The science behind the probability of extraterrestrials

[Dauphin]

Re the Anthropic principle. Just six numbers.

[Boris Godunov]

Originally posted by Dauphin
Comet activity has been suggested as a seeding for life though (extreme case being panspermia, not so extreme being just the provision of raw materials for life), so increased activity could lead to a peak of life bearing planets before it becomes overly destructive.

Like Lotm, I was referring to advanced forms of life - i.e., sentient species. While such activity could be a breeding ground for primitive life, it could also reach a point of diminishing returns where the conditions made anything other than extremophile life forms unlikely. A world that gets hit by a comet every 50,000 years probably isn't going to produce an intelligent species.

[Dauphin]

I appreciate your point and agree to an extent. Although if that logic were applied to the Earth 4-5 billion years ago you would conclude that it wouldn't be able to produce sentient life (which at the time it wasn't). It was having major impacts on a continual basis - as the moon is testament to, and of course there was one huge one called Orpheus.

[DanS]

I think it's fun to play around with the Drake Equation, but it is unscientific in the extreme to try to say much of anything based upon a sample of 1.

[MichaeltheGreat]

Re: The science behind the probability of extraterrestrials

Originally posted by MrFun
Here is something to start off with. The linked Wikipedia article below explains why lifeforms would be less likely with silicon or ammonia elements. Does anyone disagree and think differently?

Building blocks for lifeforms

Ammonia is not an element. And where exactly do you think the "amin" root of amino acids comes from? (Amines, i.e. NRRR, NHRR or NH₂R groups)

Silicon is ridiculous as a prospective base for evolution of life at any level, due to its physical properties at the atomic level and the resulting effect on silicon chemistry.

Carbon is widely abundant and supports so much more chemistry than any other element or root compound that it is pretty pointless to assume life that isn't carbon based.

[MichaeltheGreat]

Originally posted by Boris Godunov
Yeah, I read that we happen to be in a relatively stable, safe portion of the galaxy that is not subject to the turmoil that most parts are. For one thing, most areas are subject to a lot more asteroid/comet activity, which makes the presence of a life-bearing planet a more hazardous proposition.

We don't have a clue about asteroidal or comet activity around any other star, so we have absolutely no idea of such things on a galactic scale, as they are purely local phenomena particular to individual star systems.

There are some nasty areas of the galaxy near its core and in some energetic star-forming regions, but most areas of the galaxy are likely to be pretty quiet, although we can't see much of anything on a small scale in other spiral arms.

The common hazards at the galactic and stellar scale are:

Excessive gravitational interactions in dense regions of the galaxy (mostly near the core of the galaxy, but that has a lot of older stars anyway)

Local gravitational interactions from systems of multiple stars, which severely limit the number of potentially stable planetary orbits, especially those close enough to any of the stars to receive infalling radiation.

Large amounts of high energy radiating in active star forming regions. This isn't necessarily a big deal, except in extreme cases, as any star near main sequence should produce adequate stellar wind to deflect most or all extrasolar radiation up to the mid to far ultraviolet range, which is typically the highest energy level found in most star producing regions.

If you take all these factors into consideration, there are still many millions of candidate stars in this galaxy alone.

[Boris Godunov]

Originally posted by Dauphin
Although if that logic were applied to the Earth 4-5 billion years ago you would conclude that it wouldn't be able to produce sentient life (which at the time it wasn't).

Ah, but that's precisely the point of the "rare earth" hypothesis. While conditions were volatile here 4-5 bya, due to our sun being formed and the relative youth of the galaxy, things settled down for a long stretch of time that is unusual for the galaxy. Most regions of the galaxy experience far more turbulence on a far more regular basis. Consistently calm areas like ours are very rare, hence the arising of sentient beings will likely be rare.

[MrFun]

Re: Re: The science behind the probability of extraterrestrials

Originally posted by MichaeltheGreat


Ammonia is not an element. And where exactly do you think the "amin" root of amino acids comes from? (Amines, i.e. NRRR, NHRR or NH₂R groups)

Silicon is ridiculous as a prospective base for evolution of life at any level, due to its physical properties at the atomic level and the resulting effect on silicon chemistry.

