Limits for the habitable zone

  1. What are the inner and outer limits of the habitable zone in distance for the sun?
  2. jcsd
  3. Drakkith

    Staff: Mentor

  4. Would these models take into account different albedos, geological activity and atmospheric pressure? The habitable zone from what i understand is complex.
  5. Chronos

    Chronos 9,758
    Science Advisor
    Gold Member

    Those are naive estimates based on unsupported guesses on statistical parameters.
  6. Im my mind i would think that if Earths atmospheric pressure was raised to 2 bar it would raise the boiling point making the habitable zone closer to the sun. Even if there were more clouds in the atmosphere they would reflect more of the suns rays making the zone closer as well. The albedo of Venus is .70 while Earth is .30-.35 which means Earth recieve more sunlight.
  7. Drakkith

    Staff: Mentor

    I guarantee you it isn't that simple.
  8. I just find it intesting that Earth is considered on the inner part of the zone when we have had ice ages that covered a great deal of our planet.
  9. Life could exist at 3au if it evolved there but the ecology of that life system would be very different to that of the earth. The length of the days and years are intrinsically bound up with life forms on earth. If for some reason the earth moved to 3AU with or without the moon I believe 99.9% of life on earth would cease to exist for a myriad of ecological and biological,seasonal reasons, but having said that, some forms life would likely "find a way" and evolve on a different evolutionary ladder.
    Last edited: Mar 1, 2013
  10. The other thing to take into consideration is that the earth has a molten iron outer core which gives 'us' a magnetosphere. This prevents our delicate atmosphere from being blown away by the solar wind! Take away 'our' magnetic field, and life as we know it here would be vastly different regardless where we sit in the habitable zone.

  11. I agree Damo, we seem to have hit a billion to one sweet spot. What I wonder are the odds that it exists anywhere else in the universe? The thought that we are alone is a very foreboding one don't you think!?
    Last edited: Mar 1, 2013
  12. This article shows how they go about calculating the habital zone. It also includes atmospheric and other considerations.

    Its also nice and recent. Covers alot of considerations.
    Last edited: Mar 1, 2013
  13. I have no doubt that there is life outside our miniscule point of reference in our local group of stars, let alone the unimaginable expanse of our galaxy, to the incomprehensible size of the local group of galaxies, and then onto 99.99% of the rest of the universe which is outside of that! Have no fear, we are not alone!:smile:

    The 'uniqueness' of our present solar system layout is based on what 'we' think would be typical, and it is hard to imagine that all the critical bits of the gigsaw are common to other systems, but the more that is found about other systems, the less unique our one becomes. It all comes back to the idea that our sun is just a typical star in a typical patch of space, which formed a set time in the past from a nebula of typical space lego. We are not so special.

  14. I don't disagree that there's a possibility Damo, after all "we" happened. It's the probability of life even remotely evolving along our lines. Most people envisage highly sophisticated extraterrestrials with advanced warp-drive ect, how often do people consider that the Human race are in fact the most advanced beings in the universe?
  15. Drakkith

