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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