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View Full Version : Pluto-Charon = Earth-Moon



ToSeek
2005-Jan-28, 06:47 PM
Pluto-Charon origin may mirror Earth and Moon (http://www.spaceflightnow.com/news/n0501/27pluto/)


The evolution of Kuiper Belt objects, Pluto and its lone moon Charon may have something in common with Earth and our single Moon: a giant impact in the distant past.

This suggests to me that one of the key elements of the "Rare Earth" hypothesis isn't so rare after all.

badprof
2005-Jan-28, 07:03 PM
As one of my students from last semester commented during a discussion about an unanswered question on the solar system. "the answer to every question is 'something big hit it!!'" :D

Maurice

Manchurian Taikonaut
2005-Jan-29, 05:18 PM
This suggests to me that one of the key elements of the "Rare Earth" hypothesis isn't so rare after all.

that sounds like a good bit of info for a strong debate, is Earth so rare afterall ?

voyager_3
2005-Jan-29, 05:31 PM
Makes it all the more important that we get a mission out there!

Padawan
2005-Jan-29, 05:51 PM
Makes it all the more important that we get a mission out there!

My thoughts exactly when I saw the title of this thread!


ToSeek, thanks for the link man! :)

Plat
2005-Jan-29, 07:02 PM
That rare Earth theory has been losing credibility year after year, as our technology improves we find evidence of our supposedly "rare" planet not being so rare after all

ToSeek
2005-Jan-29, 07:32 PM
That rare Earth theory has been losing credibility year after year, as our technology improves we find evidence of our supposedly "rare" planet not being so rare after all

The key aspect of the Rare Earth Hypothesis to me is the unlikelihood of a stable environment for a hospitable planet to last the billions of years needed for complex life to develop. (Evidence is that it's harder for life to become multicellular than it is for life to emerge in the first place: the latter happened in a geological eyeblink, while the former took billions of years.) Though numerous extra-solar planets have been found, hardly any are at a habitable distance from their Sun.

Ilya
2005-Jan-30, 02:36 AM
That rare Earth theory has been losing credibility year after year, as our technology improves we find evidence of our supposedly "rare" planet not being so rare after all

Only to those who never understood Rare Earth theory in the first place. While I agree that Ward and Brownlee's definition of what constitutes a "habitable planet" is far too narrow, it is actually the less important constraint on the development of complex lifeforms and civilization.

The more important constraint is occasional catastrophic events, such as asteroid impacts and nearby supernovae. Life adapts to the local conditions - if a planet has 100 atm surface pressure, water will be comfortably liquid at 200 C, and life will evolve accordingly. In other words, life fits the box that it came in. But Ward and Brownlee's main point - and I think in that they are right, - is that ANY box gets violently shaken every once in a while. And the more complex life forms are, the less of a shake it takes to destroy them. Permian Extinction 250 million years ago came awfully close to wiping out every multicellular creature on Earth. Recent evidence supports the asteroid impact hypothesis; if that asteroid massed twice as much, Earth would be back to bacteria - and would still be there now.

Good analogy are card castles. On any given day thousands of people throughout the world build card castles. Necessary "environment" is easy to come by - a flat surface with no wind, and several card decks. Yet very few 5' tall card castles exist anywhere in the world. 10' tall ones probably do not exist at all - even though there are endlessly trying fanatics. The probability of something knocking them down before they grow that complex is too high - and grows higher as the castle grows. A vibration from a passing truck won't harm a 10-card castle, but will bring down a 100-card one.

Maddad
2005-Jan-30, 03:38 AM
Though numerous extra-solar planets have been found, hardly any are at a habitable distance from their Sun.While this is true, its implication is false. We have found hot Jupiters because that is what our detection capability allows us to see. We could not have found Earth-like planets because even if they were there, we could not see them.

ToSeek
2005-Jan-30, 04:25 AM
Though numerous extra-solar planets have been found, hardly any are at a habitable distance from their Sun.While this is true, its implication is false. We have found hot Jupiters because that is what our detection capability allows us to see. We could not have found Earth-like planets because even if they were there, we could not see them.

A fair point, but I would argue that implications of Platinum Rhymer's statement are also false: just because we are finding lots of planets does not let us conclude that there are lots of stable, Earthlike planets out there.

W.F. Tomba
2005-Jan-30, 06:18 AM
But Ward and Brownlee's main point - and I think in that they are right, - is that ANY box gets violently shaken every once in a while. And the more complex life forms are, the less of a shake it takes to destroy them.
I agree with this in general. However, if some form of intelligent life happens to survive long enough to establish a sustainable presence outside its home planet, it suddenly becomes much harder to destroy, and could have a very good chance of survival over long periods of time. So if you decide that there's a high probability of a space-colonizing civilization having developed at least once in our galaxy, then the overall chance of finding intelligent life could be fairly good.

eburacum45
2005-Jan-30, 10:05 AM
The possibility that the Pluto/Charon system was formed in the same way as the Earth/Moon system makes just one step in the development of an Earth-like planet a little less unlikely.
But even given a larger set of earth-like terrestrial worlds, the occurrence of catastrophic events will still have an effect on the persistence of a complex biosphere.

