exact? 24 hours becomes 25 hours, for example?
The radius of Prosperina's hill sphere is 620 000 km - roughly 1.6 Earth-Moon distances - so any moons of Prosperina need to stay within that, or they become moons of Minthe instead.
With a 3-day orbit, Erebus's semi-major axis would be 42 000 km, so well within the hill sphere. Erebus would be tidally locked to Prosperina.
A dozen smaller moons is certainly a lot - Pluto only has a third as many - but it's not necessarily unrealistic, especially considering that Prosperina's hill sphere extends roughly ten times as far as Pluto's furthest moon anyway.
Yeah that is pretty much what I was going for. Prosperina is a much larger planet than Pluto is, it is interesting to have a double planet system where both planets have rings and Moons. I imagine that discovery of this in this alternative world would be quite fascinating.
That's a fun idea. Prosperina's radius is about 0.68 Earth radii (4350 km), and, assuming its rings have the same icy composition as it, they should extend out to 1.66 Earth radii (10 620 km) (the Roche limit). This is around a quarter of the way to Erebus's orbit. That means there won't be a Cassini Division-type gap in the rings at a major resonance with Erebus, since Erebus orbits too far out.
A few of Prosperina's subsatellites would probably orbit within the rings and shepherd gaps there.

Minthe's orbital period would be about 6500 years.
Yes, that makes sense. With all this information in mind, would you be capable of editing it into my description that I showed? You can even make your own additions to it, I would be curious to see if you have anything else in mind for this. I am all open for suggestions and ideas.
 
Then it would be easy to just add an hour to the orbital periods of these planets so that it doesn't seem so exact? 24 hours becomes 25 hours, for example?
Well, that might not work, because it's important to keep it so that Hecae's orbit is exactly twice as long as Persephone's, Nox's is three times as long, etc. (which doesn't happen with, say, 13:25:37). Maybe something more like 12.17 : 24.34 : 36.51 : 73.02 - keep the proportions the same, but no longer perfect numbers of hours. It is realistic to measure orbital periods with that much precision or even more - just look at Dysnomia, which we've measured to within ten seconds despite it being almost 100 AU away.
It's just occurred to me that, although it would be just outside the Roche limit, Persephone's planetward half would be only a few thousand km away from it, and tidal interactions with the other moons could well push it over the edge to make it break apart. To combat this, I think it might be wise to raise Persephone from its current ~50 Mm into a 58.2 Mm orbit, with a new orbital period of 15.47 hours, unless this would mess up anything you had planned. This would reduce the tidal forces on it by around a quarter, so it would definitely still deform significantly, but probably be less likely to break apart.
Yeah that is pretty much what I was going for. Prosperina is a much larger planet than Pluto is, it is interesting to have a double planet system where both planets have rings and Moons. I imagine that discovery of this in this alternative world would be quite fascinating.
Have the rings of Prosperina been discovered by humans yet (you mentioned Minthe's have)? What about Erebus? How precisely have we measured the orbits, masses, radii, etc. of all of these bodies?
Prosperina might be nearly as bright as Minthe if its rings are more prominent than Minthe's.
Yes, that makes sense. With all this information in mind, would you be capable of editing it into my description that I showed? You can even make your own additions to it, I would be curious to see if you have anything else in mind for this. I am all open for suggestions and ideas.
Well, I can't edit your posts :). That said, I can give an updated description below.
Minthe and Prosperina are a binary planet in an eccentric orbit around the Sun that takes them between 340 and 360 AU. Their orbit is inclined 17º relative to the ecliptic. The pair have captured many objects [including OTL Sedna] as distant, irregular moons within their hill sphere, which extends out to 6.3 AU, although most of these moons have likely not yet been discovered.

The two are separated by 9.0 lunar distances, or 3 500 000 km. They are not tidally locked to each other, which is unique for solid bodies in the outer solar system. Both worlds are known to have a ring system; that of Prosperina is much more prominent than that of Minthe. Neither has any atmosphere of note.

Minthe is by far the larger of the two, at 6.3 Earth Masses and 2.6 Earth Radii. Its rings extend from roughly surface level to about 44 000 km from the planetary center. It has four known moons of its own: Persephone, Hecae, Nox, and Mors, in a 1:2:3:6 mean motion resonance. Persephone is roughly similar in size to Pluto. Its orbital period is 15.47 hours, giving it a semimajor axis of around 58 000 km. Hecae is slightly larger than Persephone, being more comparable in size to Eris. Its orbital period is 30.94 hours, with an SMA of around 92 000 km. Nox is the smallest of Minthe's known moons, at only 500 km radius, although many minor moons likely exist which have not yet been detected. It orbits in 46.41 hours, with an SMA of 121 000 km. The last of Minthe's known moons is Mors, which is similar in size to Persephone. Its orbital period is 92.82 hours, with a semimajor axis of 192 000 km.

Prosperina, the smaller of the twin planets, is only Mars-size, at 0.11 Earth masses, although its icier composition means that its radius is 0.68 times Earth's. Prosperina's rings are brighter than Minthe's. They extend out 10 600 km from the center of the planet. Prosperina only has one known moon of its own, Erebus, which is about 500 km in radius. Erebus has a 72.66-hour orbit with a semimajor axis of about 42 000 km.

