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Space Forum / SETI / July 2005Drake increases estimate to 50K communicating E.T. civilizations
On the space.com website there is a Drake equation streaming video link with Frank Drake and Seth Shostak talking about the equation. I believe that Dr. Drake estimated many years ago around 10,000 transmitting ET civilizations in our galaxy. In the video he has increased the number to 50,000 and estimated the average distance between communicating ETI's at one thousand light years apart. _______________________________ E.T. phone home (hopefully we'll be in the beam!) Jason H. Jason, > Jason H. wrote: > On the space.com website there is a Drake equation streaming video link [quoted text clipped - 6 lines] > > E.T. phone home (hopefully we'll be in the beam!) Jason H. (and we'll be looking at the right spot in the sky, at the right frequency and the right time :)) In Frank Drake & Dava Sobel's book "Is Anyone Out There", written in 1992 (wow, how time flies), Frank Drake said, in the preface, that he estimated there were 10000 advanced civilizations in our galaxy alone. Well, he also said he expected that contact would be made by the year 2000. Rats, we missed that one! I'm just stating what was predicted in the book and I'm not really being critical of Dr Drake whom I admire like Carl Sagan, Freeman Dyson, and many others who had tinkered and spent lifetimes working in the SETI field ... I think Shostak has indicated 2025 as the next threshold for contact. I am sceptical now of that happening but I am looking forward to the detection of an Earth like planet(or planets) before that time (I might live that long just out of pure stubbornness!). [....] > I think Shostak has indicated 2025 as the next threshold for contact. I am sceptical now of that happening but I am looking > forward to the detection of an Earth like planet(or planets) before that time (I might live that long just out of pure > stubbornness!). Indeed, I think Seth indicated this (2025) number last year or so. Did he give any indication as to how he got to that number ? It seems to me that if Drake states that the nearest ETI is an average of 1000 LY away, then the only way to get contact in 20 years would be either of 2 options : (1) We detect the ETI's at 1000 LYs away, or (2) ETIs do some serious 'blind-beaconing', targeting millions of potential ETI star systems for millions of years. I think (2) is very unlikely, and for (1) we need a massive antenna array. I think 20 years is very optimistic. I'd vote for (1), in 200 years. We'll all be dead and gone then, but hey, we are the pioneers. Odds are always against pioneers. We might be lucky ! But I'm not going to hold my breath. I'm with you Alfred : I hope to see the day that we positively detect Earth-like planets in the 'habitable zone' around a star. Rob > Rob Dekker wrote: >>"Alfred A. Aburto Jr." <aburto@sbcglobal.net> wrote in message news:t_tCe.19$Fk4.0@newssvr21.news.prodigy.com... [quoted text clipped - 6 lines] > Indeed, I think Seth indicated this (2025) number last year or so. > Did he give any indication as to how he got to that number ? I don't recall exactly. I think it was in regard to the enhanced capabilities of the Allen Tellescope (multiple beams and greater sensitivity --- more stars searched at greater ranges). > It seems to me that if Drake states that the nearest ETI is an > average of 1000 LY away, then the only way to get contact [quoted text clipped - 6 lines] > I think (2) is very unlikely, and for (1) we need a massive antenna array. > I think 20 years is very optimistic. I'd vote for (1), in 200 years. They're out there. Surely several centuries down the road there will be pretty good statistics on life on other planets (or not) ... When we find the first earthlike planet it will certainly generate alot of interest (in studying it and learning more about it. Maybe if things look really interesting, one might even send a robot probe to investigate in more detail). This is all real long term thinking though. Lots of things could change for the better or worse during that time... > We'll all be dead and gone then, but hey, we are the pioneers. > Odds are always against pioneers. We might be lucky ! > But I'm not going to hold my breath. > > I'm with you Alfred : I hope to see the day that we positively detect > Earth-like planets in the 'habitable zone' around a star. Hey NASA and ESA lets get cracking :-) with those terrestrial planet finder missions! > Rob > > Rob Dekker wrote: >>>"Alfred A. Aburto Jr." <aburto@sbcglobal.net> wrote in message news:t_tCe.19$Fk4.0@newssvr21.news.prodigy.com... [quoted text clipped - 9 lines] > I don't recall exactly. I think it was in regard to the enhanced capabilities of the Allen Tellescope (multiple beams and greater > sensitivity --- more stars searched at greater ranges). Ah yes. That makes sense. I think he meant to say that IF there is a beacon out there aimed at us, THEN we will be able to find it within 20 years. That is reasonable, and although the odds are against a beacon being aimed at us, it is very certainly the best thing we can do right now. >> It seems to me that if Drake states that the