Planet-Formation Model Indicates Earthlike Planets Might Be Common

Russell Wallace wrote:

if anything, a mass extinction every now and then
is positively beneficial.

Note that you are speaking as the (accidental?) winner of many of
these events. The dinosaurs (as well as 95%+ of the rest of the
species that have ever lived on Earth) would strongly disagree.
Are mass extinctions strong drivers of evolution? Probably, but
they very well may happen in any situation that allows life (& the
resulting complex ecosystems). The original poster was probably more
concerned with things like habitable obliquities, which may be a good
point.

--
Brian Davis

(Brian Davis) wrote in message . com...
Russell Wallace wrote:

if anything, a mass extinction every now and then
is positively beneficial.

Note that you are speaking as the (accidental?) winner of many of
these events. The dinosaurs (as well as 95%+ of the rest of the
species that have ever lived on Earth) would strongly disagree.
Are mass extinctions strong drivers of evolution? Probably, but
they very well may happen in any situation that allows life (& the
resulting complex ecosystems). The original poster was probably more
concerned with things like habitable obliquities, which may be a good
point.

I was speaking about situations like Mars. The planet may have been
capable of supporting life for a while, but not for billions and
billions of years.

If you're only interested in microb life, then it doesn't matter. But,
as we saw with the evidence for microb life on Mars, the general
public has almost zero interest in it.

Regards,

Philip

(Brian Davis) wrote in message . com...
P. Ussyliquor wrote:

In order for life to evolve it needs billions of years
of relative stability.

Note that life took hold on Earth in *under* 0.5 billion years, and
that's based on a thin and rough geologic record - it may have been
faster than that. Life does not need billions of years. Complex life
that can, say, use the internet, *that* may require billions of
years...

Earth has stability because it has:
1. a very large moon acting as a stabaliser to the planetary wobble.

Actually, that's a pet peeve of mine. It's *only* true here, in the
current solar system. If Earth's mass or rotation rate were different,
or Jupiter was at a different location, Earth's obliquity would *NOT*
be subject to a chaotic, high-obliquity variations. In short, there is
nothing that says a large Moon is needed as an angular momentum bank
to push the precession frequency up above resonance - slowing the
rotation to push the angular momentum *below* resonance would work as
well, for instance.
Heck, in another billion years or so, the presence of the Moon
forces us *into* such a high-obliquity state.

2. a very large object (Juptier) that acts as a vacuum cleaner to suck
up the majority of in-coming asteroids and comets.

This has been argued both ways as well. Yes, Jupiter does "clean
things up" - but it does it partially by throwing them in-system. I
*suspect* the simulation mentioned did have a jovian-class planet, but
I'm not sure; anybody have the paper?

Also, since these simulations are based on data from our solar system,
and almost exclusively our solar system (since we know almost nothing
about other systems), OF COURSE it is going to predict systems similar
to ours.

I suspect the main point of this paper is to try to get a handle on
the amount of water & other volitles accreted, but I'm not sure...
yet.

My point is, that when astronomers talk about "earth-like" planets,
they are usually only refering to lumps of rock of a certain size and
perhaps a certain distance from a star.

And you need a LOT more than the right distance from the sun to have a
planet that can sustain life.

These models do NOT predict Earth-like planets as opposed to Mars-like
planets.

But when you announce "Earth-like" planets, the general public doesn't
think "dead rocks with some ice caps" they think of planets that can
sustain life.

Regards,

Philip

P. Ussyliquor wrote:

My point is, that when astronomers talk about "earth-like" planets,
they are usually only refering to lumps of rock of a certain size and
perhaps a certain distance from a star.

And you need a LOT more than the right distance from the sun to have a
planet that can sustain life.

We once calculated it out in one of my astronomy classes. The estimates
were about one in a hundred stars.

But, there's a catch. Consider how long we have been around as humans.
We know that there was a cataclysm about 35-40K years ago that wiped out
almost all humans. We can all trace our genes back to less than a dozen
individuals at that time. Then there was the Ice age. Then, before that,
there were three mass extinctions.

Given all of that, we figured there's about a 1/10 chance of there being
somewhat evolved life that hasn't completely died off due to whatever factors.
Most planets will be like Mare - once capable of supporting life, but now
an irradiated barren rock.

Then you have to consider that we've only been even moderately technologicaly
advanced for 5-10K years at best. The last 2-3K is where most of our
development has taken place. Also, any advanced civilization is very likely
to destroy itself given enough time. Let's say a 10,000 year timespan.

Out of 2-3 billion years, that's an awfully small window. Let's say 2 billion
years divided by 10,000 years. That's a 1 in 200,000 chance that we've even
hit the right era where they are at a level near ours. More advanced races
would likely see us as violent oddities to be studied - and definately avoided,
so contact with them would be very rare.

...

What it worked out to was our class came up with the figure that at any time
in our galaxy, there are about ten starts with life at a level near ours,
but they are all over the place and travelling there at even a significant
fraction of light would take so long that there'd likely be nothing there
when we get there. Finding the right ten out of the entire galaxy is also
ubsurd - even if we could scan every single one, it would take hundreds of
years to catalog them all.