All of the known moons of Minthe and Prosperina are believed to be tidally locked to their respective planets.

Although only seven bodies in the Minthe-Prosperina system are known, it is suspected that, between them, the planets may have the most moons of any in the solar system, because their hill sphere is larger than that of any other planet, and they are suspected to have captured many minor outlying bodies within it.

It might be best, if we want to continue this exchange, to do so in DMs, so as to avoid hijacking this thread.
 
Redoing my alternate Venus.
May I present the planet Neptune (OTL Venus). Neptune is the second planet of the Solar System and the closest to Earth, having an orbit of 0.78 AU
Named after the Roman God of the sea duo to it's slight blue appearance in the sky. Neptune is often called the brother planet of Earth.
With Neptune having 81.5% the mass, 95% the radius of the Earth and duo to a similar density also a similar gravity to Earth, it's no wonder that the two worlds are often considered as twins.
Neptune has a 20.3 hour day with a 19.8° axial tilt. Giving it a similar day to Proterozoic Earth.
With a atmosphere only 0.81 percent that of Earth's comprising of 98% Nitrogen, 1,36% Argon and 0,35% Co², the temperature is still a hot 43.5°C.

It's insides are similar to Earth. A iron-nickel core roughly taking 20% of it's mass being around 500°C cooler than the core of Earth, a thick mantle and a thin crust. And a magnetic field strenght of around 0.256 gauss. It's geomagnetic pole having an tilt of around 6 degrees.

Despite it's slight blue atmosphere, it's not a water world like Earth. Instead it could be described as a planet wide muddy desert, with small surface patches of turquoise, iron rich water and a flat surface. Duo to the dryness of Neptune, clouds are few. Concentrating on the few lakes of Neptune and it's more wet poles.
Duo to the lack of a large hydrosphere, plate tectonic as known on Earth couldn't form in Neptune. The weather being mostly sandstorms, thunderstorms and in rare cases rain. Thus the only highest points on Neptune are it's volcanoes.
If one wants to find water on Neptune, one needs to go deep. To the underground lakes, rivers and wet patches if Nepture.
And this is also where one can find the majority of the primitive single celled life of Neptune.
And with primitive, one means it. They are comparable to the life found on Earth before the Great Oxidation Event. Maybe it was because of bad luck, their relative small living area or not having the need for multicelular and oxygen producing evolution. Either way, they have even less time than life on Earth, with it's home world being even closer to the aging sun.

Neptune is by far the closest planet to our home. But still, it is a harsh, unforgiving world. Even to his own offsprings. Let alone the offsprings of Earth who will in the next decade try to land a manned mission on Neptunes surface.
Truly, Neptune is the center of attention in the media and astronomical and astrobiological scene. Having received many rovers on his land, lakes and even floating in in the atmosphere. Not forgetting the countless satellites around him, observing both the surface and underground of Neptune.
There are strong discussions about either preserving the ecosystem of Neptune or to take the opportunity and turn Neptune into a second Earth.

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This is an interesting idea! Does Neptune have moons, or does it lack them like OTL Venus? Are those white patches on the surface clouds, or are they volcanoes tall enough to be snowcapped? Are there polar ice caps? If so, are they permanent, or do they disappear in the summer and reform in the winter?
Does Neptune have the exact same orbit as OTL Venus, or is it slightly changed?
Its atmosphere is only 0.81% the pressure of Earth's? That's only a little more than Mars! It really is inhospitable. How does water stay liquid on the surface?
If the other planets of the solar system are unchanged - what is OTL Neptune called ITTL?
 
This is an interesting idea! Does Neptune have moons, or does it lack them like OTL Venus? Are those white patches on the surface clouds, or are they volcanoes tall enough to be snowcapped? Are there polar ice caps? If so, are they permanent, or do they disappear in the summer and reform in the winter?
Does Neptune have the exact same orbit as OTL Venus, or is it slightly changed?
Its atmosphere is only 0.81% the pressure of Earth's? That's only a little more than Mars! It really is inhospitable. How does water stay liquid on the surface?
If the other planets of the solar system are unchanged - what is OTL Neptune called ITTL?
No, Neptune doesn't have a moon. The impact that in OTL caused the slow rotation hit at a different angle which didn't caused a slow rotation but also no formation of large permanent moons.
The white patches are clouds. Sadly Universe Sandbox 2 doesn't have such advanced customizations of surfaces yet.
Neptune has small polar ice caps duo to only having an axial tilt of around 20%, thus making the poles cold enough for miniscule ice caps.
Neptune has an orbit of 0.78 AU, just like Venus did for a while during the early days of the Solar System. In this TL the planet simply stayed there instead of migrating back.
That was a mistake on my part. It actually has an atmospheric pressure of 81% to that of Earth's. So the water question answers itself.
Haven't thought of a name for OTL Neptune. It could be Janus or Oceanus.
 
@DUWANG

What if there was an Earth Mass planet instead of Mars, like an Earth sized Mars-like world instead of OTL Mars?

