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Drakes Equation



Forwarded from SPACE TECH list:

Date: 10 Sep 1995 16:54:39 GMT
>From: Frank Crary <fcrary@rintintin.Colorado.EDU> 
Subject: Drakes Equation

In article <9507288096.AA809642490@ncr.disa.mil>, 
Terry Colvin <colvint@ncr.disa.mil> wrote:
>>Despite searching for gas giants with Hubble etc around nearest 100 stars, no 
>>giant planets have been spotted.
>      Using our curent technology to detect planetary bodies orbiting other 
>stellar systems is hellishly difficult. Planets are dark, cool, low-mass 
>bodies compared to the stars they orbit. It's much too early to suggest 
>jovian planets are rare. Our own solar system sports two full-fledged jovian 
>bodies (Jupiter and Saturn) and two sub-jovian examples (Uranus and Neptune).

It's dangerous to make assumptions from one solar system. But you 
are correct about how difficult it is to detect planets
around other stars. In fact, the current searches would _not_ 
have been capable of detecting our own solar system if the 
shoe had been on the other foot (i.e. if we'd been at one
of those other stars and looking at the solar system.) All 
we can say is that there don't seem to be solar systems 
with planets much more massive than Jupiter but in a 
Jupiter-like orbit, nor planets the size of Jupiter 
orbiting much farther from their stars. (E.g. something
5 AU from a star and several times the mass of Jupiter 
or something the mass of Jupiter but orbiting at 10 or 
15 AU.)

>>Also, super computer simulations of forming
>>solar systems easily produce inner rocky planets, but not gas giants.

>     I don't recall seeing these data. Please cite the source.

I'd very much like to see the reference as well. This is 
completely opposite to computer models I am aware of. As
far as I know, Jupiter is believed to have formed very early 
and in a way that would ought to occur in other solar 
systems. Numerical models then _assume_ the existence
of Jupiter in examining the formation of the terrestrial 
planets. Without Jupiter, you'd get several thousand 
"planetesimals" ranging in size from large asteroids
to things the size of the Moon. But these objects wouldn't 
be on crossing orbits and it would take them a very
long time to merge into the terrestrial planets (i.e. 
too long, like a billion years; far to long for this 
to be what happened.) When Jupiter's gravity is
added to the simulations, it perturbs the orbits of these 
planetesimals and puts them on crossing orbits. That 
speeds up the formation of the terrestrial planets and 
the results seem to be both accurate and repeatable. 
(That is, over dozens of simulations starting with 
different initial conditions, the models almost
always end up with three to six terrestrial planets, 
an asteroid belt closer to Jupiter and the more 
massive terrestrial planets are near the center,
the way Venus and the Earth are, with the less

massive ones, e.g. Mercury and Mars, either closer 
or farther from the Sun.)

                                         Frank Crary
                                         CU Boulder