SETI A Moon's Significance in Life?


Larry Klaes (lklaes@bbn.com)
Thu, 24 Jun 1999 12:57:28 -0400


>X-MSMail-Priority: Normal >X-Mailer: Microsoft Outlook Express 4.72.2106.4 >X-MimeOLE: Produced By Microsoft MimeOLE V4.72.2106.4 >Date: Thu, 24 Jun 1999 19:20:33 +1000 >Reply-To: History of Astronomy Discussion Group <HASTRO-L@WVNVM.WVNET.EDU> >Sender: History of Astronomy Discussion Group <HASTRO-L@WVNVM.WVNET.EDU> >From: Ann Fieldhouse <ann@MAGNA.COM.AU> >Subject: A Moon's Significance in Life? >To: HASTRO-L@WVNVM.WVNET.EDU > >On 19 June Richard and Tatyana Wilds <Wilds@NETWORKSPLUS.NET> wrote: > > >>This is just the kind of "science" that is ruining the credibility of the >>profession (what little is left in the public view). I was taught that >>sciece is "observable and repeatable." It is nice to have ideas about >>"coincidences", but this is not science. Many of us have worked very hard >>to deal with the numbers and experiments that are required to do quality >>science. The public view of science, at this point, is so eroded through >>the use of unsubstantiated "coincidence" that I fear the scientific >>community is facing an ever greater deterioration of public financial >>support for science programs simply through its increasing unreliability. >>So much of what we call science today is not observable and repeatable and, >>therefore, not science, that we stand to delude ourselves in the extreme. >>What ever happened to science by the numbers? > >My son raised this topic of the New Scientist Article on a closed kids >astronomy group here in Australia and got the following interesting >response. The author has agreed to us posting his comments to your list, >but anonymously, as he is not a member of Hastro-L, and hence would not be >able to discuss the matters he raises further on this list. > >This is his response: > >"> Edwin Nelson wrote: >> >> What do you think about this New Scientist Article by Marcus Chownd? >><SNIP> > >> If Gonzalez is right, then all extraterrestrials, wherever they are, >> are likely to live on planets like ours that experience total >> eclipses. But since an unusually large Moon is rare, he says, this >> suggests that both ETs and total eclipses are very rare indeed. >> >> From New Scientist, 19 June 1999 >> >> >http://www.newscientist.com/cgi-bin/pageserver.cgi?/ns/19990619/newsstory5.h >tml >> >> Dr Gonzalez's Home Page is at: >> http://www.astro.washington.edu/gonzalez/ >> >> You can see some articles by him at : >> http://www.reasons.org/resources/FAF/index.html >> A site called "Reasons to Believe" >> >> What do you think? > >Hi Edwin, > >It's an interesting theory - and it makes some sense. But like a lot of >theories, it needs a lot more examination before it can be anywhere near >proven. > >I can see a few potential problems with it though which I'll outline >below (I'll quote where the problems are and explain what I think they >are). > >> So are we just extraordinarily lucky? Guillermo Gonzalez of the >> University of Washington in Seattle thinks not. He points out that our >> distance from the Sun, and hence its apparent size, is a necessary >> condition for us to be here. "If we were a little nearer or farther >> from the Sun, the Earth would be too hot or too cold and so >> uninhabitable," says Gonzalez. > >OK - let's look at this, first of all. > >Gonzalez is quite correct in his assertion that if we were closer to the >sun or further away from it, it would heat us more or less - and so life >like ours would not be possible. > >But how much closer could we be - or how much further away? > >Earth is about 150,000,000 km from the sun. > >The two planets that come closest to us are also the two planets that >have the most similar distance between themselves and the sun - Venus >and Mars. > >Venus is 72% of our distance from the sun or about 108,000,000 km. Mars >is 1.88 times our distance from the sun or about 282,000,000 km. > >Let's look at Mars first - because generally speaking, with the >exception of Earth, it is regarded by scientists as the place with the >best potential for life of some sort. > >The surface of Mars is cold - very cold by our standards. A hot day on >Mars is around -30 degrees Celsius. A very cold night about -80 degrees >Celsius. I know these temperatures sound very cold - and they are. But >there are places on Earth that get very nearly that cold - and life >exists there. As an example, some inhabited parts of Alaska have >experienced temperatures