P. Ussyliquor wrote:

My point is, that when astronomers talk about "earth-like" planets,
they are usually only refering to lumps of rock of a certain size and
perhaps a certain distance from a star.

I apologize then - I saw you list several objection to the
potential habitability of such a body, and tried to address them. You
are in a sense correct - a "habitable" planet to an astronomer is
indeed generally a rocky world with a limited atmosphere, with a
surface temperature & pressure that supports liquid water. And,
honestly, that's a pretty good definition: for instance, the only
"earth-like" planet in the solar system by this broad definition is,
well... Earth.

And you need a LOT more than the right distance from the sun to have a
planet that can sustain life.

Perhaps. Again, as you noted before, there is a derth of hard data
here. What other conditions (other than those listed above) would you
consider essential? I'd say an active internal convection (the source
of such heat is not as critical) is required, but beyond that... as I
noted, a large moon is probably a red herring. Presence/absence of a
jovian class object is questionable as well.

These models do NOT predict Earth-like planets as opposed to Mars-like
planets.

Actually (from what I can dig out of the press release - not much
to go on, I agree), they are exluding Mars-like planets. Such a planet
doesn't retain a hydrosphere, which is something they seem to
specificly look at.

But when you announce "Earth-like" planets, the general public doesn't
think "dead rocks with some ice caps" they think of planets that can
sustain life.

More's the pity for the general public - keep in mind, this is a
press release, and it's not even one I've seen picked up on the AP or
other wires. Cut them some slck, it seems like some very interesting
research.

--
Brian Davis

In article .net,
Joseph Oberlander wrote:

But, there's a catch. Consider how long we have been around as humans.
We know that there was a cataclysm about 35-40K years ago that wiped out
almost all humans. We can all trace our genes back to less than a dozen
individuals at that time. Then there was the Ice age. Then, before that,
there were three mass extinctions.

Given all of that, we figured there's about a 1/10 chance of there being
somewhat evolved life that hasn't completely died off due to whatever factors.
Most planets will be like Mare - once capable of supporting life, but now
an irradiated barren rock.

Five big extinctions (possibly six depending on how the Pleistocene
works out): Cretaceous-Tertiary, End Triassic, Permian-Triassic (the big
kahuna, against which other MEs are mere hicoughs), Late Devonian and
Ordovician-Silurian. Note how in all case, including the Permian one (which
killed something like 95% of all species), were followed by a recovery
of diversity, although not of disparity.

What I take from that is that terminating all life is actually
difficult and that given survivors, variation, selection and time empty
niches will be refilled and indeed new ones discovered.
--
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With Social Credit you will take one step forward."

Camil Samson

(Russell Wallace) writes:

On 13 Dec 2003 11:37:07 -0500, (Herman Rubin)
wrote:
In article ,
Russell Wallace wrote:

Solid planets three times Earth's diameter? i.e. with more than 27
times the mass? (Presumably considerably more than 27 times, since the
core would be pretty compressed at that stage.) Is that really
possible?

If the density is the same, gravity, which is proportional
to radius times density, would be three times as great.

*nods* 3G would be survivable to life forms adapted for it, of course
- but the problem I'm thinking of isn't the surface gravity, it's the
availability of 27 Earth masses of solid material?

According to the "core accretion model," such a large "silicate" object
forms the "core" that tiggers "runaway" accretion of "ices" and "gases"
to become a gas giant. If such a core were to form inside the "ice line,"
there would be no "ices" and not enough "gases" to form a gas giant,
so intead you get a "superterrestrial planet."


-- Gordon D. Pusch

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Matthew Montchalin writes:

On Fri, 12 Dec 2003, Russell Wallace wrote:
Solid planets three times Earth's diameter? i.e. with more than 27
times the mass? (Presumably considerably more than 27 times, since the
core would be pretty compressed at that stage.) Is that really
possible?

They probably wouldn't rotate as fast, would they?

Actually, to the extent that our own (apparently anomolous) solar system
is any guide, and to the degree the numerical simulations are accurate,
large planets tend to rotate _faster_ than small planets, because being
more compact, they have smaller radii of gyration (moment of inertia per
unit mass).


-- Gordon D. Pusch

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(P. Ussyliquor) writes:

In order for life to evolve it needs billions of years of relative
stability.
Just being the right distance from the sun doesn't provide that.

Earth has stability because it has:
1. a very large moon acting as a stabaliser to the planetary wobble.

This is still just a hypothesis; it has not been established.


2. a very large object (Juptier) that acts as a vacuum cleaner to suck
up the majority of in-coming asteroids and comets.

This is still just a hypothesis; it has not been established.


What percentage of their simulations have that?

Since it has not been established that _either_ of these conditions are
essential, this statistic is irrelevant.


Also, since these simulations are based on data from our solar system,
and almost exclusively our solar system (since we know almost nothing
about other systems), OF COURSE it is going to predict systems similar
to ours.

On the contrary:

1.) They a based on fundamental physical first principle, astronomical
observations of many other protoplanetary disks, and a number of
simplifying assumptions.

2.) The simulated systems only "resemble" ours in the most generic sense
immaginable --- i.e., they wind up with some number of "rocky" inner
planets, and some number of "gassy" outer planets, with orbits of varying
degrees of circularity.


-- Gordon D. Pusch

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