Re: Drake increases estimate to 50K communicating E.T. civilizations

Alfred A. Aburto Jr. wrote:
>
> Humm, did the doppler shift method (via Marcy, et.al.) fail to find
> extra-solar planets around Alpha Centauri? I think they did, but memory
> is not serving me well here.
>
> Also, how about the habitable zone around Proxima Centauri? I know it is
> a small red dwarf star, but still it does have a habitable zone. Proxima
> is the 3'rd star of the Alpha Centauri system.

There is nothing to rule out the possibility of a planet existing in
the HZ around Proxima of course, but with respect to the planet then
being "habitable" for life, there are two things to keep in mind.
Firstly, the HZ would be so close - something like just 10% of the
Sun-Mercury distance in our solar system - as to make the planet
perpetually *tidally locked* in its orbit. That would mean one side
will be permanently in daylight, facing Proxima, and the other side
will experience continuous night.

The second thing is, Proxima is what's known as a "flare star", so that
it doubles its luminosity every so often. That would render conditions
on the surface of any of its planets highly catastrophic to any life
managing to take a hold on it.

>
> Another question is the stability of planetary orbits in a system with 2
> stars orbiting one another (Alpha Centauri A & B). In this system stable
> planetary orbits probably (vague ancient memory surfaces here) exist
> only fairly near to the stars and may be closer to the stars than their
> estimated habitable zones (treating the HZ as if the stars were single
> stars)...

According to papers quoted on the Solstation website (link below), both
Alpha Centauri A and B can hold terrestrial-sized rocky planets within
their habitable zones without them suffering significant adverse
gravitationl perturbations, so as to de-stabilise any life forms
evolving there. Even at their closest approach to one another
(periastron passage) every 80 years, the two stars are still 11 AUs
apart (a touch further than how far Saturn orbits the Sun in our solar
system).

I quote:
"In a binary system, a planet must not be located too far away from its
"home" star or its orbit will be unstable. If that distance exceeds
about one fifth of the closest approach of the other star, then the
gravitational pull of that second star can disrupt the orbit of the
planet."

The habitable zones around Alpha Centauri A and B are well outside
those dynamical thresholds, and easily stable enough for life to evolve
in peace and comfort.

More here:

http://www.solstation.com/stars/alp-cent3.htm

Abdul Ahad

.

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