The science behind the probability of extraterrestrials

[MichaeltheGreat]

(edit - for Dan's crosspost)

[DanS]

NASA's gonna haf to get it's ass in gear to make huge space telescopes. We need to see! The pace that they are setting and the costs that they are charging is disappointing.

[MrFun]

from the excerpt provided by Boris:

"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. "


I am curious -- how was it that other animals and plants could survive such a catastrophic disaster? Why didn't ALL life become extinct on Earth? You can't say that it was because dinosaurs were cold-blooded reptiles and that this factor had something to do with their extinction. Obviously, we have reptiles with us today.

Anyone care to try to explain?

[lord of the mark]

may i propose yet another narrow band?

If you get too far from the idea that the earth is unique, you end up with Carl Sagan, talking about Beellions and Beellions of galactic civilizations. You will soon spin off into a cold region inhabited by trekkies, FTL travel enthusiasts, etc.

If you get too close to the idea that the earth is unique, you not only violate the Copernican theorem, you end up spinning rapidly toward intelligent design, creationism, etc. Before you know it you'll be protesting in front of abortion clinics, and the only way of avoiding total brain death will be to divert towards ultraorthodox Judaism.


The safe range is really quite narrow.

[lord of the mark]

Originally posted by MrFun
from the excerpt provided by Boris:

"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. "


I am curious -- how was it that other animals and plants could survive such a catastrophic disaster? Why didn't ALL life become extinct on Earth? You can't say that it was because dinosaurs were cold-blooded reptiles and that this factor had something to do with their extinction. Obviously, we have reptiles with us today.

Anyone care to try to explain?

1. only tiny little animals (both reptiles and mammals) with lower resource requirements survived - it was awhile before we got megafauna again.
2. We lucked out - it could have been a WORSE collision.

[MrFun]

And plants species that survived -- they survived without photosynthesis during the time that sunlight was blocked out?

[Boris Godunov]

http://palaeo.gly.bris.ac.uk/Communication/Goddard/page1.html

Plants
Studies have been carried out on terrestrial plants in North America and these figures can be extrapolated to cover global extinction levels, although it must be remembered that North America may have been relatively close to the impact site at Chicxculub and so worse affected than other areas of the world. Taking this into account, studies of the leaf fossil record indicate species-level extinction as high as 75% in vegetation from the southern Rocky Mountains, with a northward decline to 25% in polar broad-leaved deciduous forest from central Alberta. This decreasing effect with increasing latitudes is due to temperate latitude vegetation being better adapted to extremes of climate combined with the presence of dormancy mechanisms, allowing seeds to remain dormant until conditions improved. Groups of both land and sea plants with well-developed dormancy mechanisms appear to have survived better than those without, this is more evidence for the extinctions being due to one major event and not a gradual extinction. However, as with many of the animal extinctions, many species of plants were in decline before the KT. Some groups of plants, despite declining in abundance at the KT survived into the Eocene only to then become extinct. This is more evidence for a general climatic change that appears to have begun at least by the Early Maastrichtian. Overall it should be concluded that the Kt had a limited effect in terms of extinctions on plant life.

[MrFun]

thanks, Boris

[MichaeltheGreat]

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.

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.

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.

[MichaeltheGreat]

Originally posted by DanS
NASA's gonna haf to get it's ass in gear to make huge space telescopes. We need to see! The pace that they are setting and the costs that they are charging is disappointing.

Hopefully Kepler will do the job, and we're getting better and better interferometric results from Keck and ESA.

[MichaeltheGreat]

Originally posted by MrFun
And plants species that survived -- they survived without photosynthesis during the time that sunlight was blocked out?

Sunlight would only partly be blocked out on a global scale - think of houseplants near, but not in, a small window.

[chequita guevara]

Originally posted by DanS
Yeh, that leaves us with only 35 billion stars.

I'm not saying lie elswhere is improbable. Just that certain regions of the galaxy aren't worth looking at in that regard. If half a precent of those systems are capable of supporting life, that's 175 million star systems.

[Lancer]

Already exist do skull sucking locust aliens. Coming for us they are.

[Boris Godunov]

Originally posted by MichaeltheGreat
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.

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.

Another point is that mass-extinction phenomena and collisions are not bad things.

Every 300 million years or so? Perhaps not. Every 50,000 years though? Probably bad.

[lord of the mark]

Originally posted by Lancer
Already exist do skull sucking locust aliens. Coming for us they are.

ach, a gitte yoddishe kinde!!! Gitte shabbos!!!