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How special is the Solar System? (Forwarded)



 
 
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  #1  
Old August 7th 04, 03:40 AM
Rodney Kelp
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Default How special is the Solar System? (Forwarded)

The next nearest star 4.3 light years away, Alpha Centuri A is a million
miles across or 1,227 times the size of our sun. It could have a very large
solar system. Assuming the galaxy is 100,000 light years across go in to it
from here about 25,000 light years (halfway to the center) and the stars are
much closer together making solar system planets also close together.There
must be billions of planets in this galaxy alone. No matter how they are
formed there has to be millions of similar types and many binary star
systems and mulit-star systems. I think the count is roughly 100 billion
stars in this galaxy. Now think of 125 billion galaxies estimated by Hubble
and you see very many stars and planets.

"Andrew Yee" wrote in message
...
Royal Astronomical Society Press Notice

Issued by Dr Jacqueline Mitton, RAS Press Officer
jmitton -at- dial.pipex.com
tel: +44 (0)1223-564914

CONTACTS

Dr Martin Beer
University of Leicester, UK
Tel: +44 (0)116 2231802
Email:

Prof. Andrew King
University of Leicester, UK
Tel: +44 (0)116 2522072
Email:


Dr. Mario Livio
Space Telescope Science Institute, USA
Tel: +1 410 338 4439
Email:


Dr. Jim Pringle
University of Cambridge, UK
Tel: +44 (0)1223 337513
Email:


************************************************** ************

Date: 3rd August 2004

PN04-30

HOW SPECIAL IS THE SOLAR SYSTEM?

On the evidence to date, our solar system could be fundamentally different

from
the majority of planetary systems around stars because it formed in a

different
way. If that is the case, Earth-like planets will be very rare. After

examining
the properties of the 100 or so known extrasolar planetary systems and

assessing
two ways in which planets could form, Dr Martin Beer and Professor Andrew

King
of the University of Leicester, Dr Mario Livio of the Space Telescope

Science
Institute and Dr Jim Pringle of the University of Cambridge flag up the

distinct
possibility that our solar system is special in a paper to be published in

the
Monthly Notices of the Royal Astronomical Society.

In our solar system, the orbits of all the major planets are quite close

to
being circular (apart from Pluto's, which is a special case), and the four

giant
planets are a considerable distance from the Sun. The extrasolar planets
detected so far -- all giants similar in nature to Jupiter are by

comparison
much closer to their parent stars, and their orbits are almost all highly
elliptical and so very elongated.

"There are two main explanations for these observations," says Martin

Beer. "The
most intriguing is that planets can be formed by more than one mechanism

and the
assumption astronomers have made until now -- that all planets formed in
basically the same way -- is a mistake."

In the picture of planet formation developed to explain the solar system,

giant
planets like Jupiter form around rocky cores (like the Earth), which use

their
gravity to pull in large quantities of gas from their surroundings in the

cool
outer reaches of a vast disc of material. The rocky cores closer to the

parent
star cannot acquire gas because it is too hot there and so remain

Earth-like.

The most popular alternative theory is that giant planets can form

directly
through gravitational collapse. In this scenario, rocky cores -- potential
Earth-like planets -- do not form at all. If this theory applies to all

the
extrasolar planet systems detected so far, then none of them can be

expected to
contain an Earth-like planet that is habitable by life of the kind we are
familiar with.

However, the team are cautious about jumping to a definite conclusion too

soon
and warn about the second possible explanation for the apparent disparity
between the solar system and the known extrasolar systems. Techniques

currently
in use are not yet capable of detecting a solar-system look-alike around a
distant star, so a selection effect might be distorting the statistics --

like a
fisherman deciding that all fish are larger than 5 inches because that is

the
size of the holes in his net.

It will be another 5 years or so before astronomers have the observing

power to
resolve the question of which explanation is correct. Meanwhile, the

current
data leave open the possibility that the solar system is indeed different

from
other planetary systems.

NOTES

1. Currently around 100 extrasolar planets are known which have been

detected
through the wobble of their host stars caused by the motion of the planets
themselves.

2. The paper has recently been accepted by the Monthly Notices of the

Royal
Astronomical Society but no publication date has yet been set.