    Staff: Mentor

    That would be an interesting thing to discover.
  16. Chronos

    Chronos 9,758
    Science Advisor
    Gold Member

    The technology required to detect a civilization like our own is a daunting task. Even assuming a planet with a history and conditions identical to our own does not guarantee an identical evolutionary outcome. After all, we humans were on the brink of extinction a mere 75,000 years ago. I sort of like the 1 in a billion odds. Sounds reasonable. The problem arises when you factor that into the billions of years this galaxy has existed. That suggests there may have been as many as 400 civilizations as advanced as our own in the history of our galaxy. It is also pretty obvious they were spread out over billions of years. This makes 400 in a billion look like a pretty small number. I would hazard to guess that only a very few occupy our galaxy at any given point in time. And that raises another question, how many of those very few actually have the resources and inclination to agree on the merits of investing capital to contact ant hills in Africa? So my point is human like intelligent life is probably extraordinarily rare in our galaxy at any given point in time, and, few [if any] are near enough to succeed in a SETI endeavor.
  17. I agree whole heartedly. Life, and intelligence are 2 vastly different things as we are well aware. I would expect that anyone with a basic understanding of the evolutionary process on our planet (which has enabled life to get to the stage where it is today), wouldn't be expecting any life we potentially find in the nearest 1000 suitable systems to contain anything more than the simplest bacteria, if anything at all.
    It might be possible that at the present time we may be the most advanced species in our Galaxy, but that will be dependent on a few things. If interstellar space travel proves to be the massive hurdle it appears to us to be at present (which affects all intelligent species around the Milky Way the same), and there is no solution to the practical speed limits imposed by physics regardless of technological advances, this would mean that any intelligent species could be wiped out completely by one single asteroid, never to explore again. This would vastly reduce the timescale that 'other' life is actually around for us to see it. As Chronos has pointed out, using probability doesn't make the likelyhood of co-existing intelligence in the Galaxy at the same time promising. One thing though, if you consider that the timeframe of evolution 2nd and 3rd generation star systems and corresponding planets are in the same ballpark time wise as our sun, I think this would shorten the odds of intelligent life existing at the same time as us.
    If interstellar space travel is a scientific step away, then I would expect other civilizations to be already around the Galaxy.
    Who knows, our planet may have already have been canvased in the past billion years as a potential 'life harboring planet'. If this were the case, I would expect that the visitors would leave some form of calling card, probably not on the surface as they would be smart enough to understand the geology of the planet, but somewhere (like the surface of the moon) where whether and geological process wouldn't erase their calling card.

  18. Life in general is extremely sensitive. Even the simplest 'hardcore' lifeforms (extremophiles) are prone to temperature and pressure.

    We can envision a more complex and intelligent lifeforms because it is 'allowed' prior to our latest understanding. It is mathematically possible and indirectly provable.

    We are one transition(life) of that massive computation(nature). Such transitional products are not divisible to one type even given extreme sensitivity because that processes(life) is extremely adaptive too. If time is the only considered variable in comparison to our present knowledge then it is more unlikely (quantitatively) that we are the peak of cognitive evolution.
  19. mfb

    Staff: Mentor

    An asteroid cannot destroy a whole solar system, but intelligent life can destroy (or even use) an asteroid.

    The average lifetime of intelligent and technological species is a completely unknown factor. Is it of the order of 100-1000 years, because they all tend to kill themselves as soon as nuclear power and similar things are available? Is it of the order of millions of years? Is there a reasonable probability that the species (or some descendants) will exist for billions or trillions of years?

    Life exists everywhere on our planet (excluding the molten interior), and there are indications that life from earth might be able to live permanently on Mars. I would not call this "sensitive" - we don't even know where the limit of our life is. Life which evolved in different conditions would be able to adapt to different conditions.
    Sure, you cannot take a bacterium which is adapted to live in rocks 3km below the surface and expect it to survive somewhere in the antarctic ocean. So what?
  20. Not really. All P-life 'as we know it' has that degree of sensitivity until given the opportunity(It will happen eventually) to adapt gradually under conditions(local/General) while most of them died and became extinct. Case of Mars. In general. It is different to earth but such local criteria has similarity to earth which is compatible to certain life. We can say that simple lifeform thrived and has lesser biological limitations compared to most complex P-life that requires a larger condition to exceed.

    I agree we don't know the limits to life but we do have a positive hint and a critical analysis on conditions to life (P-based life)to which we can draw local limits like biological potential and/or multiple adaptations of certain species. Question like; "Do they have the intermediate profile that enable them to adapt in those conditions?"

    Well. Adaptation-mutation is a gradual process you don't expect a fish to develop limbs or wings overnight (Tiktaalik roseae) or drain your fish tank and expect them to grow limbs or more so survive :biggrin:. Same is true with bacterium adaptation?
    Last edited: Mar 5, 2013
  21. mfb

    Staff: Mentor

    I don't get your point. There is no place where all organisms can survive. How does that matter if we consider places where life is possible?
    There is no place on earth where life is impossible (again, excluding the molten interior).

    What is "P-life"?

    If Mars is similar to Earth, there are similar planets on many stellar systems.

    Sure. Life on other planets will be adapted to the conditions of that planet, as it always faced those conditions during its evolution.
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