down to -50C (-62C seems to have been the >record). These are places where life exists. > >So while Mars would not be a comfortable place for us to live in terms >of temperature, it would be possible. And I'm talking about life like >ours - on a colder planet, life could have evolved that was better able >to survive in that weather. It's happened here - when the Earth got >suddenly cold 65,000,000 years ago, the dinosaurs died out - and the >mammals who could survive colder weather survived. > >Now we look at Venus. The surface of Venus is, unlike Mars, somewhere >man could not survive. Temperature is only one reason why this is so - >but it's a big enough one. The surface temperature of Venus is around >460 degrees Celsius - and there is no way we could survive that. > >But the temperature of Venus is not caused only by the fact it's closer >to the sun than us. In fact, that's a rather small difference. What >causes Venus's huge temperature increase over Earth is it's atmosphere. >The amount of Carbon Dioxide and Sulfur Dioxide in the Venusian >atmosphere causes a massive greenhouse effect that heats the planet >incredibly. There's a lot of evidence that at one stage, Venus and the >Earth were very similar in terms of atmosphere - but for some reason, >this changed. > >If Venus had the same atmosphere as Earth, it's surface temperature >would be only 10 degrees higher than our own - in fact, because it is >slightly smaller than us, with lesser gravity, the atmosphere would be >thinner and this would lead to cooling. Venus could very easily have had >the same basic temperature as Earth - if it had had the same atmosphere. > >Distance from the sun makes up only part of the conditions necessary to >life - 25% either way could easily make no real difference, depending on >other factors. > >> At the same time our existence depends on an unusually large moon >> since its pull stops the Earth wobbling around too much on its axis >> and causing wild and catastrophic swings in climate like those on >> Mars. Our Moon, which is unusually large compared to those in almost >> all other planet-moon systems, probably formed from molten material >> blasted from the Earth during the impact of a giant body more than 4 >> billion years ago. > >Is our moon unusually large? No, not really. Gonzalez article says there >are a total of 65 moons in our solar system - I'll use that figure >because it's about right (there is some dispute). > >There are 5 moons larger than our own in the solar system - Titan >(Saturn); Io, Ganymede, and Callisto (Jupiter); and Triton (Neptune). In >addition, Europa (Saturn) is nearly the same size as the moon. > >So there are 7 moons of comparable size to our own in the solar system - >or about 10%. Our moon is not that unusual. > >Now what does set our moon apart is the size of it compared to Earth - >the other large moons all orbit planets far larger than Earth. Our moon >has a diamater about a quarter of our planets. Out of the six large >moons mentioned above, the biggest fraction any of those have is Triton >to Neptune and that's only about 7% of the diameter. > >So our moon is unusually large when compared to the planet it orbits - >but even then it's only number 2. Chiron, the moon of Pluto is nearly >half the diameter of it's mother planet. > >So, if we assume our solar system is representative of the Universe as a >whole - and that's the only assumption we can make because we have so >little data, then moons the size of ours account for around 10% of all >moons - and large moon/planet relationships, around 3-4%. These numbers >may seem small - but viewed on the scale of our galaxy, we're talking >millions of planets - and billions in the Universe. > >> If Gonzalez is right, then all extraterrestrials, wherever they are, >> are likely to live on planets like ours that experience total >> eclipses. But since an unusually large Moon is rare, he says, this >> suggests that both ETs and total eclipses are very rare indeed. > >This is the biggest flaw I can see. > >Let us assume that Gonzalez is correct in his basic idea - that >extraterrestials are most likely to live on planets that experience >total eclipses. How rare would this make ETs? > >First of all, we need to start with Drake's Equation. This is a formula >developed by a Californian astronomer named Frank Drake in an effort to >estimate how many planets have life (and specifically intelligent life >that is civilised enough to theoretically