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  #2  
Old August 7th 04, 04:05 PM
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Default How special is the Solar System? (Forwarded)


Rodney Kelp wrote:

[Forwarded press release]

HOW SPECIAL IS THE SOLAR SYSTEM?

On the evidence to date, our solar system could be fundamentally

different
from
the majority of planetary systems around stars because it formed in

a
different
way. If that is the case, Earth-like planets will be very rare.


If I was an astronomer, I'd be very wary of making this kind of
statement. They've almost always proved wrong in past. Given that we
can't detect Earth size planets yet, nor are very good at finding
multi-year orbit big planets, this is just pure speculation. And this
type of speculation, that the Earth, Sun, or solar system is in some
way special, has a very bad track record. The principle of mediocrity
applies here (assume we are average unless there is some strong
evidence against it), and there is no evidence against it yet.
Lou Scheffer

  #4  
Old August 7th 04, 07:24 PM
Alain Fournier
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Default How special is the Solar System? (Forwarded)

wrote:
Rodney Kelp wrote:


[Forwarded press release]

HOW SPECIAL IS THE SOLAR SYSTEM?

On the evidence to date, our solar system could be fundamentally


different

from

the majority of planetary systems around stars because it formed in


a

different

way. If that is the case, Earth-like planets will be very rare.



If I was an astronomer, I'd be very wary of making this kind of
statement. They've almost always proved wrong in past. Given that we
can't detect Earth size planets yet, nor are very good at finding
multi-year orbit big planets, this is just pure speculation. And this
type of speculation, that the Earth, Sun, or solar system is in some
way special, has a very bad track record. The principle of mediocrity
applies here (assume we are average unless there is some strong
evidence against it), and there is no evidence against it yet.
Lou Scheffer


Yes there is evidence against it. The evidence isn't very strong
but we are now getting some serious data on the matter. Well
the evidence is somewhat strong that our solar system is
fundamentally different from the majority of planetary systems
around stars. It is much weaker about Earth-like planets being
very rare. Most planets found to date are Jupiter sized or bigger
in an elliptical orbit with periapsis less than 1 AU. If Jupiter
had an elliptical orbit with periapsis less than 1 AU, then
Earths orbit wouldn't be stable and Earth wouldn't be.

Alain Fournier

  #5  
Old August 7th 04, 07:46 PM
Greg D. Moore \(Strider\)
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Default How special is the Solar System? (Forwarded)


"Alain Fournier" wrote in message
...

Yes there is evidence against it. The evidence isn't very strong
but we are now getting some serious data on the matter. Well
the evidence is somewhat strong that our solar system is
fundamentally different from the majority of planetary systems
around stars. It is much weaker about Earth-like planets being
very rare. Most planets found to date are Jupiter sized or bigger
in an elliptical orbit with periapsis less than 1 AU. If Jupiter
had an elliptical orbit with periapsis less than 1 AU, then
Earths orbit wouldn't be stable and Earth wouldn't be.


Keep in mind that most of the techniques used are more LIKELY to find that
sort of system than ours.

Mostly they focus on the wobble of a star, a star with a Jupiter class
planet close in will have a more easily detectable wobble than one further
out, for two reasons:

1) Mass closer causes a larger effect.
2) Shorter orbit causes wobbles more often.


Alain Fournier



  #6  
Old August 7th 04, 09:22 PM
Andrew Nowicki
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Posts: n/a
Default How special is the Solar System?

"How special is the Solar System?" by M. E. Beer1.,
A.R. King1, M. Livio2 and J. E. Pringle2 is posted
at: http://arxiv.org/pdf/astro-ph/0407476

MY COMMENT

The high eccentricity of the extraterrestrial gas giants
implies that all or nearly all extraterrestrial planets
have eccentric orbits and that solar system is uncommon.
In addition to the nearly circular orbits (except for
Pluto), the solar planets are almost evenly distributed
as predicted by the Titius-Bode Law. The Titius-Bode Law
also works for moons orbiting solar planets, but does not
work well for Neptune and Pluto:
http://astrosun2.astro.cornell.edu/a.../bodes_law.htm

Apparently the solar system accretion disk was not
disturbed by interlopers when planets and moons
formed 4.5 billion years ago, except for the most
distant planets: Neptune and Pluto. We still do not
know if planetary orbits are inherently unstable.
It seems that planetary systems having many planets
should be less stable than planetary systems having
few planets. The absence of massive bodies in the
middle of the solar system (known as the main
asteroid belt) may have stabilized the solar system.
If planetary orbits are inherently unstable than solar
system is uncommon and SETI is a waste of time. Simple
forms of life may survive on a somewhat unstable planet,
but they cannot create a technological civilization.

