# Sagans List and life in the universe **URL:** \ **Category:** Great Debates\ **Created:** [November 12, 2010, 12:10am UTC](https://boards.straightdope.com/t/sagans-list-and-life-in-the-universe/560372 "2010-11-12T00:10:09Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![MrDibble](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mrdibble/32/114_2.png) [@MrDibble](https://boards.straightdope.com/u/MrDibble)\ **Post date:** [November 13, 2010, 8:44am UTC](https://boards.straightdope.com/t/sagans-list-and-life-in-the-universe/560372/21 "2010-11-13T08:44:58Z")
> [@Lord\_Ashtar](#): > > I must say, I never considered a black hole as a possible energy source for life. I wonder how that would work? “Black” holes put out _a lot_ of energy, all across the EM spectrum, including the visible. Certainly some put out enough for photosynthesis (but the X-Rays, Gamma rays and the like make this not a nice place for Earth-autotrophs). Remember that a collapsar can be orbited by a planet/s just like any other large gravitational body. Some sort of life could, for instance, develop around active galactic nuclei, but it would have to be so very different from “life as we know it” [:dubious:] as to be very hard to describe beforehand. I don’t \*know \*how such an entity would look, but I do know that it wouldn’t be impossible. Some sort of broad-spectum planet-bound burrowing radiovore with silicate organ-analogues and metal-shielded casing? Or diaphanous gas-cloud entities with atom-thick gold “leaves” for trapping radiation? Who knows… ---
**Author:** ![MrDibble](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mrdibble/32/114_2.png) [@MrDibble](https://boards.straightdope.com/u/MrDibble)\ **Post date:** [November 13, 2010, 8:49am UTC](https://boards.straightdope.com/t/sagans-list-and-life-in-the-universe/560372/22 "2010-11-13T08:49:42Z")
> [@Grateful-UnDead](#): > > What??? > > What I am saying is that we specify a particular requirement. > > Then taking all the planets in the universe, we delete those that don’t meet that requirement. I can confidently say that you’re _never_ going to find a naturally-occurring planet that doesn’t have oxygen, carbon and nitrogen in some form. ---
**Author:** ![wintertime](https://avatars.discourse-cdn.com/v4/letter/w/c68b51/32.png) [@wintertime](https://boards.straightdope.com/u/wintertime)\ **Post date:** [November 13, 2010, 10:17am UTC](https://boards.straightdope.com/t/sagans-list-and-life-in-the-universe/560372/23 "2010-11-13T10:17:07Z")
Speaking of silicon-based life: While we have found oxidized silicon at other places, I am not aware that we can say the same about silanes or silicones (?)… can you think of other molecules that could be an indication of a silicon-based biochemistry? ---
**Author:** ![MrDibble](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mrdibble/32/114_2.png) [@MrDibble](https://boards.straightdope.com/u/MrDibble)\ **Post date:** [November 13, 2010, 4:38pm UTC](https://boards.straightdope.com/t/sagans-list-and-life-in-the-universe/560372/24 "2010-11-13T16:38:13Z")
> [@wintertime](#): > > Speaking of silicon-based life: While we have found oxidized silicon at other places, I am not aware that we can say the same about silanes or silicones (?).. can you think of other molecules that could be an indication of a silicon-based biochemistry? Not really - but that’s the problem in a nutshell: you’re (unconsciously) expecting a silicon-based life to use an analogue to carbon-based oxidation/solvent biochemistry, when it need do no such thing. Perhaps it uses electro-chemistry instead, or something else out of left field. Look at the models of abiogenesis that use replicating clay minerals as a substrate. Those clays are partway to living (replicating, growing) without a biochemical basis. ---
