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kzb
2013-Jun-06, 05:06 PM
This is an interesting paper. Can there be habitable planets in globular clusters, open clusters, in the galactic bulge and in the galaxy centre? This paper looks at the existance of planets in habitable zones given the probabilities in these areas of stellar close approaches.

The answers seem to be probably no, yes, yes, and no chance respectively. (Asimov's famous short story, later developed into a full novel, unfortunately looks an unlikely scenario.)

I was also interested to read about the theorised history of our own system with regard to stellar interactions and the evidence that it has happened.

Habitability in Different Milky Way Stellar Environments: a Stellar Interaction Dynamical Approach

http://arxiv.org/abs/1306.0464

Selfsim
2013-Jun-07, 07:11 AM
Why stop with considering only stellar interactions?

Just a glimpse at the complex effects Saturn's shepherd moons have on its rings, and the (theoretical) chaos in the orbits of our own planets over time, would suggest there's a lot more to it than just stellar interactions(??)

'Habitability' is a very pliable, bendy thing ... with very little considerations beyond the most basic astronomical physical parameters supporting it.

What are we really trying to do with this 'habitability' thing, anyway?

grapes
2013-Jun-07, 09:46 AM
The answers seem to be probably no, yes, yes, and no chance respectively. (Asimov's famous short story, later developed into a full novel, unfortunately looks an unlikely scenario.)
Since it's the title of the thread, I'm interested in how you came to the conclusion in the parentheses--is that a result of info in the paper? What do you mean by unlikely?

kzb
2013-Jun-07, 11:44 AM
Since it's the title of the thread, I'm interested in how you came to the conclusion in the parentheses--is that a result of info in the paper? What do you mean by unlikely?

Nightfall was set in a multiple star system in the central region of a globular cluster, as I remember it (not sure if this applies to the later novel, come to think of it).

The paper says that any planets in habitable-zone orbits around stars in globular clusters would be disrupted by multiple stellar encounters. I said "probably no", because there is the caveat that in the outer region of a globular cluster, there is a fair chance that such planets would survive. But then it goes on to say that the orbits of most of the outer stars will take them through the central region periodically. So the odds are stacked against the existence of the Nightfall habitable planet, but it may not be absolutely impossible.

kzb
2013-Jun-07, 11:53 AM
Why stop with considering only stellar interactions?

Just a glimpse at the complex effects Saturn's shepherd moons have on its rings, and the (theoretical) chaos in the orbits of our own planets over time, would suggest there's a lot more to it than just stellar interactions(??)

'Habitability' is a very pliable, bendy thing ... with very little considerations beyond the most basic astronomical physical parameters supporting it.

What are we really trying to do with this 'habitability' thing, anyway?

Yes this paper is not the be all and end all. But it does raise the profile of one of the factors in considering where habitable planets might exist. I found it interesting that habitable planets are quite possible within the galactic bulge (or bar), as long as the system is not too close to the galactic nucleus. At least when only stellar encounters are the only factor. Like you say, there are a lot more factors.

TooMany
2013-Jun-07, 05:12 PM
Yes this paper is not the be all and end all. But it does raise the profile of one of the factors in considering where habitable planets might exist. I found it interesting that habitable planets are quite possible within the galactic bulge (or bar), as long as the system is not too close to the galactic nucleus. At least when only stellar encounters are the only factor. Like you say, there are a lot more factors.

kzb, do you any estimate of the stellar density in the habitable areas of the bulge versus local density?

kzb
2013-Jun-08, 05:22 PM
kzb, do you any estimate of the stellar density in the habitable areas of the bulge versus local density?

Around here it is 0.13 pc^-3. In Fig 3 of the paper they have the galactic centre (not nucleus) as up to 100 pc^-3. This area is outlined as having too many encounters for habitability. Outside about 1kpc galactic radius it says things become OK. The stellar density is not the only factor though, the velocity dispersion comes into it as well.

publiusr
2013-Jun-08, 06:17 PM
Now if a conventional system wandered into a cluster, that might make for a different story--a tidally locked planet close to a red dwarf is close enough not to be tugged too badly.

ASTRO BOY
2013-Jun-09, 02:22 AM
Habitability in Different Milky Way Stellar Environments: a Stellar Interaction Dynamical Approach

http://arxiv.org/abs/1306.0464



Habitability is obviously hard to ascertain and define.......It has a habit of surprising us and I believe will continue to surprise us as we venture further afield.