Carbon is widely abundant and supports so much more chemistry than any other element or root compound that it is pretty pointless to assume life that isn't carbon based.

First, sorry for the misuse of words when I mentioned ammonia.


Second, you presumed that I already believe that ammonia or silicon-based lifeforms are possible when in fact, all I did, was posted a link that explained why such lifeforms are not possible, and merely asked what others thought.

So thank you for posting your thoughts. By the way, you seem to be slacking -- you post much longer posts than this usually.

[chequita guevara]

I've read there's really only a small belt around the galaxy that would be suitable for supporting life, including approximately 35% of our galaxy's stars. Too close in to the core and you have too much radiation. Too far out and you don't have enough supernovae to seed the stars with heavier elements.

[MrFun]

Originally posted by Boris Godunov


Ah, but that's precisely the point of the "rare earth" hypothesis. While conditions were volatile here 4-5 bya, due to our sun being formed and the relative youth of the galaxy, things settled down for a long stretch of time that is unusual for the galaxy. Most regions of the galaxy experience far more turbulence on a far more regular basis. Consistently calm areas like ours are very rare, hence the arising of sentient beings will likely be rare.

But maybe we're mistakened about the rarity of calmer areas of the universe.

Just because we haven't discovered other areas of the universe that could be as calm as ours, doesn't mean they are not out there. You said that most regions in our galaxy has much more turbelence compared to our part of the galaxy. We could very well discover a galaxy that is different from ours though, in that it could have calmer areas.

[MichaeltheGreat]

Originally posted by Boris Godunov


Ah, but that's precisely the point of the "rare earth" hypothesis. While conditions were volatile here 4-5 bya, due to our sun being formed and the relative youth of the galaxy, things settled down for a long stretch of time that is unusual for the galaxy. Most regions of the galaxy experience far more turbulence on a far more regular basis. Consistently calm areas like ours are very rare, hence the arising of sentient beings will likely be rare.

You're confusing galactic scale and age and stellar system scale and age.

Out on the arms of a standard spiral galaxy, there's not a hell of a lot going on after the first few billion years of the galaxies age - most of the compression of gas that results in active star forming regions has occurred (and that star formation has absorbed/dispersed the shock waves of the compression), large numbers of first and second generation supernovae have occurred, seeding the area with heavier than iron elements and creating additional shockwaves to drive gas compression, etc. Most of the area of our galaxy's spiral arms is essentially free of galaxy scale hazards and disruptions.

The rest of it is a matter of local stellar system characteristics, and of course, a young main sequence star in its first few hundred million years of life is not going to be an environment for intelligent life at that time. The point is that out of the total lifespan of the typical main sequence star in a single-star system, there will be a few billion relatively calm years for things to happen.

[MichaeltheGreat]

Originally posted by chegitz guevara
I've read there's really only a small belt around the galaxy that would be suitable for supporting life, including approximately 35% of our galaxy's stars. Too close in to the core and you have too much radiation. Too far out and you don't have enough supernovae to seed the stars with heavier elements.

If you take 35%, (it's probably less than that, but we've got 10^11 stars to play with, so what the heck. , and assume 99.9% of those are unsuitable as even candidates, due to being multiple star systems, non-main sequence, etc., then you still end up with millions of candidate stars.

[DanS]

I've read there's really only a small belt around the galaxy that would be suitable for supporting life, including approximately 35% of our galaxy's stars. Too close in to the core and you have too much radiation. Too far out and you don't have enough supernovae to seed the stars with heavier elements.

Yeh, that leaves us with only 35 billion stars.

[DanS]

Good cross-post.

[Boris Godunov]

Originally posted by MrFun
But maybe we're mistakened about the rarity of calmer areas of the universe.

Oh, absolutely, they could be. I was just reiterating what those who propose a "Rare Earth" say. They could very well be wrong.