nearest ETI is an >> average of 1000 LY away, then the only way to get contact [quoted text clipped - 10 lines] > ... When we find the first earthlike planet it will certainly generate alot of interest (in studying it and learning more about > it. Maybe if things look really interesting, one might even send a robot probe to investigate in more detail). A probe would certainly take hundreds or thousands of years to get there and back. So we probably will try everything with 'observation' from Earth. You raised a thought though : If we find such a planet, and it is, say 500 LYs away, then we might be tempted to send a (beacon) signal to it. But since it will take 1000 years until we might hear something back, we probably stop transmitting after we have said all we wanted to say in a first 'Hello' message repeated 1000 times (total duration maybe 1 month or so)... Then, if there is no answer after 1000 years, we could try it again (if we still exist ourselves). But there is no use transmitting to a dead planet, so ETI beaconing might happen very few and far between. It's a big space out there. > This is all real long term thinking though. Lots of things could change for the better or worse during that time... > [quoted text clipped - 6 lines] > > Hey NASA and ESA lets get cracking :-) with those terrestrial planet finder missions! Righton ! >> Rob > >> I'm with you Alfred : I hope to see the day that we positively detect > >> Earth-like planets in the 'habitable zone' around a star. > > > > Hey NASA and ESA lets get cracking :-) with those terrestrial planet finder missions! > > Righton ! Ditto! We need look no further than the two ripe suns of Alpha Centauri A & B right on our doorstep. Until the launch of NASA's TPF, which could be several years out, the European Southern Observatory (ESO), the Keck Telescopes and the Anglo Australian Telescope (AAT) are all well placed to scan the habitable zones of each system. I discussed the magnitudes and separations of potential exoplanets around those two stars, a while back here:- http://www.astroscience.org/abdul-ahad/extrasolar-planets.htm An optimistic projection... if somewhat of a tiny ray of hope in the eternal darkness... Abdul Ahad > AA Institute wrote: > [quoted text clipped - 21 lines] > > Abdul Ahad 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. 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)... Al > 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 [quoted text clipped - 3 lines] > 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 > AA Institute wrote: > [quoted text clipped - 14 lines] > will be permanently in daylight, facing Proxima, and the other side > will experience continuous night. What if a planet was Earth sized or bigger with an atmosphere and oceans? Would tidal lock kill possibilities still? I wonder? ... just thinking out loud so to speak ... > 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. Humm, but with a planet with an atmosphere and/or oceans maybe the flares would present challenges beat rather than lost perhaps? Who knows? Life seems to thrive on challenges. Today I read in Scientific American about a microbe that uses arsenic rather than oxygen and thrives in highly saline water/arsenic environments (Searles Lake in California --- where only recently anyone thought to look for life there). The flare may be the "bolt of lightning", or the "spark" needed to start life. It seems to me, on Earth, life started in an extremely hostile environment, not some quiet and safe place ... just some thoughts ... >>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 [quoted text clipped - 26 lines] > > http://www.solstation.com/stars/alp-cent3.htm Yes, this is what I remember ... the HZ are ok places then ... just need to find planets in them ... > Abdul Ahad > >>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 [quoted text clipped - 30 lines] > life. It seems to me, on Earth, life started in an extremely hostile > environment, not some quiet and safe place ... just some thoughts ... Yes Al, there are limitless possibilities and that's why we should always keep an open mind. But if we're searching for ETI - *intelligent* civilisations - then working on a law of averages, with prioritisation of what limited resources we have, it's probably going to be more fruitful to focus a search for New Earth in the vicinity of the Alpha Centauri A+B pair first. Since these two stars are more 'sun-like', it's where we as humans would feel naturally most comfortable. And I'm not just being romantic here! :-) Abdul Ahad "AA Institute" <abdul.ahad@ntlworld.com> wrote in message [...] > > 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 [quoted text clipped - 8 lines] > will be permanently in daylight, facing Proxima, and the other side > will experience continuous night. That's interesting Abdul ! Mercury is in tidal lock with the sun, but it is clearly outside (or actually way inside) the HZ for the sun. Interesting is that for Proxima, the HZ apparently overlaps with the tidal-lock zone... Is there a simple formula which expresses the 'tidal-lock-zone' of a particular star ? So, can we determine when a stars' HZ is totally within a tidal lock