On multi stellar systems I did find this.......

http://astrobites.org/2012/10/26/can-binary-star-systems-harbor-habitable-planets/

an extract from that paper is as follows.......

"In this paper, Eggl et al. delve into that question in detail by calculating the habitable zones around stars in the 19 nearest binary systems and determining whether orbits within the habitable zone would be stable. They find that Earth-size planets could exist on stable orbits in 17 of the 19 systems considered. That impressive success rate (89%) implies that real Earth-mass habitable planets in binary star systems might be almost as common as planets orbiting binary stars are in science fiction!"

eburacum45
2013-Jun-09, 12:59 PM
Stellar encounters would be problematic for many different types of potentially habitable (biocompatible) worlds. If a planet with liquid water has its orbit perturbed enough, the chances that life on that planet will survive are reduced; this would also apply to planets which are biocompatible because of liquid ammonia, or ethane, and so on. However a planet with tidally heated moons could be perturbed quite significantly without changing the sub-surface conditions on those moons very much. If life can emerge and/or persist on a tidally heated moon of any sort, that life might survive a close encounter, even one which propels the planet into interstellar space.

eburacum45
2013-Jun-09, 01:15 PM
One interesting idea that pops up in that paper is that an old cluster like M67 might have a distributed Oort cloud, shared between many or most of the component stars. An old cluster might have numerous stable planetary systems, and if life develops in one of them, it might be possible for life to spread to many other planets in that cluster via lithopanspermia. Or, alternately, this scenario might itself be vanishingly rare.

kzb
2013-Jun-10, 12:12 PM
Stellar encounters would be problematic for many different types of potentially habitable (biocompatible) worlds. If a planet with liquid water has its orbit perturbed enough, the chances that life on that planet will survive are reduced; this would also apply to planets which are biocompatible because of liquid ammonia, or ethane, and so on. However a planet with tidally heated moons could be perturbed quite significantly without changing the sub-surface conditions on those moons very much. If life can emerge and/or persist on a tidally heated moon of any sort, that life might survive a close encounter, even one which propels the planet into interstellar space.

To be fair, the paper concerns only "surface life". It is all about the habitable zone.

If you allow that tidally-heated moons are habitable (I agree biocompatible is a better word), then really almost anywhere would be habitable/biocompatible. Even tidal heating from a massive planet is not necessary apparently, because people are speculating on Ceres and Pluto having sub-surface oceans. There is also a paper on rogue planets retaining subsurface oceans.

Probably 95% of stellar systems would be biocompatible (and with an average of more than one biocompatible world per star), plus there are all the rogue planets of the correct size and age. You are probably looking at about a trillion biocompatible worlds in the galaxy.

I'm not disagreeing, this is an interesting point in itself, but the paper is not going down that route.

kzb
2013-Jun-11, 05:54 PM
kzb, do you any estimate of the stellar density in the habitable areas of the bulge versus local density?

Sorry my previous answer was a bit hasty. Figure 12 (left hand panel) is what you want for the bulge density.

Since they are saying habitabilty starts at around 1000pc radius, you can estimate on that plot that the stellar density at that radius is around 2 pc^-3. The funny thing is, the units are in M-sun pc^-3, rather than just n pc^-3. I don't know if this is a mistake in the paper or not. If it really is M-sun, you'd have to correct for the average stellar mass. Anyhow, as an order of magnitude I believe that is about 20 times the local density.

I've also followed up the reference where the galactic model is sourced. It's from 1998, and it is only a model. The units are not direct in that either.

TooMany
2013-Jun-11, 08:59 PM
Yes that is odd (M-sun) but maybe that is what they mean, density in terms of solar mass versus simple number density. There is a dependence on mass as well as stellar density in determining perturbations of planetary systems. I would presume that low mass stars could live together more comfortably than high-mass stars. If the mass spectrum is uniform, then it makes sense to use M-sun pc^-3 for the density.

Indagare
2013-Nov-17, 06:30 PM
Nightfall may not be entirely realistic. There are four stars that are in two sets of close binary system, the main yellow star and the tiny red star. If the orbits are all around some common barycenter then it might be possible, but even then the main world would almost certainly be too hot for life as we know it here - not to mention the fact that there's some other planet in that system with an extremely long orbit.

Of course, the point of the story was not really whether or not life on such a world was possible, but more what would happen if creatures that were used to continual daylight suddenly experienced darkness.