This does not mean that the most extreme case of the Rare Earth scenario is necessarily correct; other solar systems containing suitable terrestrial planets (with oceans and large moons) might actually be less hazardous environments than our own solar system. If Jupiter was just a little bigger (or perhaps smaller) the asteroid belt might not exist at all, and the space around the inner planets in such a system might be relatively free of potential impactors.
Similarly the threat from supernovae seems to be somewhat over-estimated; to be a real danger to an established complex biosphere on a planet with a suitable magnetic field, a supernova has to be no more than a few tens of light-years away; to sterilize the planet it would need to be closer still.

So the 'Very Rare'Earth hypothesis might not be correct; however earth-like planets are probably at least 'Medium Rare'.

synthomus
2005-Jan-30, 02:36 PM
And the more complex life forms are, the less of a shake it takes to destroy them. Permian Extinction 250 million years ago came awfully close to wiping out every multicellular creature on Earth. Recent evidence supports the asteroid impact hypothesis; if that asteroid massed twice as much, Earth would be back to bacteria - and would still be there now.

[-( I think that's mainly a popular scientific myth.

An extinction wiping out 95% of all multicellular species is terrible news for all thoses highly specialized and environmentally adapted species that have to perish in the event of a big global climatic catastrophe like after an asteriod impact. Yet it's a bit naive to only look at the numbers and deduce from that: oops, we nearly hit the 100% extinction mark so it must have been very tight, we were probably just lucky enough to have some multicellular species left. I believe that's underestimating the quantitative fact that 5% still is a lot of species as well as disregarding the special qualities of the surviving species. Obviously the survivors were a lot more tenacious and adaptable or they wouldn't have survived the event in the first place. For that reason I think it would take a lot more destructive power to doom these 5% than it might seem at first glance.

Yes, a big mass extinction sets the evolutionary clock back for a while as many evolutionary inventions get lost forever. Yet it's highly unlikely that it crushes out basic achievements like multicellularity. There is multicellular life at deep sea vents feeding from microbes which make a living from the vents! It alone would suffice to start a new global evolutionary explosion once the dust has settled.

In evolution basics like primitive microbial reproduction or multicellularity count. Once they've developed they're here to stay. All that fancy looking evolutionary products we marvel at in our flora and fauna world are a piece of cake compared to those basics. If a mass extinction caused a widespread tabula rasa, it only would instigate the surviving multicellulars to develop even more fancy kind of things.

Apart from solar instability, beeing kicked out of orbit by a passing star, getting hit by a really big asteroid/planetoid-sized body or getting fried by a near gamma ray burst there aren't any other scenarios I consider potentially life-threatening to multicellular life as a whole. The average sized asteroid, comet or any supervolcano isn't fit for the job.

Ilya
2005-Jan-30, 10:31 PM
But Ward and Brownlee's main point - and I think in that they are right, - is that ANY box gets violently shaken every once in a while. And the more complex life forms are, the less of a shake it takes to destroy them.
I agree with this in general. However, if some form of intelligent life happens to survive long enough to establish a sustainable presence outside its home planet, it suddenly becomes much harder to destroy, and could have a very good chance of survival over long periods of time. So if you decide that there's a high probability of a space-colonizing civilization having developed at least once in our galaxy, then the overall chance of finding intelligent life could be fairly good.

True. Which is why one the Drake Equation thread I put the probability of intelligent life ever developing very low, but its lifetime (or "communicating lifetime" for the purposes of DE) as rather high.

Ilya
2005-Jan-30, 10:41 PM
Apart from solar instability, beeing kicked out of orbit by a passing star, getting hit by a really big asteroid/planetoid-sized body or getting fried by a near gamma ray burst there aren't any other scenarios I consider potentially life-threatening to multicellular life as a whole. The average sized asteroid, comet or any supervolcano isn't fit for the job.

Point taken. Simple, unspecialized multicellular organisms are almost as tough as bacteria. But that does not really invalidate the "Rare Earth" hypothesis -- first, some parts of the Galaxy periodically unleash events deadly even for bacteria and second, even in other relatively hospitable parts planets populated by tube worms and cockroaches should greatly outnumber those featuring apatosaurs, tigers, or digger wasps*.