zone ? Thus, if we assume for now that life (at least intelligent life) is unlikely inside the tidal lock zone, and unlikely outside the HZ, then there should be a large class of brown dwarfs that are unlikely to be able to have intelligent life planets... > 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. Variable stars, and variable circumstances are not necessarily excluding life. The best progress in evolution on planet Earth was made during the 'catastrophic' events like the KT boundary. So, I think that for life, it is more important how often these flare's happen than the simple fact that the do (happen). If there are violent outbreaks of 100% increase in radiation on the star every 100 years or more often, then I think the likelyhood of intelligent life developing there might be slim. Although I must say that we dont know much about this. Maybe life can built-in a protection system which allows easy survival for ANY frequently occurring event... How little we know... ... > Mercury is in tidal lock with the sun, but it is clearly outside (or > actually way inside) the HZ for the sun. Interesting is that for Proxima, > the HZ apparently overlaps with the tidal-lock zone... > Is there a simple formula which expresses the 'tidal-lock-zone' of a > particular star ? > So, can we determine when a stars' HZ is totally within a tidal lock zone ? That is an interesting notion, but I'm not sure we can draw any hard borders around a star as to where a planet would become tidally locked. Mercury is not *entirely* locked in that manner, as it still has an axial rotation period which is different to its orbital revolution. Much will also depend upon how much the planetary body departs from being a complete sphere in its geometric profile; if there is a significant "lobe" on one hemisphere of such a body, then that can cause it to become tidally locked more readily and at a greater distance (radius vector) than would otherwise be the case. Unless someone here knows the dynamical intricacies more precisely and can point out to the contrary, I don't think there is any *definite* grounds for saying a planet in the HZ around Proxima Centauri is going to be tidally locked for certain. There is a high *likelihood* of that outcome, but it's not a foregone conclusion. > Thus, if we assume for now that life (at least intelligent life) is unlikely > inside the tidal lock zone, and unlikely outside the HZ, then there should > be a large class of brown dwarfs that are unlikely to be able to have > intelligent life planets... I think brown dwarfs and many red dwarfs are automatically excluded from the top of SETI lists on the grounds that they are too "exotic" compared to more mainstream main sequence stars like the Sun? > > 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 [quoted text clipped - 15 lines] > built-in a protection system which allows easy survival for ANY frequently > occurring event... Where the seeds of life have already taken a firm *hold* and the germination process has been triggered, the sprouting seedlings would appear to be as solid as steel in their endurance of, and adapatability to, harsh conditions. The planetary surface probably has to be sufficiently *fertile* (i.e. with the right conditions, such as what existed on the early Earth) for that to happen. A lot of it really depends on whether we view life as having been "created" or "seeded" here a long time ago. If it was "created" (out of non-living compounds), then we're talking of some highly unique event that probably does not recur with a greater probability than... say one in a billion. In that case, life is indeed a very rare phenomena in the universe and the nearest ETI civilisation may be... well, in the Andromeda spiral, over 3 million light years away! OTOH, if the seeds of life were already created - some time soon after the birth of the universe - and are constantly drifting through space in realtive abundance waiting for a *fertile* planetary shore on which they can land and take a hold, then life is a lot more commonplace than we think, and the nearest ETI civilisation may be just over on the other side of the interstellar pond around Alpha Centauri. The question is, and I've asked this often before without getting any strong vibes either way, which is the popular view amongst the SETI community here in the early years of the 21st century? Which was the more common view back in the 1960s, when professor Frank Drake and others started the SETI program? Is at (A) Created, or (B) Seeded?! Cheers! Abdul Ahad > Rob Dekker wrote: > "AA Institute" <abdul.ahad@ntlworld.com> wrote in message [quoted text clipped - 20 lines] > particular star ? > So, can we determine when a stars' HZ is totally within a tidal lock zone ? The tidal force is inversely proportional to the cube of the distance of the planet from the star (in this case) and it is proportional to the mass of the star. If we let dMercury be the distance of Mercury from the Sun, and dHZ be the distance of the HZ from a star, and Msun & Mstar be the mass of the sun & star, and then let: a = Mstar/Msun b = dMercury/dHZ then c = a * b^3 > 1 will indicate tidal locking is in effect (using Mercury as a