A earth sized Mars would be a pretty different planet; it would have a magnetic field, a thicker atmosphere with a lot of nitrogen, it would be geological active, there would be seas, lakes and rivers. Also with a magnetic field it would have lost far less water over its history.

But let’s say it do start out with less water. I suspect that most of the water would be trapped in ice caps. The northern lowland would be home to swallow lakes and shallow inland seas, it would have a temperate to subarctic climate, if life was introduced there it would turn into forest and taiga. The southern highland on the other hand would become a giant alpine desert. Valles Marines would turn into a temperate to mediterranean region. If Hellas crater still exist, it would create a deadly high temperature salt desert like Death Valley just far far worse, only in the winter would be survivable for humans.
 
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A earth sized Mars would be a pretty different planet; it would have a magnetic field, a thicker atmosphere with a lot of nitrogen, it would be geological active, there would be seas, lakes and rivers. Also with a magnetic field it would have lost far less water over its history.

But let’s say it do start out with less water. I suspect that most of the water would be trapped in ice caps. The northern lowland would be home to swallow lakes and seas, it would have a temperate to subarctic climate, if life was introduced there it would turn into forest and taiga. The southern highland on the other hand would become a giant alpine desert. Valles Marines would turn into a temperate to mediterranean region. If Hellas crater still exist, it would create a deadly high temperature salt desert like Death Valley just far far worse, only in the winter would be survivable for humans.

I do not think Mars would have had the same formations if it was an Earth mass planet. It would have looked completely unrecognizable, having a substantial atmosphere, geological activity, etc. It would not have a large Moon like Earth, though, perhaps it's larger gravity would have captured more asteroidal moons over time. Let us presume that Mars has seas, lakes, rivers, and has more water than OTL.
 
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I do not think Mars would have had the same formations if it was an Earth mass planet. It would have looked completely unrecognizable, having a substantial atmosphere, geological activity, etc. It would not have a large Moon like Earth, though, perhaps it's larger gravity would have captured more asteroidal moons over time. Let us presume that Mars has seas, lakes, rivers, and has more water than OTL.

In that case, it’s simply Earth which receive 40% of our Earth’s sunlight. It would create a equator in a eternal temperate springtime, while the ice capes would reach Rome and Atlanta. Central Asia would be covered by massive taiga or mammoth steppe.
 
In that case, it’s simply Earth which receive 40% of our Earth’s sunlight. It would create a equator in a eternal temperate springtime, while the ice capes would reach Rome and Atlanta. Central Asia would be covered by massive taiga or mammoth steppe.
Makes sense. Would an Earth mass planet in the place of Mars be stable in terms of the gravitational influences in the solar system?
 
Makes sense. Would an Earth mass planet in the place of Mars be stable in terms of the gravitational influences in the solar system?

I don’t see why not, but I must admit I would find earth sized Mars with roughly the same geographic features [1] more interesting., than a world of polar caps, dry steppes and eternal spring time.

[1]Minus Olympus Mons.
 
I don’t see why not, but I must admit I would find earth sized Mars with roughly the same geographic features [1] more interesting., than a world of polar caps, dry steppes and eternal spring time.

[1]Minus Olympus Mons.

Do you think you could make such a world in Universe Sandbox to see how it would turn out? Earth mass, habitable planet (not a world of the same geographic features) in the place of Mars?
 
Do you think you could make such a world in Universe Sandbox to see how it would turn out? Earth mass, habitable planet (not a world of the same geographic features) in the place of Mars?

I doubt it, I would have to buy and learn how to use the program first.
 
Somewhere in this hypothetical universe is a galaxy made not naturally but through artificial means, with a diameter of 16 million parsecs. This "Z-Kardashev Galaxy" is comprised solely of copies of one same solar system: a single Kardashev-type star with a diameter of 1,860,000 miles and a temperature of 72,000,000,000,000 degrees Fahrenheit, which results in a luminosity of 2,657,599,202,344,462,135,749,473,441,629,625,102,087,912,417 times brighter than the sun. In spite of its massively hyper-high heat, the Kardashev-type star is actually inactive, meaning the heat is trapped within it and can't escape. Theoretically, it will expand the giant star's lifespan to 100 billion billion years (no, this is not a typo.)


In turn, orbiting it from within its habitable zone are co-orbital binaries of one same star type: K5, 74% as wide, 69% as massive and only 16% as bright as our sun. Orbiting each of the binaries P-style, in turn, are co-orbits of Earth- to Venus-sized rocky planets.


With the specifics now in place, questions ensue:


  1. How many Kardashev-type stars can fit within the 16-million-parsec parameters of the Z-Kardashev Galaxy?
  2. How far and how wide is the Kardashev-type star's habitable zone?
  3. How many K5 binaries can co-orbit each other within the 100-billion-billion-year timespan?
  4. How many of the K5 binary co-orbitals can fit inside the Kardashev-type star's habitable zone?
  5. How many Earth- or Venus-sized planets can fit within each K5 binary's gravitational pull within the 100-billion-billion-year timespan and not get themselves tidally locked?
 
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