Help requested on long-term habitable-zone stars

[AotrsCommander]

I am currently writing another entry for a massive lore project (you may have seen the previous thread on the shape of the galaxy).

I am usually pretty good about getting the details accurate (or at least explaining around them), but my sources I am working from to write up are from about 2005-2006 and the understanding (both mine and of astrophysics generally) has improved a great deal since then.

I have managed a reasonable job of re-lensing things with better knowledge, but, as I came to the close of my work-day, I reached this passage (originally from an adventure I wrote), with the pertinent problems highlighted.

You have entered a system that, surprisingly, is documented in something other than minimal detail, if not well. The system is very old, this vicinity at the edge of the galaxy, being home to the oldest stars. It has no name, merely a designation. It consists of a main sequence single star, now slowly expanding and cooling, before it will eventually collapse into a white dwarf. There are seven remaining planets in the system (how many have been swallowed by the burgeoning star you will never know), three gas giants and four rocky worlds. Two of these – the outer most ones – are merely chunks of rock and ice.
The other two are not.
The first, according to your data – which appears to be over a thousand years old, and complied by a wandering explorer – is a terrestrial water planet with an oxygen/nitrogen atmosphere that is well within breathable limits and is very close to one G; it has a 37 hour day. Life has evolved there, but is very, very old (in fact it is noted as being among the oldest ecosystems known). The world is getting slowly cooler as time creeps on, as the heat from the star is slowly fading. Now, the only equatorial region and it’s attendant continent is free of ice, and the rest of the world is locked in thickening ice. At the equator, the temperature is roughly temperate.

This is, as they say, clearly bollocks, because it contradicts itself. Unless I am wrong, it would not be possible for a planet to be in such a position that it could have been habitatable before, during and implied at the close of the main sequence red giant stage.

If this was a main sequence star, then, obviosuly, at the point it went into the red giant stage, a planet that had evolved life in the main sequence stage would be thown out of it during the giant stage. From my reading, while it is possible for a planet to have developed life during the red giant stage - perhaps even for a few billion years - when the expansion of the star moved the habitability sonze outward, it would not be for long enough to be "among the oldest ecosystems known."

So then, the question becomes how to re-interpret this. (Fortunately, the first thing to note is that this was for a band of adventurers, who, more even that perhaps kost adventurers, were idiots; at least to the point of ths passage being interpreted as their own unreliable-narration-perceptions.)

As the context of the adventure largerly thematically hinges on this being a "really old ecosystem" more than the stellar data, can you suggest to what we might re-interpret what they found.

The star is not not given a classification (that helps!) and we can potentially disgard "main sequence" - PCs-Don't-Read-Astrogation-Data-Properly, so what we would be looking at is a something that would create a very long-term habitable zone.

I initially thought "just use a red dwarf," and be done with it (as they have, as I understand, the longest-term habitability prospects), but THAT presents a problem those generally require a tidelocked planet, which the lfie-bearing on is not (it has a day length of 37 hours, though no year length noted, fortunately).

The second planet (a more venus-life hot world being terraformed by the aliens the PCs were following) is also noted as being closer to the star than this planet. (Following what at the time must have seemed a logical progression.)

Can anyone offet any suggestions on a more accurate way to approach this? (I grant you, with the knowledge available now, the one of the oldest known" is probably not going to be possible (I can to some extent work around that by narravtor bias on the thousand-year old data of course), but I do need a situation whereby a planet, far enough out to be (at least) the second world and not tidelocked, would be able to sustain a (mostly) terrestrial biosphere for an unsually long time.

Suggestions would be very welcome!

 

[Algr]

As the star expands into its red dwarf stage, it would get hotter, not colder. The habitable zone would shift outward. Planets that were formerly too cold for life could then develop it. The stars mass, and hence gravity, would only minimally reduce over time, so planetary orbits would slowly get larger. Inner planets could be consumed, but any ejection would not likely be caused by the star. 37 hours is a reasonable day for a planet that is far from its sun, and is old enough to only get warm due to the sun entering its red giant phase. A star larger than Sol might be a good choice here. If some other presence destabilizes the solar system, then a planetary collision might invigorate your planet and allow it to have a magnetic field, moon, atmosphere, or other features unlikely in a very old system.

This might help, it is one of my favorites:


Best watched on a large monitor. Use your spacebar to pause and read - and youtube has keys that let you frame advance and reverse.

Edit: Some notes on that video: Earth becomes uninhabitable long before the sun reaches the red giant stage. This is seemingly due to the magnetic field shutting down and atmospheric thinning. (Don't worry, we still have a billion years or so.) This could be an issue for your old planet. Also note that the video's clock runs at different speeds in different eras, elongating the sun's pre-ignition phase, and shortening it's life as a white dwarf.