We need better computer simulations of orbital
stability -- these simulations are more important
than all the microwave SETI research.

PS. I wonder if the Moon (Luna) acts like a vacuum
cleaner in a sense that it hurls deadly asteroids
away from the Earth.

__________________________________________________ ______________


RELATED ARTICLES


Computer simulations of orbital stability are difficult.
For example, the following paper is based on simulations
made on a supercomputer having 128 processors, and yet it
neglects possible inclinations as well as planetary systems
having more than 3 planets:
Stability of Terrestrial Planets in the Habitable Zone of
Gl 777 A, HD 72659, Gl 614, 47 Uma and HD 4208
http://arXiv:astro-ph/0403152


Excerpt from "The Stability Of The Orbits Of Earth-Mass Planets
In And Near The Habitable Zones Of Known Exoplanetary Systems"
by Barrie W Jones, David R Underwood, P Nick Sleep,
http://www.astrophys-assist.com/educate/cgino617.pdf:
"We have shown that Earth-mass planets could survive in
variously restricted regions of the habitable zones (HZs)
of most of a sample of nine of the 93 main-sequence exoplanetary
systems confirmed by May 2003. In a preliminary extrapolation
of our results to the other systems, we estimate that roughly
a third of the 93 systems might be able to have Earth-mass
planets in stable, confined orbits somewhere in their HZs."
This is a poor quality article. It does not explain
how they calculated the orbital stability.


Excerpt from "Dynamical Stability and Habitability of a
Terrestrial Planet in HD74156" by M. Colleen Gino,
http://www.astrophys-assist.com/educate/cgino617.pdf:
"The dynamical stability of the system must be taken into
account as well, particularly in light of the impact that
large planets can have on the orbit of the terrestrial planet.
For a terrestrial planet to remain habitable, there is a
dynamical requirement that other planets in the system don’t
gravitationally perturb the planet outside of its habitability
zone. In a recent study involving 85 of the known extrasolar
planetary systems, Menou and Tabachnik (2003) found that more
than half of these systems, primarily those with distant
eccentric giant planets, are not likely to support terrestrial
planets and are therefore dynamically inhabitable. Marcy and
Butler (2000) give similar evidence for the likelihood of
terrestrial planets to be scattered gravitationally from the
high eccentricity of Jupiter-like planets that exist between
2 – 3 AU. Under such circumstances the circular orbits and the
long term survival of terrestrial planets is not guaranteed."
  #7  
Old August 8th 04, 01:28 AM
Sander Vesik
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Default How special is the Solar System? (Forwarded)

Dave O'Neill dave @ nospam atomicrazor . com wrote:

"Alain Fournier" wrote in message
...
wrote:
Rodney Kelp wrote:



Yes there is evidence against it. The evidence isn't very strong
but we are now getting some serious data on the matter. Well
the evidence is somewhat strong that our solar system is
fundamentally different from the majority of planetary systems
around stars. It is much weaker about Earth-like planets being
very rare. Most planets found to date are Jupiter sized or bigger
in an elliptical orbit with periapsis less than 1 AU. If Jupiter
had an elliptical orbit with periapsis less than 1 AU, then
Earths orbit wouldn't be stable and Earth wouldn't be.


I suspect we've too few samples to draw any real conclusions. I remember
talking to an astronomer about extra-solar planets in the early 90's and he
was not then prepared to say they existed at all because the data was too
flimsy.


Umm... Earth style planets are coupe of orders o magnitude what we can
detect. Presently it would probably have to massively collide with
something for us to detect. This is about to change in not too distant
fture so peple saying such things now is particularily odd.


Given the size and scope, Earths might be rare but I do find it hard to
believe they are unique.


It depdends on what you mean by "Earths" - given that this system has
2 2/3-s, I don't really see why they should be rare.


Dave


--
Sander

+++ Out of cheese error +++
  #8  
Old August 8th 04, 01:29 AM
Andrew Nowicki
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Default How special is the Solar System?