**Author:** ![wintertime](https://avatars.discourse-cdn.com/v4/letter/w/c68b51/32.png) [@wintertime](https://boards.straightdope.com/u/wintertime)\ **Post date:** [November 13, 2010, 5:38pm UTC](https://boards.straightdope.com/t/sagans-list-and-life-in-the-universe/560372/25 "2010-11-13T17:38:30Z")
> [@MrDibble](#): > > Not really - but that’s the problem in a nutshell: you’re (unconsciously) expecting a silicon-based life to use an analogue to carbon-based oxidation/solvent biochemistry, when it need do no such thing. Ah, you might have assumed I hold a certain position when I need do no such thing to ask this question. 😉 The lack of any freely and widely available molecules that could be building blocks of silicon-based life is per se no evidence that such life _cannot_ exist. Their lack, however, is a reason to say that a) the existence of the analogous forms you mentioned is not supported so far by our observations and b) that we might not find the (chemical) traces we’d expect from silicon-based life, if it is indeed built upon different principles. If someone proposed some kind of electro-chemistry, he’d still have to show how this could work to elevate the thought above wild speculation into hypothesis-territory; this way, we’d also get an idea about the characteristic traces that such a life form would leave behind, so that we would be able to identify it in the first place. ---
**Author:** ![septimus](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/septimus/32/410_2.png) [@septimus](https://boards.straightdope.com/u/septimus)\ **Post date:** [November 13, 2010, 7:10pm UTC](https://boards.straightdope.com/t/sagans-list-and-life-in-the-universe/560372/26 "2010-11-13T19:10:42Z")
Two other reasons the Earth’s Moon might have been helpful in the development of advanced life: (1) Due to the feedback of conserved momentum sharing, the Moon helps stabilize the Earth’s axis (obliquity of the ecliptic) or so I’m told. Otherwise climate would fluctuate much more erratically. (2) Tides and waves, partly due to Moon’s gravity, create tide pool fluctuations which assist the development of land life from sea life. ---
**Author:** ![Grateful-UnDead](https://avatars.discourse-cdn.com/v4/letter/g/ccd318/32.png) [@Grateful-UnDead](https://boards.straightdope.com/u/Grateful-UnDead)\ **Post date:** [November 13, 2010, 10:55pm UTC](https://boards.straightdope.com/t/sagans-list-and-life-in-the-universe/560372/27 "2010-11-13T22:55:44Z")
> [@MrDibble](#): > > Some sort of life could, for instance, develop around active galactic nuclei, but it would have to be so very different from “life as we know it” [:dubious:] as to be very hard to describe beforehand. I don’t \*know \*how such an entity would look, but I do know that it wouldn’t be impossible. Some sort of broad-spectum planet-bound burrowing radiovore with silicate organ-analogues and metal-shielded casing? Or diaphanous gas-cloud entities with atom-thick gold “leaves” for trapping radiation? Who knows… This is one of the issues that can be adressed in general terms, and it doesn’t require any specific knowledge of the particular life form. Regardless of the nature of the final life form, it would be fundamental that it would require chemical bonds be formed, producing molecules of varying complexity. However, if the environment is such that the radiation intensity is such that those bonds are broken as soon as they are formed, then no life of any kind will develop. Therefore, one of the requirements on the list would be that the planet be a minimum distance of “D1” from the galactic centre. Add to this the requirement that there would need to be a certain minimum matter density for a planetary system to form in the first place, and we can see that there would be a “donut” of space within the galaxy within which the planet can form, and chemical bonding could occur. Just these two requirements alone have culled a large volume of the galaxy. Having said that, your idea of a “gas cloud entity” is very interesting. Would a diffuse, gaseous cloud be considered “life”? How would that work? ---