 

[AotrsCommander]

Hmm.

Difficult. My understanding at the time was mostly a 90s-era understanding of really sort of school-age astrophysics (hell, even when I wrote the stuff in 2005/6 is was the better part of two decades ago we know a lot more now than then).

I sort of, from said explanations, assumed a main sequence star went along from O to M, as if were, but that's not QUITE how it works, is it? At least, the progression is not along the line of the HR curve like I believed, but looking at it properly the other day, that's not what happens.

But, this is problem with doing things without fully understanding the underlying principles!

(Before I started this project, I was still thinking of a spiral galaxy being like in all the pictures - with only stars in the arms and the reality is not very like that. Hell, even looking at it earlier this week, I still didn't QUITE get my head around it when I looked at it a month or two ago.)

Actually having a planet with a really long habitability envelope (that is not tide-locked around a red dwarf which are - to my understanding, stable and long-lived) is going to be quite hard.

On the flip side, watching through the video, the other planet in the system was a venus-like world being terraformed (by super-exotic technology), but it was noted as having very high volcanicity and the surface being mostly cooled lava flows... So perhaps as much by accident as design, I got THAT about right.
Okay. So one thing it that it appears as though my mental logic that as the star gets cooler it gets dimmer is entirely backwards - from what I can tell from the video, the stars (or at least Sol-like main sequence) would get hooter and brighter and then cooler and still brighter until the helium flash when it get cooler and dimmer and the SUPER hot and really dim in the death phase.

So there's not really a point (until it is really well too late) when the luminense (and therefor the heat on the planet) is getting LESS, even when the star is cooling off and heading towards the giant stage.

It seems a bit dubious, then, but what about a planet that was in EXACTLY the right place such that, as the star starts to lose mass, the planet... Moves further out, sustaining the habitability? Now the obvious problem with that is, according to the video, by the time the sun starts losing mass, the habitability envelope would have already long gone for something that started in it, and my own personal bane, stellar wind[1] is increasing and likly blowing the atmosphere off shortly anyway.At the moment, all I can think of it a planet on such a weird eccentric orbit (perhaps due to some impact or other events, which did NOT wipe life out miraculously[2]) that carried it from further in the solar system to further out. (Which the video suggested by virtue of Jupiter having apparently changed orbits.)Otherwise back to base principles: the net result is a planet with an unusually long habitability period, now slowly coming to and end as it isn't getting enough light (orbit slowly spiralling out?) as its getting colder (some sort of final ice age?) I think also part of my current thinking might be influenced in that regard by Serina, the world of birds speculative zoology site, wherein the end time sare the planet cooling off and that seemed to match the idea I'd had most of twenty years ago). But now I'm not entirely sure, like, how that would work, given the steadily increases luminosities would seemed to indicate more stellar flux (and stellar wind) the later and older the star gets.

Am I completekly off-base here?
[1]I spent months on this forum working out a plausible, reasonably well-researched tidelocked planet for this sort of thing, and then at the last hur ran right into "but what about the stellar wind?" and realized that I couldn't even make balpark calculations for it and had to resort to hand-waving after all that.

[2]In fairness, the precursor racehavign retrocasually engineered probabilities across the entuire universe to make it the way it is and the number of habitable earth-like-close planets (right down to causing multiple earth, identical to a point) to spawn, that is not without at least some in-universe plausibility.

 

[Algr]

As the star expands into its red dwarf stage

Ugg, I mean red giant stage. Too late to edit.

----
One way to have a planet get colder is to have it pulled away from its star by a rogue planet entering the system. Or you could invent some new physics - the star is still in main sequence, but cooling because of "internal convections" in a way that you don't spend much time explaining.

Can the planet simply enter an ice age somehow? The surface gets really bright, or some new kind of plant evolves that sucks up all the greenhouse gasses really quickly. (These might be too easy to fix with sci-fi technology though.)

 

[AotrsCommander]

Ugg, I mean red giant stage. Too late to edit.

----
One way to have a planet get colder is to have it pulled away from its star by a rogue planet entering the system. Or you could invent some new physics - the star is still in main sequence, but cooling because of "internal convections" in a way that you don't spend much time explaining.

Can the planet simply enter an ice age somehow? The surface gets really bright, or some new kind of plant evolves that sucks up all the greenhouse gasses really quickly. (These might be too easy to fix with sci-fi technology though.)

It's okay, I realized what you meant from context!

Food for thought. I'll have a proper think tomorrow (when I'm back on work-time), but several things suggest themselves.