Here is the most far-fetched speculation I have ever made:

HOW WE BECAME THE FIRST INTELLIGENT SPECIES IN THE UNIVERSE

4.5 billion years ago an accretion disk formed around
the young Sun. The accretion disk had uniform density
except low density in its center. When planets formed,
the center became the main asteroid belt. The absence of
large mass in the main asteroid belt divided the solar
planets into two nearly isolated planetary systems. The
separation stabilized planetary orbits. Stable, nearly
circular planetary orbits are rare outside the solar
system. They are very important because they ensure stable
climate which is necessary for advanced forms of life.

...quick fast forward...

About 4 billion years ago two planets collided and were
transformed into the Earth and its Moon. The young Earth
was pelted with debris produced by the collision. When the
debris entered the atmosphere, it broke into dust which
fell into primordial oceans and was transformed into clay
minerals. The primordial oceans were hot and covered with
a thick layer of hydrocarbons. Clay crystals floated on
the surface of the oceans due to surface tension. These
crystals were alive and they were our ancestors. They bred
when they broke into smaller crystals and they died when
they sank. Their lifestyle was "survival of the weakest."
The weakest crystals broke easily, so they bred faster than
strong crystals, and they did not sink because their small
size kept them afloat due to the surface tension. The
weakest clay crystals were microscopic tubes with helical
defects. The surface tension attracted nucleotides into the
tubes. The helical defects polymerized the nucleotides into
RNA chains. These long, strong RNA chains pulled the tubes
apart, thereby breaking them and breeding them.

...quick fast forward...

Natural bush fires are common in periodically dry habitats.
From 2.9 million to 2.4 million years ago Africa was relatively

dry, but not as dry as it is today. South and east Africa and
Sahara were covered with savanna. The dry period coincides
perfectly with the existence of Australopithecus africanus.
This hominid lived in dry african woodlands surrounded by
grasslands. At present time July is the driest month on the
Mediterranean coast of Africa. 2000 kilometers south, in
southern parts of Mali, Niger, Chad, and Sudan, July is the
wettest month. South Africa has the same wet/dry weather
pattern: June is the wettest month in Kaapstad but it is the
driest month in Johannesburg. If the same weather pattern
existed in the past, Sahara and south Africa were ravaged by
natural bush fires. Large fires suppress rain, so they can
burn for a very long time. In some years favorable winds must
have spread bush fires in these zones for several weeks -- it
was only natural for the apiths to walk in front of the fire,
scavenging fried carcasses, and shoo the competing birds.
With a little artificial help the fires followed the dry
season and burned for thousands of years.

Australopithecus africanus fossils abound with carbon-13.
This means that either they ate large quantities of carbon-13
enriched foods such as grasses and sedges, or they ate animals
that ate these plants, or both. Grasses have large quantities
of silica crystals which scratch tooth enamel. Fossil teeth of
the Australopithecus africanus do not have scratches compatible
with eating grasses and they do not have sharp edges that can
cut raw meat. Australopithecus africanus could not eat sedges
because sedges grow in wet places only. This means that
Australopithecus africanus ate cooked meat, probably snakes
and lizards killed by wildfires.

Australopithecus afarensis was a bipedal hominid and immediate
ancestor of the Australopithecus africanus. It inhabited dry
bushland, riparian woodland, probably with seasonal floodplains,
and riverine forest habitats. Australopithecus afarensis could
not sleep on trees because its hands were too weak, and its feet
did not grasp well. It was vulnerable to predators at night
unless it slept inside a shelter. Chimps make tree nests, so
the idea of more intelligent australopithecines making shelters
is not far fetched. The shelter was probably just a pile of
sticks on the ground with large cavity in its center. The
shelter was a safe hiding place, so a lone apith could forage
in the vicinity of the shelter and exploit its low density food
sources, such as the gallery forest. When a lion prowled the
gallery forest, the apiths screamed "danger" and retreated into
the shelters. A gallery forest surrounded by a desert was a
perfect habitat because there were no competitors and no
predators.

Some of the shelters were destroyed by natural bush fires.
As the apiths learned how to protect their shelters from the
fires, they understood how fire works. Perhaps the firestick
farming was invented by a subspecies of Australopithecus
afarensis which evolved into the Australopithecus africanus.
The migrating Australopithecus africanus did not have time to
make the shelters, so it had to sleep in the trees. This
explains its ape-like features: curved hand and foot bones,
short legs, divergent big toes, and upward oriented shoulder
joints.