communicate with other >planets). It's only a theory - and it's only an estimate. But it's >widely used by astronomers when they discuss the extraterrestial life. > >Drake's Equation is designed to work out how many civilisations >currently exist in our galaxy. It consists of seven factors. > >The Astronomical Factors >a) Rate of Star Formation In the Galaxy >b) Fraction of Stars With Planetary Systems >c) Average number of planets potentially suitable for life orbiting >stars with planetary systems. > >The Biological Factors >d) Fraction of planets where life is possible, on which life has evolved >e) Fraction of planets with life, where that life is intelligent > >The Sociological Factors >f) Fraction of planets with intelligent life where that life has >developed civilisations able to communicate off the planet. >g) Average life of one of those civilisations. > >The equation itself is: > >N (Number of civilisations able to communicate off planet currently in >our galaxy) = a*b*c*d*e*f*g > >So we now need to work out the values for all those letters. > >The rate of star formation is something we pretty much know from >observations. There's enough maths in this post already, so I won't do >the maths for that but roughly 10 new stars are formed in our galaxy >each year. > >So a = 10 > >The fraction of stars with planets orbiting them is pretty high - so far >planets have been found orbiting three of the twenty stars closest to us >- and all of them may have planets for all we know, we just can't see >them. Astronomers normally guesstimate that at least 10% of stars have >planets, so we'll use that figure here. > >b = 0.1 > >Number of planets that could theoretically support life. Well, we know >in our own solar system one planet could definitely support life (our >own) and two or three others might have done - so the fraction seems to >be around 0.11 - 0.33 if our own solar system is typical. Again, let's >play it safe and use the low figure - 0.1 > >c = 0.1 > >Many biologists believe that if a planet has the right conditions for >life, then life will develop at some stage on it. We know Earth has life >- we have reason to believe Mars and Venus could have had it once too. >So the chance of life on a suitable planet could be very high - but >again, we'll use a low figure. 0.1 again. > >d = 0.1 > >Now, we need to look at the chance intelligent life will develop on a >planet with life - again many biologists think that given enough time >this is certain. Life evolves and gets better over time - eventually >intelligence will develop. Once again, I'll use a 0.1 figure - but it >could be much higher. > >e = 0.1 > >Now - what is the chance that intelligent life will develop enough to >communicate with other stars? This is very hard to answer. We've just >reached that stage - after being intelligent for somewhere between >100,000 and 1,000,000 years. I'll use the 0.1 figure again - but this >figure is more rubbery than the others. > >f = 0.1 > >Finally, how long will such a civilisation last. Our own civilisation >has had the theoretical capacity to send and receive messages from the >stars for about 100 years (since the development of radio). How long >will our civilisation last? Simple answer is we don't know - but let's >assume our civilisation will last 100,000 years at at least our current >level of technology - based on evolutionary patterns, this seems viable. >Man will not change too significantly over the next 100 milleniums. > >g = 1000 > >If we plug these numbers into Drake's Equation, we discover there are 10 >civilisations in our galaxy with at least our level of development. > >This is a small number - but remember, I used 0.1 over and over again in >that equation. In many cases, the value could be much higher - 0.9 or >higher, even. > >And there are 1,000,000,000 galaxies in the Universe - if each of them >have 10 civilisations, that makes 10 billion civilisations around the >stars. > >If we then add Gonzalez's ideas into the equation, we have to add a >feature h - the fraction of planets where solar eclipses occur. Let's >assume it's about 0.1 again, just as it is in our solar system. > >That's 1,000,000,000 civilisations of aliens out there. > >The universe is so vast, even very rare things happen all the time." > >I would be interested in any responses people here might have to this. > >Ann Fieldhouse (Nelson) >ann@magna.com.au >ann@fieldhouse.com.au >



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