**Author:** ![Grateful-UnDead](https://avatars.discourse-cdn.com/v4/letter/g/ccd318/32.png) [@Grateful-UnDead](https://boards.straightdope.com/u/Grateful-UnDead)\ **Post date:** [November 13, 2010, 11:02pm UTC](https://boards.straightdope.com/t/sagans-list-and-life-in-the-universe/560372/28 "2010-11-13T23:02:59Z")
> [@MrDibble](#): > > I can confidently say that you’re _never_ going to find a naturally-occurring planet that doesn’t have oxygen, carbon and nitrogen in some form. Yes, and if it turns out that any or all of these elements are fundamental requirements for life, those requirements immediately put some physical parameters on the supporting planet. Specifically, the planet would require the presence each of these elements, and they would have to be present in certain minimum abundance. So, as a coarse screen, how many planets in the galaxy/universe have oxygen/carbon/nitrogen in abundance “A”, “B”, “C”? So, now we have a “donut” within a galaxy that encloses the volume of space that can support life due to material and radiation constraints, now we add the requirement for C, N, O in specific minimum abundances, and the number of supporting planets gets smaller. ---
**Author:** ![Blake](https://avatars.discourse-cdn.com/v4/letter/b/a9a28c/32.png) [@Blake](https://boards.straightdope.com/u/Blake)\ **Post date:** [November 14, 2010, 3:40am UTC](https://boards.straightdope.com/t/sagans-list-and-life-in-the-universe/560372/29 "2010-11-14T03:40:57Z")
> [@Grateful-UnDead](#): > > Therefore, one of the requirements on the list would be that the planet be a minimum distance of “D1” from the galactic centre. Yes, but that distance is probably incredibly short. As Mr Dibble suggests, life could develop underground even at ridiculously high radiation levels. Radiation “such that [chemical] bonds are broken as soon as they are formed” is highly ionising radiation. It can’t travel far through solid matter because, well it interacts with chemical bonds, thus breaking them. So long as the planet isn’t actually made molten by the radiation it receives. molten then there is nothing at all stopping even carbon and water based life from existing on it. You really don’t have to get too far from galactic centre to find solid planets, so I doubt that restriction has nay meaningful impact at all on the list. If we posit life that isn’t carbon and water based then even being molten isn’t an obstacle. > [@](#): > > Add to this the requirement that there would need to be a certain minimum matter density for a planetary system to form in the first place, and we can see that there would be a “donut” of space within the galaxy within which the planet can form, and chemical bonding could occur. Can you provide evidence for this claim that outlying star systems don’t have any planets? > [@](#): > > Just these two requirements alone have culled a large volume of the galaxy. No, they haven’t. The first has negligible effect. The second is unproven. ---
**Author:** ![Blake](https://avatars.discourse-cdn.com/v4/letter/b/a9a28c/32.png) [@Blake](https://boards.straightdope.com/u/Blake)\ **Post date:** [November 14, 2010, 3:44am UTC](https://boards.straightdope.com/t/sagans-list-and-life-in-the-universe/560372/30 "2010-11-14T03:44:58Z")
> [@Grateful-UnDead](#): > > So, as a coarse screen, how many planets in the galaxy/universe have oxygen/carbon/nitrogen in abundance “A”, “B”, “C”? 100%. Or so close to as make no difference. Aside from noble gases, you have just names 3 of the 5 most abundant elements on the universe. I can’t see any way that a planet could not have those elements in abundance. ---
**Author:** ![Grateful-UnDead](https://avatars.discourse-cdn.com/v4/letter/g/ccd318/32.png) [@Grateful-UnDead](https://boards.straightdope.com/u/Grateful-UnDead)\ **Post date:** [November 14, 2010, 7:11am UTC](https://boards.straightdope.com/t/sagans-list-and-life-in-the-universe/560372/31 "2010-11-14T07:11:41Z")
> [@Blake](#): > > Yes, but that distance is probably incredibly short. As Mr Dibble suggests, life could develop underground even at ridiculously high radiation levels. Radiation “such that [chemical] bonds are broken as soon as they are formed” is highly ionising radiation. It can’t travel far through solid matter because, well it interacts with chemical bonds, thus breaking them. > > So long as the planet isn’t actually made molten by the radiation it receives. molten then there is nothing at all stopping even carbon and water based life from existing on it. You really don’t have to get too far from galactic centre to find solid planets, so I doubt that restriction has nay meaningful impact at all on the list. > > If we posit life that isn’t carbon