So far the Australopithecus africanus fossils were found only
in east and south Africa, but east Sahara was also good habitat
for the migrating fire apes. In Egypt the lowest level of Nile
is in April and May, while the highest level is in September
(Asuan) and October (Cairo). Low water level is attractive to
hunting hominids because it concentrates prey animals in a small
area and it dries up reeds and grasses. The fire apes migrated
north in the spring along the Nile River. They had to return to
south Sahara in the fall. Microwave images of Sahara taken by
satellites show dry, ancient bed of a river that flowed through
the center of Sahara in the south-west direction. The lowest
level of water in this river was in late summer or fall. This is
exactly what the fire apes needed to migrate to south Sahara in
the fall. The fire apes followed the dry season as they migrated
counterclockwise around the present day Chad.

What tools did the fire apes use? Their most difficult task was
spreading the fire. They were not smart enough to start a fire,
so they must have invented some ways of transporting it. A single
stick taken from the fire does not burn longer than a minute. A
bundle of parallel, long sticks or reeds (like fasces) burns much
longer. Hollow bones could also be used to transport fire.

There are biological arguments in favor of the fire ape theory:
- We cannot eat red meat unless it is cooked or ground.
- We have sweat glands. Sweat mixed with soot protected the
fire apes from the fires.
- The smell of fire is probably offensive to the sensitive
olfactory organs of the wild animals, but we tolerate it so
well that some people inhale smoke for fun.
- People, including hunters, have weak sense of smell.
- Cooked meat is devoid of parasites, so it is safe to eat.
- Playing with fire is dangerous -- this explains why we are
the only species having Homo level of intelligence.
- Humans have eyelashes, but they are not well adapted to
life in a desert, so their eyelashes have different purpose
than camel's eyelashes.
- Broad nasal aperture of Australopithecus africanus and
external nose of genus Homo may indicate environment polluted
with dust or smoke.
  #9  
Old August 8th 04, 01:31 AM
Sander Vesik
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Default How special is the Solar System? (Forwarded)

Alain Fournier wrote:

Yes there is evidence against it. The evidence isn't very strong
but we are now getting some serious data on the matter. Well
the evidence is somewhat strong that our solar system is
fundamentally different from the majority of planetary systems
around stars. It is much weaker about Earth-like planets being
very rare. Most planets found to date are Jupiter sized or bigger
in an elliptical orbit with periapsis less than 1 AU. If Jupiter
had an elliptical orbit with periapsis less than 1 AU, then
Earths orbit wouldn't be stable and Earth wouldn't be.


No - its just that that type of objects and star systems are what
we can detect best so of course they are most numerous. Oh, and even
with a hot super-Jupiter you could get terrestrial planets in habitable
zone given favourable presence of smaller gas giants.


Alain Fournier


--
Sander

+++ Out of cheese error +++
  #10  
Old August 8th 04, 09:32 AM
Alain Fournier
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Posts: n/a
Default How special is the Solar System? (Forwarded)



Sander Vesik wrote:
Alain Fournier wrote:

Yes there is evidence against it. The evidence isn't very strong
but we are now getting some serious data on the matter. Well
the evidence is somewhat strong that our solar system is
fundamentally different from the majority of planetary systems
around stars. It is much weaker about Earth-like planets being
very rare. Most planets found to date are Jupiter sized or bigger
in an elliptical orbit with periapsis less than 1 AU. If Jupiter
had an elliptical orbit with periapsis less than 1 AU, then
Earths orbit wouldn't be stable and Earth wouldn't be.



No - its just that that type of objects and star systems are what
we can detect best so of course they are most numerous. Oh, and even
with a hot super-Jupiter you could get terrestrial planets in habitable
zone given favourable presence of smaller gas giants.



We can detect large planets close to the star. But why
are they in elliptical orbits. We can detect circular
orbits as well as elliptical orbits. I agree that
there could still be lots of Earth like planets. But it does
seem likely that our solar system is atypical, if only because
our planets close to the sun are in circular orbits. It
still is early to declare what is a typical solar system
but we are now getting some data. And that data is not
pointing towards our solar system being typical.

Alain Fournier

 




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