1) rogue planet as you say

2) some sort of star interaction with the star that caused it to lose mass/luminosity (micro black hole? Other passing star?)

3) due to interaction with another star passing through its original system, the planet was captured by the new (current) from its old one, extending the habitability again (albiet at the cost of a Permian-level extinction event in the process). This would be probably the most instantly appealing one, if perhaps the most difficult to hand-wave. As you could then say that it basically got a second roll of the dice, extending the habitability envelope by 50-100%.

4) Some form of RCB variable star with exceptional longevity. (This would also have precident, sinc the tidelocked world I mentioned before was in orbit around a variable star whose brightness varied over the course of civilisations, causing repeated collaspes as while the ecosystem was prepared for it, the civilisations weren't.)

5) "Just" a severe ice-age, (perhaps partly caused by the planet's core itself cooling...?)

The terrain that is focussed on "in-story" was a series of basically oversized giant's causeway basalt slabs, creating a highland area of geometric "tiles," so cooling of some sort following major volcanic activity is a logical factor.

 

[AotrsCommander]

Revised attempt. Does this seem a little more credible (or at least consistent.) Split a bit of difference. Treading the line of plausibility a bit, but at least I know I'm doing so and hopefully I'm still in the "extremely unlikely, but at least statistically possible" end of things.

After a few days of travel – marked again by a high speed FTL jump and a long in-system transit, the Agamemnon reached the freighter’s destination.

This was, to their surprise, a system that was documented in something other than minimal detail, if not well. The data was from the Lazerblaster data base and again was about a thousand years old, coming from the pre-Dormancy Gap period.
The system was a single main sequence star, with only a (meaningless to modern astrogation) designation. There were seven planets; three gas giants and four rock planets. Two of these – the outer most ones – were merely chunks of rock and ice.

The other two were not.

The first was a terrestrial habitable planet (which would now be classified as an HPE-B) with an oxygen/nitrogen atmosphere that was well within breathable limits and was very close to one G and with a 37 hour-long day. Life had evolved there. The biosphere was reported as being extremely old, the Agamemnon’s data entry suggested it was one of the oldest biospheres known. (It should be noted that there is a missing “of that star classification” in that superlative. Some planets around habitable red dwarfs have extremely old biospheres, though in fairness, the explorer who recorded the Agamemnon data millennia ago might not have been personally familiar with them.)

Data suggested the world was getting slowly cooler as time crept on, entering a permanent ice-age due to a combination of the cooling of the core and an erratic orbit which would only stabilise when the planet was at the fringes (or perhaps outside of) the planet’s habitability zone. In the current era, the only equatorial region and its attendant continent was free of ice, and the rest of the world was locked in thickening ice sheets. At the equator, the temperature was roughly temperate.

[talk about the venus hothouse planet omitted]

As an aside, the system itself is a somewhat unique one. The Agamemnon band merely skimmed the astrogation data they had and did not question why the habitable world was noted as being so old. But the actual reason for this is fascinating. From the astrogation data, it appears it the remaining main sequence star was part of a binary system, and the at least three of the seven planets initially orbited the other star. The system’s larger star, just as it was beginning to each the end of its main sequence and enter the giant stage, captured a rogue micro-black hole into an extremely close orbit. This began to draw off mass from the star. The habitable world had been on the interior of the habitable zone of the larger star (and getting hotter and hotter), and as the star’s mass was eroded by the micro-black hole, the orbit expanded.

Eventually, the primary either collapsed entirely or went into a mini-nova state, is unclear which. This caused the remaining planets (and perhaps others) to be ejected from the system – but they were caught by the secondary star. Incredibly, the habitable planet not only survived being dislodged from its initial home, but life survived the several million year trip as a rogue planet entirely from the heat from the planet’s own originally highly volcanic core. (At the cost of an almost total mass extinction, of course.) This explains the massive geometric tiles found on the surface. That the planet ended up in the habitable zone of the second star – even if erratically and perhaps not permanently – is a chance that might cause one to believe Harbinger probability engineering was involved. But what it achieved was functionally increasing the world’s habitability for multicellular life by perhaps as much as 50%.

(One of the researchers of this data requested – and was personally granted permission by none other than Lord Lungrender himself – to leak this information to the wider galactic community directed towards the Shardan, in the hopes that they would take an interest and perhaps preserve the planet one day, since it might be unique in the galaxy.)

Implied but not stated that the previously highly active core hasn't recovered from the period it was "between" stars (when life would have been basically mostly below ground or around ocean seeps and vents) which is why it's cooling now.

