Inhabitable planets at doable distances ever or somewhere?

  1. In our time epoch and location in space, it seems that even if we received signals of some extraterrestial civilization, or detect some extrasolar inhabitable planet, unless we would achieve some huge breakthroughs it would be impossible to ever visit each other.

    But can we conceive of other places where star density is much higher than in our vicinity, or perhaps much earlier epochs when the universe was much smaller so all stars were much closer to each other, where several inhabitable planets may have been close enough to each other so as to make space travel between them for any eventual advanced civilizations developing in them feasible?

    A related question, would life have been possible at those much earlier epochs when star systems were much more densely packed? As from which age of the universe would heavy elements be present in enough quantity so as to permit life to emerge in suitable planets? Or were cosmic cataclysms too frequent for any emerging life to survive for long enough?
  2. jcsd
  3. mathman

    mathman 6,754
    Science Advisor
    Gold Member

    (1) Early universe had no elements other than H, He, and a little Li. Other elements are created in stars.
    (2) Intelligent life seems to require a long time to evolve. Our planet is ~ 4.5 billion years old. Space travel is ~ 50 years.
  4. mfb

    Staff: Mentor

    A higher density of stars is problematic for the stability of planetary orbits.
    An easy way to get civilizations to be able to visit each other are systems with more than one inhabitated planet / moon. If Mars would have life similar to Earth, I'm sure we would have been there already.

    With just one known sample of life, this is hard to answer.
  5. Thanks, but, are not in the current epoch (I mean somewhere else, perhaps very far away but at the same epoch from the BB as we are, so there has been time enough for enough heavy elements to form and for intelligent life to develop) where stars are much closer to each other? Surely closer to the galactic centers star systems are more densely packed, right? Sitting in one of those planets one would see a relatively bright sky even in the 'night' due to the many stars at close distance, and suns with inhabitable planets might not be too far away?
  6. I would not automatically rule out the possibility of inter-stellar travel for humans. I agree it would require breakthroughs, but even setting aside traveling at a significant percentage of c, in order to benefit from time dilation, there are at least three possibilities:
    1. Generation ships.
    2. Hibernation.
    3. Frozen embryos and educator robots.

    You have also ignored the possibility that if there are other intelligent species, they may have much longer life spans, in which case a travel time of a century or two may not be a concern.
  7. I'm afraid "too far" is relative to travel speed. People traveling just to Europa or Titan is a very long way away. Presently it takes roughly 3-5 years to traverse the distance (roughly 4 AU) to Europa at currently practical speeds. It would take roughly 20-35 years to get to Pluto at an average of 40 AU. You might reach the Oort Cloud at a little more than 5,000 AU if you could live to be 400 years old, and we are still not even to close to the nearest star. The Oort Cloud is unimaginably big, stretching from roughly 5,000 AU away to a whopping 100,000 AU.

    Voyager 1 didn't have to accelerate people and supplies to keep them alive for almost 40 years, so coming in at well under 1 ton, it was practical to achieve speeds useful in the so-called slingshot gravity assist boost. It has attained the incredible speed of 17 km/s, roughly 38,000 mph. Getting human occupied craft to such speeds is a problem for which we have no practical solutions yet. Even at that incredible and presently impractical speed Voyager 1 has traveled for almost 37 years, it has traversed only 128 AU. This is not meant to demean it's amazing achievement, but only to put into perspective how far we are from anything remotely resembling Interstellar travel.

    In another thread there is a link to a wonderful but very humbling graphic representation of how far our earliest radio waves have reached within our galaxy at essentially lightspeed for roughly 100 years. It's BIG out there and we have barely begun.
  8. mfb

    Staff: Mentor

    Gravitational slingshots work the same way for every spacecraft mass (as long as it is tiny compared to the planet masses...), but the resulting speed is limited to the order of 10km/s (30 000 years per light year).
    Faster spacecrafts could still use this, but it would just be a tiny contribution to the total velocity.

    If reaching a star is our goal, it is pointless to send something now - it would be surpassed by faster spacecrafts long before it even comes close.

    If you have planets in stable orbits there (and without constant bombardement by asteroids) - that is a big "if".
  9. Chronos

    Chronos 10,348
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    Lacking some rather stunning technological advances, the prospects of human travel beyond our solar system are slim. The energy requirements, as well as economics involved, are impractical.
  10. mfb

    Staff: Mentor

    We had many stunning technological advances in the past. I would be surprised if we didn't have some in the future.
  11. I'm aware that gravity slingshots are also practical for manned craft but that doesn't change the entry speeds required to reach the velocity of Voyager (let alone higher ones) and vastly more mass requires vastly more energy which translates into longer burns which equals heavier fuel requirements.

    My main point is that given these vast distances chemical rockets will not suffice.
  12. Interesting! Anyone worked out what percentage of stars might be relegated from having a "second earth" through being in an over-dense region? We are on the outer edge of our galaxy, which is a fairly loose spiral. Maybe only stars in such a position can have an earth?