Ilya
2013-Nov-17, 10:27 PM
The most unrealistic part IMO is that nobody has ever seen the giant planet which blocks the sun during the story. It is seven times the sun's angular diameter. Earth's Moon the is same angular diameter as the sun, yet it is often seen in daylight. Granted, during the story's events the giant planet is between Lagash and the sun, so cannot be seen, but there should be many orbital configurations when it is in half phase -- and would be obvious in "daylight" sky.

R.A.F.
2013-Nov-18, 04:30 PM
Of course, the point of the story was not really whether or not life on such a world was possible, but more what would happen if creatures that were used to continual daylight suddenly experienced darkness.

I can't help but imagine the blind being unaffected, and what?...they had no windowless buildings?

I can't get past that, let alone the headache the physics involved would cause.



I love Asimov, but, for me, this is one of his least favorite stories.

DonM435
2013-Nov-18, 04:36 PM
I'd say that a dark building interior would be less terrifying so long as one knew that there was light just outside. But the idea of a natural darkness throughout the world would be tough on a mind that never considered it possible.

Noclevername
2013-Nov-18, 04:40 PM
I can't help but imagine the blind being unaffected, and what?...they had no windowless buildings?

I can't get past that, let alone the headache the physics involved would cause.



I love Asimov, but, for me, this is one of his least favorite stories.

The universal madness is the least realistic part of the story. More so than funny suns or humans evolving on an alien planet with different environmental conditions.

DonM435
2013-Nov-18, 05:50 PM
I'm only familiar with the original short story, which had the coincident setting of all five suns as a very rare event. I gather that the expanded novel has eclipses and other complications? I'll have to look for it.

R.A.F.
2013-Nov-18, 06:11 PM
The universal madness is the least realistic part of the story. More so than funny suns or humans evolving on an alien planet with different environmental conditions.

We see eye to eye. :)

ravens_cry
2013-Nov-19, 12:20 AM
There is something almost Lovecraftian about the story, of an implacable event unable to be prevented that leads to a mind-altering revelation. As for the universal madness, eh, hard to say if that's 'realistic' or not. Just because they look human does not mean they are human.

Noclevername
2013-Nov-19, 12:24 AM
As for the universal madness, eh, hard to say if that's 'realistic' or not. Just because they look human does not mean they are human.

Well, if they do have some significant mental difference from humans, it kind of ruins the point of the story.

ravens_cry
2013-Nov-19, 12:55 AM
Well, if they do have some significant mental difference from humans, it kind of ruins the point of the story.
Well, they obviously do, though I am not sure how it 'ruins' the point of the story.

Noclevername
2013-Nov-19, 12:59 AM
Well, they obviously do, though I am not sure how it 'ruins' the point of the story.

Because the story was about how people would react to nightfall, not how superficially humanoid aliens with mental differences would react. Nothing else in the story indicates they are anything other than normal humans.

DonM435
2013-Nov-19, 02:47 AM
Because the story was about how people would react to nightfall, not how superficially humanoid aliens with mental differences would react. Nothing else in the story indicates they are anything other than normal humans.

I'd read that a sore point with Isaac Asimov was that his editor, John W. Campbell, had inserted a paragraph into the first publication of "Nightfall" that mentioned Earth in passing. He felt that it was well-written, but inappropriate.

ravens_cry
2013-Nov-19, 07:26 AM
Because the story was about how people would react to nightfall, not how superficially humanoid aliens with mental differences would react. Nothing else in the story indicates they are anything other than normal humans.
They are people . . . that perhaps happen to be superficially humanoid aliens. Their reaction is likely alien, but it's somewhat plausible given their developmental history. Plenty of other science fiction stories have aliens having alien reactions to alien (or familiar) events, and they don't 'ruin' such stories.

Noclevername
2013-Nov-19, 08:43 AM
They are people . . . that perhaps happen to be superficially humanoid aliens. Their reaction is likely alien, but it's somewhat plausible given their developmental history. Plenty of other science fiction stories have aliens having alien reactions to alien (or familiar) events, and they don't 'ruin' such stories.

If Isaac Asimov had made it a story about aliens, he would have made it clear or at least hinted that they were aliens. He did not. Your conclusion that they are not psychologically human hinges entirely on the fact that their reaction to an unfamiliar panic-inducing situation was extreme.

Asimov is known for characterization that is sometimes skewed or exaggerated for effect; look at his early work especially, and the later Azazel stories. He coined the phrase "social Science Fiction" specifically for the short story version of Nightfall, indicating that it was about society, not fictional beings.

swampyankee
2013-Nov-19, 10:40 AM
The universal madness is the least realistic part of the story. More so than funny suns or humans evolving on an alien planet with different environmental conditions.