 

[Algr]

Interesting. If the planet had an advanced technological civilization in its distant past, that could explain lots of things. They may have worked to keep the planet's magnetic field strong, and as a side effect caused planetary cooling?

as the star’s mass was eroded by the micro-black hole, the orbit expanded.

This won't work. The mass would still be there in the micro-black hole, so the orbit of the planets would remain the same. All this stuff with the black hole and moving from one solar system to another sounds very complicated and implausible. The chance of any random vector in space ever passing near a star is surprisingly low. I'd step back, look what your story needs, and find the simplest answer that fits the bill.

 

[AotrsCommander]

The ancient race explanation is one that I didn't want to use; and in this instance wouldn't have worked, since the oldest known civilisation is around ony one or two million years prior*. And "Harbingers did it" is a bit too neat and easy.

Otherwise, then, back to the drawing board then.

Would something like a planetary impact be able to, for instance, move a planet from orbit to orbit of a binary (or perhaps from a circumbinary orbit to a regular one)? I know wer'e in the "very unlikely" score, but that was sort of the point for this planet, one that had rolled all natural 20s, as it were, on its checks for longevity.

(But never on the ones to develop intelligent life, perhaps dur to the repeated mass extinctions that would have allowed whatever events there were to allow it to have a longer habitability window than is typically expected.)
*This is because they used retrocasual probability engineering (or are at least extremely strongly believed to have done from available data, anyway) to make the universe always have bee the way it is. Thus, no other civilisation could predate them because they don't and never had existsed before the Harbingers. Which somewhat alarmed the Aotrs when they found something that appeared to date from BEFORE the Harbingers; which is metaphysically impossible. Only that where they found itself had a retrocasual probability generator and thus a protective field, but that doesn't make any sense because the point about retrocausal probability engineering is that it dosn't make a different present, it makes the the universe be the same as its always been because it reaches back in time to make sure everything works out exactly the way it always had done, so there was never a different past to have existed. It is, frankly, a terrifying concept, but only tangential to this case. The Harbingers could and did use it to literally make near-indentical copies of worlds down to the people up to a certain breakpoint and thus COULD do something like that, if I had wanted to go that route; but I am preferring that in this instance to have what happened be a "natural" role of the dice that falls outside Harbinger manipulation, however. (The Harbingers already have an incalcularly big impact, so much that I have taken pains to even say in-universe how potentially dangerous it is to attribute everything to them as being the most conveniantly explanatory one.)

 

[Algr]

Would something like a planetary impact be able to, for instance, move a planet from orbit to orbit of a binary (or perhaps from a circumbinary orbit to a regular one)?

An impact capable of doing this would also liquify the whole surface and perhaps create a moon. All life would be ultra-sterilized. See the Earth/Theia collision. However, a near miss with something of planetary size could move a planet via gravity. One of the Jovians could destabilize a planet in the outer solar system, shooting it inwards.

Gravitational encounters will "trade" orbital distance. For your planet to be drawn away from the star, something else would have to move closer. This would also tend to result in less circular orbits prone to more disruption, but if nothing else gets flung out of the system or into the suns, then they will eventually stabilize into more circular orbits.

Note that due to the three-body-problem, your two suns must be much closer together than any planet, or extremely far apart. For a system with three bodies to be stable, two of them must be close enough that the third can regard the others as a single object. I think the rule is 5:1 distance, but I'm not sure. So either your two stars are too close together to have any planet orbit just one, or they are so far apart that anything orbiting them both would be colder than Pluto.

If two sun-like stars were at Sun-Mars distance, there would be no stable orbits in the habitable zone, any planet there would be flung out of the system or collide with one of the stars.

 

[AotrsCommander]

Okay.

So, scrap the binary idea totally. Instead, then, how about an impact on another stellar body from, say, a captured rogue planet (or perhaps just one with a moon that spiralling in) that shoved said planet further in, and thus eased the habitable world out, as the star came towards the red giant stage (which it would be otherwise too hot for life, if I recall the video correctly) and the erratic orbit is starting to stabilise (but which would leave it on the very edge of the increasingly limited habitable zone?

(Maybe I'm a bit fixated on the idea of orbit changes, but the video introduced the concept to me and sometimes, these things don't let go...!)

 

[Algr]

When Sol becomes a red giant, the habitable zone will move out past Saturn. It will also be quite unstable over time, so the idea of a single planet keeping anywhere near it is rather unlikely. Orbital changes are plausible, but one of that extreme is rather unlikely. Close encounters with those jovians are unlikely to result in any stable orbit. By comparison, the binary star system isn't helpful, but doesn't actually cause any problems so long as you choose the right system. So keep the binary, loose the red giant? Or perhaps the star that enters the red giant phase isn't the one that the planet orbits? The consequences of that are beyond my knowledge.