reference --- since it is just barely tidally locked with the Sun) A rule of thumb estimate, but it should work. I'd better test this with known cases (planets & satellites) :-) > Thus, if we assume for now that life (at least intelligent life) is unlikely > inside the tidal lock zone, and unlikely outside the HZ, then there should [quoted text clipped - 22 lines] > > How little we know... [....] >> Is there a simple formula which expresses the 'tidal-lock-zone' of a >> particular star ? [quoted text clipped - 14 lines] > > A rule of thumb estimate, but it should work. Very smooth ! I like it. So, similarly, the HZ is inversely proportional to the square of the distance of the planet from the star (assuming a constant-power/m^2 is habitable), and it is proportional to the power of the star. Let dEarth be a good reference for a HZ around the sun, and Psun & Pstar be the power of the sun & star, and then let : x = Pstar/Psun y = dEarth/dHZ then z = x * y^2 == 1 will indicate that the planet receives Earth-like habitable power Putting both formula's together to get rid of the dHZ, then we will get an equation for a planet which has Earth-like 'climate' but will be get into tidal lock like Mercury is around our Sun : Pstar^2 / Mstar^3 < (Psun^2 / Msun^3 ) * (dMercury / dEarth)^6 The right hand side we can calculate, and is constant. Voila! There is our boundary for stars for which habitable planets will be tidal-locked. At constant temperature, the stars' power should roughly scale with square of its radius, and its mass with the cube of radius, so the radius of the star should not influence the outcome of this boundary too much. So I think only temperature is the defining factor. Low temperature (red/brown) stars should tend get their habitable planets in tidal lock.... Disclaimer: I did not verify any of this, I'm sort of writing as we speak... > I'd better test this with known cases (planets & satellites) :-) Me too ! [...] Actually Rob, there's a good discussion and analysis of stars and habitable zones pertaining to SETI, here: http://www.solstation.com/habitable.htm Interesting! Abdul Ahad > Actually Rob, there's a good discussion and analysis of stars and > habitable zones pertaining to SETI, here: [quoted text clipped - 4 lines] > > Abdul Ahad Thanks Abdul. There certainly has been done a lot more work on this already than I could have imagined. Rob > Rob Dekker wrote: > [quoted text clipped - 56 lines] > > [...] The Habitable distance is determined by temperature of the star. The star type (O,B,A,F,G,K,M,N ... etc) and subclass number 0,1,2,3,...,9 define the effective temperature of a main sequence star. From this, assuming a simple model, one can determine the distance from the star where the HZ lies using the Stefan-Boltzmann Law ... From the Luminosity of the stars (given type and class effective temperature) one can determine such things as the mass and radius of the star. From this information the tidal locking would fall out. Of course Kasting has a much more complex model of habitable planets and habitable zones, albedo effects and atmospheric effects. Well from his model the neat plots shown on the "solstation" web site fall out. Also Margaret Turnbull's webside is mentioned at the "solstation" site and she has detailed charts of HZ'z (based on Kastings work) and much information about the stars that will be used for the next SETI searches. Also, she needs help filling in data for her tables and she indicated in one powerpoint paper I read that she could use the volunteer help of amateur astromers using under ultilized 1m optical telescopes to help fill in her table. Nice project for amateur astronomers ... Al "Alfred A. Aburto Jr." <aburto@sbcglobal.net> wrote in message [...] > The tidal force is inversely proportional to the cube of the distance of > the planet from the star (in this case) Actually... Why is it cube dependent, Al ? Tidal force is gravity, so it should be square-dependent of distance. Right ? Rob > Rob Dekker wrote: > "Alfred A. Aburto Jr." <aburto@sbcglobal.net> wrote in message [quoted text clipped - 8 lines] > > Rob The tidal force depends on the change in the gravitational force with distance. This is what causes differential tugs on a body and thus tides... This is the derivative of the gravitational force relative to distance(r)... that is, if F is the gravitational force (k/r^2) then the tidal effects derive from dF/dr which is -k/r^3 ... k is a constant ... By the way Abdul, very nice web site (solstation)... Al > > Rob Dekker wrote: >> "Alfred A. Aburto Jr." <aburto@sbcglobal.net> wrote in message [quoted text clipped - 12 lines] > and thus tides... This is the derivative of the gravitational force relative to distance(r)... that is, if F is the gravitational > force (k/r^2) then the tidal effects derive from dF/dr which is -k/r^3 ... k is a constant ... Thanks for refreshing my mind. Actually, I should have know, because one of my favorite web-sites discussed tidal forces very nicely, and contains the R^-3 formula for tidal force : http://www.talkorigins.org/faqs/moonrec.html > By the way Abdul, very nice web site (solstation)... > Al |
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