The universal madness isn't due to the darkness -- they've got caverns, closets, and basements -- but because of the stars.

ravens_cry
2013-Nov-19, 11:06 AM
If Isaac Asimov had made it a story about aliens, he would have made it clear or at least hinted that they were aliens. He did not. Your conclusion that they are not psychologically human hinges entirely on the fact that their reaction to an unfamiliar panic-inducing situation was extreme.

Well, they have names like none any human society uses, combinations of words and numbers, they live on a planet in the middle of globular cluster with no mention of space travel. Sounds pretty alien to me. Star Trek has given us less.



Asimov is known for characterization that is sometimes skewed or exaggerated for effect; look at his early work especially, and the later Azazel stories. He coined the phrase "social Science Fiction" specifically for the short story version of Nightfall, indicating that it was about society, not fictional beings.
The way I understand it, social Science Fiction is about examining society through the lens of parable and metaphor, using science fiction as the vehicle that lens. Think of the aliens from "Let This Be Your Last Battlefield" TOS Star Trek aliens. They were aliens, yes, but they were a parable about the foolhardy petiteness of human conflicts over what are usually minute details. It was about alien's reactions, yet it very much was social Science Fiction, yes?

Noclevername
2013-Nov-19, 01:03 PM
Well, they have names like none any human society uses, combinations of words and numbers,
As did most future humans in sci-fi at the time.


they live on a planet in the middle of globular cluster with no mention of space travel.
Previously addressed. Asimov deleted a line about Earth because of aesthetic reasons, not plot related ones.


Sounds pretty alien to me. Star Trek has given us less.

Read more Golden Age SF, it was not nearly alien enough to trigger a "they are not human" vibe to me.



The way I understand it, social Science Fiction is about examining society through the lens of parable and metaphor, using science fiction as the vehicle that lens. Think of the aliens from "Let This Be Your Last Battlefield" TOS Star Trek aliens. They were aliens, yes, but they were a parable about the foolhardy petiteness of human conflicts over what are usually minute details. It was about alien's reactions, yet it very much was social Science Fiction, yes?

And the Trek aliens had entirely human psychological reactions; obsession, hatred. The story would not have worked as allegory if the message was "these aliens do not think like humans".

Noclevername
2013-Nov-19, 01:08 PM
The universal madness isn't due to the darkness -- they've got caverns, closets, and basements -- but because of the stars.

The cause of the madness doesn't matter to me, the fact that everyone on the planet had an identical, equally extreme reaction does. Psychology Does Not work That Way. If it had been a physics error fans would be jumping all over it.

R.A.F.
2013-Nov-19, 02:53 PM
The cause of the madness doesn't matter to me, the fact that everyone on the planet had an identical, equally extreme reaction does. Psychology Does Not work That Way. If it had been a physics error fans would be jumping all over it.

Yep...the only reason I'm giving Asimov a pass on this is because he was all of 21 years old when he wrote it.

Of course if everyone on the planet didn't have the same reaction, then there wouldn't be much of a story to tell...

Githyanki
2013-Nov-20, 02:44 AM
I had to wiki nightfall; and I was sad they didn't mention Pitch-Black; that movie was obviously based off of nightfall and had an awesome set!

Noclevername
2013-Nov-20, 03:00 AM
Of course if everyone on the planet didn't have the same reaction, then there wouldn't be much of a story to tell...

The few sane people left, struggling to survive among a planet of madmen in the unfamiliar night without going mad themselves? Nah, no mileage there at all ;)

DonM435
2013-Nov-20, 04:57 AM
Yep...the only reason I'm giving Asimov a pass on this is because he was all of 21 years old when he wrote it.

Of course if everyone on the planet didn't have the same reaction, then there wouldn't be much of a story to tell...

Hey, it's a short story! It has to make its point without surveying every case.

Noclevername
2013-Nov-20, 05:13 AM
Hey, it's a short story! It has to make its point without surveying every case.

It was later turned into a novel, with Robert Silverberg and (a much older and more experienced) Asimov collaborating on the expanded material. IMO it was not as good as the original story, just padded for length. But the population-wide madness was kept. IIRC, I think the clichéd "universal sanity returning at dawn just in time to light up the ruins" scene was also added as an ending, making the psychology even more strained.