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MichaeltheGreat
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Apolyton Grand Executioner
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mooning the house that Ruth built.
Oct 1999
time: 21:36
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quote:
Originally posted by Boris Godunov
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.
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That's something in the nature of sheer quackery or unfounded dark matter speculation.
We don't have the slightest basis for detecting any information about relative density of small objects outside our solar system - hell, it's been enough of a struggle to positively identify brown dwarfs or directly image a planet (very close and superjovian mass, no less), and those are objects billions of times the mass of asteroids/comets.
quote:
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. |
That's somewhat true, but not entirely, and in many cases, the presence of Jovian planets may actually be detrimental, assuming earthlike planets have a mechanism for forming without them.
Asteroids may be remnants of planetary collisions but are more likely excess accretion disk material. As such, they generally have fixed orbits, and form belts due to minor gravitational interaction over time - we have asteroid belts between every inner solar system planet, and between Jupiter and Saturn, although objects are pretty sparse in all of these belts other than the well known one between Mars and Jupiter. Presumably, tens of millions of orbits swept out the excess accretion disk material in the same orbit as each inner planet, because the belting effect we see is consistent with that explanation, and not as easily explained by any other.
Comets and odd items (planetary ejecta, such as the approximate Mars-size body that sheared the moon's material off the earth) have many probable origins, but their "odd" orbits are almost certainly a result of gravitational ejection and recapture - in other words, interaction with Jovian scale planets is a large part of what has put them into potentially colliding orbits in the first place.
Models of accretion disks which account for conservation of energy and angular momentum don't give rise to lots of oddball orbits - that's an interactive phenomena, and a stellar system with a single sun and no Jovian planetary mass would have a harder time accreting small planets, but would not have near as much in terms of relatively large object collisions and disruption of orbits.
In our solar system, gravitational interactions of the Sun, Jupiter and Saturn are such that neither Mercury nor Mars are in truly stable orbital paths - they are very close to stable, but not completely, so that's another potential negative to Jovian planets.
What we've seen from discovery of extrasolar planets, particularly the pulsar planetary system B1257+12 indicates we don't know near enough about the possible ways in which planets form to really know if the gravitational interactions from a Jovian mass object are necessary to create compression and banding in accretion disks necessary to form earthlike planets - there may be other ways this effect can happen.
Another point is that mass-extinction phenomena and collisions are not bad things. If you look at the earth's history, each mass extinction phenomena for which we have definite knowledge in the fossil record opened up many new ecosystems and opportunities. Wiping out all species and starting from scratch isn't ideal, but wiping out a lot of dumb species that have achieved dominance through something other than intelligence is useful in creating evolutionary opportunities for the survivors.
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Lancer
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Oregon Coast, USA! or Bohol, Philippines!
Apr 1999
time: 05:36
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Already exist do skull sucking locust aliens. Coming for us they are.
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MichaeltheGreat
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Apolyton Grand Executioner
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mooning the house that Ruth built.
Oct 1999
time: 21:36
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quote:
Originally posted by Boris Godunov
The basis, IIRC, has to do with the Oort Cloud and the density of stars in the galaxy. As the density of stars becomes greater, the Oort Clouds that surround their solar systems become an increasing risk to neighbors, as comets will be drawn into their systems at a much higher rate.
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AFFFFF-LAC! 
1) We don't know squat yet about the number of objects, mass distribution, shape or extent of the Oort cloud.
2) We also don't know squat about the origin of the Oort cloud, and the extent to which it is dependent on having a string of gas giant planets. This also means that assuming Oort clouds form in other planetary systems with dissimilar configurations, we have no idea of its likely longevity and density over lifetime.
3) In the areas where density of stars is a problem, we don't have enough data to suggest that accretion disks will even form sufficiently to produce planets in a configuration that would create lots of Oort-cloud type objects, and increased radiation from extrastellar sources and termination shock effects would certainly have adverse effects on the size and stability of any Oort-cloud type objects, making objects large enough to cause damage even scarcer than would be the case otherwise.
4) For every disruption/ejection event, the possible orbital deflection is essentially random if the origin of the disruption/ejecting is extra-stellar system in origin, so the probability of collision with an earth mass planet close in to its star is extremely small.
5) Even in "dense" areas of the galaxy, stellar distances are such that disruption/ejection events would be extremely rare, except in cases where two stars proper motions are such that they interact. This is not uncommon on the scale of the entire galaxy, but extremely rare for any given star.
6) The areas where proper motion between stars makes disruption events likely are already pretty much no-brainers with respect to hosting life, namely the galactic core, globular clusters and active star forming regions with hot, young stars.
quote:
Every 300 million years or so? Perhaps not. Every 50,000 years though? Probably bad.  |
Then again, every 50,000 years means you'll pretty quickly sweep your orbit clear, and there aren't that many random colliders coming from other directions over a long period of time, so the likelihood is pretty high of getting a peaceful billion or two years in before the star goes red giant.
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MichaeltheGreat
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Apolyton Grand Executioner
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mooning the house that Ruth built.
Oct 1999
time: 21:36
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quote:
Originally posted by MrFun
I certainly agree with you and IW that we need to open our minds to forms of life by thinking outside of the box -- to a reasonable extent.
So given this, I have a question -- is it fair to assume that no matter what form life takes in any type of terrestrial planet, that water will be a fundamental, prerequisite for that planet's lifeforms? |
Water is a fairly weak solvent (but a good solvent for many organic molecules) and an excellent mixing media for suspension and exchange of objects in the size range from large complex molecules to microorganisms, but I wouldn't presume it's an absolute requirement for all primitive lifeforms. However, since "terrestrial" planets are by definition water-abundant, it's a safe presumption that water-bearing environments will be orders of magnitude more viable and diverse ecosystems.
We have iron, methane and sulphur eaters on earth, but they all live in aqueous ecosystems. We have stuff that lives in every temperature and pressure range of liquid water other than superheated geothermal resources (stuff on the edge or boundary doesn't count, I'm talking about the superheated 350°-600° stuff down in the vents.), so it's pretty clear that liquid water under virtually any condition will be a viable environment for something.
I just don't think it's a given that it's the exclusive environment. Gently heated rocks with trapped methane bubbles and oxygen bearing compounds might potentially provide an ecosystem for something, as would a liquid methane sea.
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03-06-2005 22:24
(edit - for Dan's crosspost)
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