* We tend to think of insects as indestructible, and many of them are. But Tarantula Hawk digger wasp is about as specialized a species as can be. Not likely to survive a major cataclysm.

synthomus
2005-Jan-31, 01:50 AM
Point taken. Simple, unspecialized multicellular organisms are almost as tough as bacteria. But that does not really invalidate the "Rare Earth" hypothesis -- first, some parts of the Galaxy periodically unleash events deadly even for bacteria and second, even in other relatively hospitable parts planets populated by tube worms and cockroaches should greatly outnumber those featuring apatosaurs, tigers, or digger wasps*.

If your intention was to describe conditions within the central bulk of our galaxy I guess you're partially right. It's a densely populated neighbourhood with a lot of gravitational disturbances that result in more frequent impact events and even disruptions of planetary orbits. Close supernovas might play their detrimental part there too. So yes, perhaps multicellular life would have a hard time to produce tigers in these realms. Yet I don't think it's necessarily an environment even too hostile for microbes as you've suggested.

Anyway, you made a valuable point which has to be considered in terms of the Drake Equation since a significant proportion of the stars of the Milky Way are located in its center region. On the other hand there are the spiral arms which are more benign to life. No need to suppose the evolution ends preferably with cockroaches here. So it all comes down to fine tuning the values of the Drake Equation.

My general problem though with Ward and Brownlee's Rare Earth Hypothesis starts with the title "Rare Earth: Why Complex Life Is Uncommon in the Universe". It's misleading because it suggests the Earth was something nearly unique in the whole universe. That's dishonest given the fact that the authors' initial point is the Drake Equation which deals only with our galaxy, not the universe on the whole. That still makes a little difference after all, you know. :) So basically the authors deal with the Drake Equation and by doing so they actually conclude we should't expect many, if any "radioactive" civilizations in our galaxy. Whatever concrete number they got as result of the Drake Equation, if they wanted to project it on the whole universe they should have multiplied the result a few hundred billion times. Which they haven't. "Why comlex Life Is Uncommon in our Milky Way" wasn't as catchy as the title they chose I guess.

Maddad
2005-Jan-31, 04:57 AM
When considering the Drake Equation and habitable zones for life, it might be useful to consider that life tends to fill the box you give it.

TriangleMan
2005-Jan-31, 12:29 PM
We only have one example of a habitable planet so it is difficult to extrapolate the robustness of life on other worlds from a sample of one. If there is life on other worlds its existence could be a lot more delicate then on the ancient Earth.

Kaptain K
2005-Jan-31, 03:05 PM
We only have one example of a habitable planet so it is difficult to extrapolate the robustness of life on other worlds from a sample of one. If there is life on other worlds its existence could be a lot more delicate then on the ancient Earth.
Or a lot less! :o

ToSeek
2005-Jan-31, 03:07 PM
My general problem though with Ward and Brownlee's Rare Earth Hypothesis starts with the title "Rare Earth: Why Complex Life Is Uncommon in the Universe". It's misleading because it suggests the Earth was something nearly unique in the whole universe. That's dishonest given the fact that the authors' initial point is the Drake Equation which deals only with our galaxy, not the universe on the whole. That still makes a little difference after all, you know. :) So basically the authors deal with the Drake Equation and by doing so they actually conclude we should't expect many, if any "radioactive" civilizations in our galaxy. Whatever concrete number they got as result of the Drake Equation, if they wanted to project it on the whole universe they should have multiplied the result a few hundred billion times. Which they haven't. "Why comlex Life Is Uncommon in our Milky Way" wasn't as catchy as the title they chose I guess.

I'd say that uncommon is uncommon. If there's one intelligent species per galaxy, on average, I'd say that's still "uncommon" even though it means there are hundreds of billions of them.

Brady Yoon
2005-Jan-31, 11:56 PM
^^(Sorry there's no copy; public computer).

Do you mean simple life or complex life? (speaking on relative terms :) )[/i]

synthomus
2005-Feb-01, 02:55 AM
I'd say that uncommon is uncommon. If there's one intelligent species per galaxy, on average, I'd say that's still "uncommon" even though it means there are hundreds of billions of them.

I guess you're somehow right because human beings are mostly interested in what they can actually touch. Drake deliberately confined his equation to our galaxy because at least there is a very slight chance to touch or communicate with whoever might share the Milky Way with us. In comparison, we are much less curious about life beyond our galaxy, as we think there's absolutely no way to break intergalactic isolation.

Still I doubt if healthy scepticism should extend as far as calling something "uncommon" that could actually exist in several billion places. Shouldn't we take our best estimates more seriously and accept that "comlex life" could be for real in billions of places and varieties in the universe, in strangely familiar as well as the most weirdest ways?

Of course, at least as long as we havn't found some bio markers in some extrasolar planetary spectra the whole issue will stick to beeing controversial as ever.

cyswxman
2005-Feb-01, 08:22 AM
I seem to remember glancing at some article that postulated the the Earth and the Moon are actually a double planet system, but I haven't seen anything else about that.