and water based then even being molten isn’t an obstacle. > > Can you provide evidence for this claim that outlying star systems don’t have any planets? > > No, they haven’t. The first has negligible effect. The second is unproven. The question of radiation intensity and composition is an interesting one; we know that one of the biggest impediments for human travel beyond the moon is the lethal radiation emanating from the sun, and other stars. So until this issue is resolved, humans won’t be going too far. With regard to the issue of radiation in general: the solar wind is composed of more than just ionizing radiation. So it is an interesting to consider what a planet which is close to the galactic centre and exposed to the intense radiation there, would look like. Would it be a solid ball like the earth, or would it be reduced to a ball of plasma? The only issue is, as you point out, is to calculate the radius of lethality. With the regard to your other comment: I didn’t say that outlying stars don’t have planets. I did say that there is an outer limit at which the density of matter would be too low for there to be sufficient mass to aggregate to make either stars or planets. This outer radius would be a matter for both calculation and observation. ---
**Author:** ![Grateful-UnDead](https://avatars.discourse-cdn.com/v4/letter/g/ccd318/32.png) [@Grateful-UnDead](https://boards.straightdope.com/u/Grateful-UnDead)\ **Post date:** [November 14, 2010, 7:15am UTC](https://boards.straightdope.com/t/sagans-list-and-life-in-the-universe/560372/32 "2010-11-14T07:15:15Z")
> [@Blake](#): > > 100%. Or so close to as make no difference. Aside from noble gases, you have just names 3 of the 5 most abundant elements on the universe. I can’t see any way that a planet could not have those elements in abundance. This is intuitively obvious. However, what I said was that if we assume that C,N,O are fundamental to life, then there would be a requirement that they each be a certain, yet to be defined, proportion of the planetary mass. ---
**Author:** ![MrDibble](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mrdibble/32/114_2.png) [@MrDibble](https://boards.straightdope.com/u/MrDibble)\ **Post date:** [November 14, 2010, 8:26am UTC](https://boards.straightdope.com/t/sagans-list-and-life-in-the-universe/560372/33 "2010-11-14T08:26:28Z")
> [@wintertime](#): > > If someone proposed some kind of electro-chemistry, he’d still have to show how this could work to elevate the thought above wild speculation into hypothesis-territory; this way, we’d also get an idea about the characteristic traces that such a life form would leave behind, so that we would be able to identify it in the first place. We know how such a life-form _could_ work - we have the elements of such a “bio”-chemistry all around us, in the form of integrated circuits, batteries, electroplating, photovoltaics, motors, etc. How such a system would actually be put together, I have no idea. Or what the traces would be. I could speculate all day long about how nano-solenoids could make a myomere analogue, or the chance of metallic deposition on a silicate substrate in an H[sub]2[/sub]SO[sub]4[/sub] ocean in a high magnetic-flux environment could produce primitive evolveable active circuits, but none of that speculation would be backed by any sort of numbers at all. So, not out of the realm of possibility, but in no way a scientific discussion. ---
**Author:** ![MrDibble](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mrdibble/32/114_2.png) [@MrDibble](https://boards.straightdope.com/u/MrDibble)\ **Post date:** [November 14, 2010, 8:33am UTC](https://boards.straightdope.com/t/sagans-list-and-life-in-the-universe/560372/34 "2010-11-14T08:33:57Z")
> [@Grateful-UnDead](#): > > Regardless of the nature of the final life form, it would be fundamental that it would require chemical bonds be formed, producing molecules of varying complexity. Again, this isn’t necessarily so. > [@](#): > > Having said that, your idea of a “gas cloud entity” is very interesting. Would a diffuse, gaseous cloud be considered “life”? How would that work? I don’t know. I could speculate - patterns or compression waveforms of varying densities could encode information, which is ultimately the fundamental unit of replication and inheritance. Think [fluidics](http://en.wikipedia.org/wiki/Microfluidics), but constrained by variant densities not hard components. ---