MTG: The notion that it's "galactic scale" isn't the only issue when it comes to rare occurences of life-sustaining worlds, but was just one aspect I mentioned as part of the whole. But I have read that extensive portions of the galaxy are within "clouds" of greatly increased comet/asteroid activity that make them much more dangerous.

Well then, what are the special attributes of Earth that we have to take into account when attempting to run this calculation?

Proper distance from the star. If a planet orbits its sun too closely or too far away, liquid water would not exist. There isn't much margin for error here: a change of 5 to 15 percent in Earth's distance from the Sun would lead to the freezing, or boiling, of all water on Earth.
Proper distance from the center of the galaxy. The density of stars near the center of the galaxy is so high, that the amount of cosmic radiation in that area would prevent the development of life.
A star of a proper mass. A too-massive star would emit too much ultra-violet energy, preventing the development of life. A star that is too small would require the planet to be closer to it (in order to maintain liquid water). But such a close distance would result in tidal locking (where one face of the planet constantly faces the star, and the other always remains dark -- as with the moon in its orbit around Earth). In this case one side becomes too hot, the other too cold, and the planet's atmosphere escapes.
A proper mass. A planet that is too small will not be able to maintain any atmosphere. A planet that is too massive would attract a larger number of asteroids, increasing the chances of life-destroying cataclysms.
Oceans. The ability to maintain liquid water does not automatically imply that there will be any on the planet's surface. It looks like Earth acquired its own water from asteroids made of ice that crashed here billions of years ago. On the other hand, too much water (i.e., a planet with little or no land) will lead to an unstable atmosphere, unfit for maintaining life.
A constant energy output from the star. If the star's energy output suddenly decreases, even for a relatively short while, all the water on the planet would freeze. This situation is irreversible, since when the star resumes its normal energy output, the planet's now-white surface will reflect most of this energy, and the ice will never melt. Conversely, if the stars energy output increases for a short while, all the oceans will evaporate and the result would be an irreversible greenhouse-effect, preventing the oceans from reforming.
Successful evolution. Even if all of these conditions hold, and simple life evolves (which probably happens even if some of these conditions aren't met), this still does not imply that the result is animal (multi-cellular) life. The evolution of life on Earth included some surprising leaps; two worth mentioning are the move from simple, single-cellular life to cells which contain internal organs, and the appearance of calcium-based skeletons. It appears like the first of these leaps took more time than the evolution from complex single-celled life to full-blown humans.
Avoiding disasters. Any number of disasters can lead to the complete extinction of all life on a planet. This include the supernova of a nearby star; a massive asteroid impact (like the one that probably caused the extinction of dinosaurs, and 70% of all other life-forms at the time); drastic changes of climate; and so on.
There are also a few attributes that seem, at first, to be completely unrelated to life and not required for its development. Ward and Brownlee argue strongly for the importance of the following attributes:

The existence of a Jupiter-like planet in the system. Apparently, Jupiter's large mass attracted many of the asteroids that would have otherwise hit Earth. Could life evolve in a system with no Jovian planet? On the other hand, too many Jovian planets, or one that is too large, could lead to a non-stable solar system, sending the smaller planets into the central sun or ejecting them into the cold of space.
The existence of a large, nearby moon. Apparently Luna, Earth's moon, is atypically large and close. Both of Mars's moons, for example, are minor rocks by comparison. What does this have to do with life? Well, it turns out that Luna kept (and still keeps) Earth's tilt stable. Without Luna, the tilt would have changes drastically over time, and no stable climate could exist. If the tilt would have stabilized on a too-large or too-small value, the results could also be disastrous; Earth's tilt is "just right".
Plate tectonics. Surprisingly enough, it seems like plate tectonics are required for maintaining a stable atmosphere. Plate tectonics play an important role in a complex feedback system (explained in detail in the book) that prevents too many greenhouse gases from existing in the atmosphere. No other planet (except maybe for Jupiter's moon Europa) is known to have plate tectonics. Is this a rare phenomenon, but required for life?

One of the interesting claims here is that, without a Jupiter-sized gas giant to "soak up" asteroids, our solar system would likely be uninhabitable due to asteroid collisions.