Another thing that can slowly move a planet outward is capturing debris from the next orbit out. This happened to Neptune early on. I don't know if this is survivable to life. It would be if the debris belt is all dust, but not with Sol's asteroid belt.

 

[AotrsCommander]

Hrm. As promising as that idea (moving the planet around) was, I am beginning to think it's not working.

Okay, change tack entirely. What about a small star; larger than a red dwarf, but smaller than Sol, sufficient that the star has a habitiable range that (at least the far boundary of which) is isn't so close it pretty much has to be tide-locked?

 

[Algr]

Yes, keeping the science simple and vague tends to be a good way to stay out of trouble. You could explain the long lasting magnetic field by saying there are lots of heavy metals in the core. Or just not say anything about it, except that the field is still there.

 

[AotrsCommander]

Okay, then how about this:

The innermost world – the one closest to Agamemnon’s exit jump-point – was a terrestrial habitable planet (which would now be classified as an HPE-B) with an oxygen/nitrogen atmosphere that was well within breathable limits and was very close to one G and with a 37 hour-long day. Life had evolved there. The biosphere was reported as being extremely old, the Agamemnon’s data entry suggested it was one of the oldest biospheres known. It should be noted that there is a missing “of that star classification” in that superlative. Some planets around habitable red dwarfs have extremely old biospheres, though in fairness, the explorer who recorded the Agamemnon data millennia ago might not have been personally familiar with them. The age of the biosphere was due to the fact the star was significantly smaller than typically found in HPE systems. It was just over the border of being a dwarf star, just big enough for the habitable zone to be out of the distance for tide-locking. This meant that, while not as long-lived a red dwarf, the star had a longer lifespan and thus a longer habitability window than most worlds around main sequence stars.

Data suggested the world was generally cool, in a permenant ice-age, with only infrequent bursts of interglacial high temperatures, possibly caused by increased emissions due to solar flare activity. Due to the world’s age, the core was also losing heat, and volcanity was low. In the current era, the only equatorial region and its attendant continent was free of ice, and the rest of the world was locked in thickening ice sheets. At the equator, the temperature was roughly temperate.

Vastly simpler (and closer, I think to what the original intention was).

 

[Algr]

Looks good. Write what you know.
That's why my stories have no characters.

 

[AotrsCommander]

That's why my stories have no characters.

I mean, that worked perfectly for Spacecraft 2000-2100AD.

(To the point I follow suite over four decades later, because it went into my hindbrain and never left...)

 

[Jarvis323]

What about leaving it a scientific mystery. I.e. scientists believe life survived through the stars phases, but only have theories as to how. There could be different theories, and which one is correct, if any of them, or even if life did survive so long in the first place, doesn't need to be certain.

 

[AotrsCommander]

There's already a lot of that in the article this is part of. (Actually, MOST of said article is about the unexplained and unknown of the particular power its looking at.) While I often do use the "we just don't know why, but [some people] think x or y" explanation, like every other tool, it can be overused.

The planet in question is mostly just set-dressing, really, for a bit of the adventure that preceeded the exploration of the alien structure on the surface (and the ones on the second planet), but my detail-OCD required that I at least fixed the 2005-paragraph to not be complete nonsense by my current understanding.

(And hey, I've learned some more stuff, that's always useful.)

 

[Nik_2213]

Possible solution could be as face-locked mega-moon of Neptunian, beyond cold edge of usual hab-zone ?? Combination of internal heat and tidal stirring keeps 'planet' active, thick atmosphere keeps it warm, Neptunian's albedo provides some 'insolation for much of the long nights. Have a 'middling' K-type star, so its red-giant phase is deferred unto 'deep time'...

Happens this is yet-another solution to 'Where are They', as getting a nicely Moon-stirred 'terrestrial' planet in hab-zone would seem much, much rarer than getting big moons of Neptunians etc...

So, Sol-system would not tick ETs' boxes for a likely abode...

 

[Nik_2213]

There's another 'gotcha', which I belatedly noticed.

About ~5 million years ago, when Sol's galactic orbit entered 'Local Bubble', Earth acquired an ominous sprinkling of supernova spoor in polar ice...

IMHO, given the fire-front of this spreading, supernova-spawned 'Bubble' would seem utterly hostile to tech and space-flight, any star-flight capable cultures would have packed their bags, decamped.

As we would ahead of eg a well-forecast super-typhoon...

So, if you need long-term habitability, you also need galactic orbits that avoid such toxic 'Bubbles'.

Good luck mapping those !!!