    This might have repercussions on the Fermi question: "Where are they?"
  13. mfb

    Staff: Mentor

    This is related to the topic of the galactic habitable zone. Reference 16 there deals with gravitational perturbations.

    There are billions of stars with an environment similar to our sun, so this does not answer the Fermi paradox.
  14. Chronos

    Chronos 10,348
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    Gold Member

    The pessimistic, yet, possible answer to the Fermi question is interstellar space travel is a technological challenge that has not been solved.
  15. Longer life spans could also result from technological advances for humans. I would be shocked if human life span was not lengthened to indefinite in a thousand years time.
  16. Chronos

    Chronos 10,348
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    Gold Member

    Cybernetic life is an intriguing possibility often exploited in SyFy. The prospect of seeding the universe with self replicating nanobots has also been examined. That still fails to explain 'where are they?'. If you reject 'we are alone' arguments, the logical conclusion must be interstellar travel is incredibly difficult.
  17. Remember that space is very big, and there are lots of things in it. Next, keep in mind that our resolution for astronomical observations over such large distances is still pretty lousy... we have a pretty difficult time finding entire planets more than a few solar systems away. More of an issue is the time frame during which we've been looking. A sixty or seventy year window over the span of hundreds of millions of years is pretty narrow... and that does not even take into account whether or not we've been looking in the right direction.
    Of course, yes, there is the fact that interstellar travel is extremely difficult. Maintaining interstellar societies, given communications constraints, is even more difficult.
    Next, consider resources. Most of space is very empty, with only radiation, and a few atoms (at most) per cubic meter, on average. This limits an organisms ability to spread. You go where the available resources can take you. Also, large societies eventually learn not to waste resources.
    So, what does all this mean? It pretty much means that we are not going to find a society outside of our local star group unless the members of that society want to be found; that they are, and have been, making a long term campaign to make contact; and that we continue to look, and make our own long term campaign to establish contact. Oh, yes... not just long term... we also have to send out messages in all directions, virtually continuously.
    Cyber or organic societies are going to be largely grouped around their available resources. We can't reliably spot planets in the scales involved... so how are we going to spot the societies? Advanced societies are going to strive to use less and less energy to communicate. Stars put out a lot of energy. Trying to sift out energy signals for stray communications signals will be virtually impossible... only intentional deep-space signals will have sufficient resolution to decypher messages.
    Over time, a few probes might make it to Earth, if a society is curious enough, and has sufficient resources, to pursue interstellar exploration on a large scale. But that does not help us if they arrrived more than a few thousand years ago... et alone a few million years ago. There would likely be nothing left for us to find... or anything left would be long buried. Archeologists are still trying to find evidence of things that happened just a few hundred years ago.
    I hope this over simplified explanation helps put things in a little perspective.
  18. In addition to the above well-defined issues (and Time is a huge one) there is also the glaring assumption that we are "listening on the right bands". Isolated neolithic level native cultures on Earth would not have detected smoke-signals from Industrial cultures, and much of what they may have seen was attributed to something else, due to sheer technological ignorance. We have only to imagine what someone from just a few decades difference could gather from us. Someone from the 1950s, even a scientist, might have great difficulty figuring out how to read a CD.
  19. mfb

    Staff: Mentor

    Well, 1950 is a bit late for that. They had proper microscopes to see the individual bits (even electron microscopes - 1931), and the general idea is very similar to vinyl records (~1920, with similar systems before). Sure, reading billions of bits is not an easy task, and the data format was not invented yet, but it would have been obvious that this thing contains a huge amount of information.
  20. Agreed that a tech savvy person from 1950 would know a CD contained a huge amount of information and even "the man on the street" would guess by it's shape and the hole in the center that it has the obvious similarities to vinyl records and stored information. I'm not trying to say that the difference is so great that it would be thought of as "magic", just that it could not be read and verified since the technology to do so (both hardware and code) did not exist yet and wouldn't for decades.The best Science could do then was state "We think it contains huge amounts of information, but of what kind and for what purpose we have yet to determine".

    One only has to go back another 50 years to where even the materials would seem near magic and it's purpose all but undecipherable, and this assumes technologies that are consistent with humans 50 - 100 years apart. Follow the progression to 500 years, 1000, 10,000. Then add in an entirely alien lifeform, and even if we assume that the Laws of Physics dictates that technological advancement must have some similarities regardless of biology and culture, the room for rather vast differences seems obvious.

    This is not postulated as a reason not to look, mind you... just a recognition of the immensity of the task so that we have sufficient perspective to not get too discouraged that we have yet to find anyone. It is far too soon to start imagining "Geez, maybe we really are alone" or worse, "Maybe no civilizations have survived much past nuclear level technology". We have taken a very small bite of a VERY large pie in any and every conceivable dimension.
  21. Chronos

    Chronos 10,348
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    Gold Member

    That does not change the fact that science, as we know it, is temporally bounded on earth. While lack of evidence is not absolute 'proof' of anything, it is proof our current knowledge is insufficient to answer a large number of questions
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