**Author:** ![Blake](https://avatars.discourse-cdn.com/v4/letter/b/a9a28c/32.png) [@Blake](https://boards.straightdope.com/u/Blake)\ **Post date:** [November 14, 2010, 8:49am UTC](https://boards.straightdope.com/t/sagans-list-and-life-in-the-universe/560372/35 "2010-11-14T08:49:49Z")
> [@Grateful-UnDead](#): > > The question of radiation intensity and composition is an interesting one; we know that one of the biggest impediments for human travel beyond the moon is the lethal radiation emanating from the sun, and other stars. So until this issue is resolved, humans won’t be going too far. What does that have to do with your list? We are talking about where life can arise and survive, not where life that has arisen somewhere radically different can survive. A camel can’t survive in the arctic either. That doesn’t tell us a damn thing about whether life can survive there > [@](#): > > With regard to the issue of radiation in general: the solar wind is composed of more than just ionizing radiation. So what? You were only talking about ionising radiation. > [@](#): > > I did say that there is an outer limit at which the density of matter would be too low for there to be sufficient mass to aggregate to make either stars or planets. Yeah, that’s what we call, in technical terms, the edge of the galaxy. > [@](#): > > This outer radius would be a matter for both calculation and observation. So you want to calculate how many planets are in the galaxy based on how many are beyond the edge of the galaxy? :dubious: > [@Grateful-UnDead](#): > > This is intuitively obvious. > > However, what I said was that if we assume that C,N,O are fundamental to life, then there would be a requirement that they each be a certain, yet to be defined, proportion of the planetary mass. No what you asked was what proportion of planets have that abundance. And the answer is close enough to 100%. ---
**Author:** ![septimus](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/septimus/32/410_2.png) [@septimus](https://boards.straightdope.com/u/septimus)\ **Post date:** [November 14, 2010, 12:48pm UTC](https://boards.straightdope.com/t/sagans-list-and-life-in-the-universe/560372/36 "2010-11-14T12:48:37Z")
> [@MrDibble](#): > > Again, this isn’t necessarily so.I don’t know. I could speculate - patterns or compression waveforms of varying densities could encode information, which is ultimately the fundamental unit of replication and inheritance. Think [fluidics](http://en.wikipedia.org/wiki/Microfluidics), but constrained by variant densities not hard components. Perhaps, but I’m doubtful. You need a stable platform for some sort of entropic pump. Can large-scale storms provide that? With permanence adequate to pass significant information to “children”? ---
**Author:** ![Blake](https://avatars.discourse-cdn.com/v4/letter/b/a9a28c/32.png) [@Blake](https://boards.straightdope.com/u/Blake)\ **Post date:** [November 14, 2010, 1:06pm UTC](https://boards.straightdope.com/t/sagans-list-and-life-in-the-universe/560372/37 "2010-11-14T13:06:56Z")
> [@septimus](#): > > Perhaps, but I’m doubtful. You need a stable platform for some sort of entropic pump. Can large-scale storms provide that? With permanence adequate to pass significant information to “children”? The Red Spot on Jupiter is a large scale storm that has lasted at least 400 years and shows no signs of abating. Since many living organisms produce daughter generations every hour or so, and some less, I would have to say that 400 years is sufficient time. ---
**Author:** ![wintertime](https://avatars.discourse-cdn.com/v4/letter/w/c68b51/32.png) [@wintertime](https://boards.straightdope.com/u/wintertime)\ **Post date:** [November 15, 2010, 10:30pm UTC](https://boards.straightdope.com/t/sagans-list-and-life-in-the-universe/560372/38 "2010-11-15T22:30:59Z")
> [@MrDibble](#): > > We know how such a life-form _could_ work - we have the elements of such a “bio”-chemistry all around us, in the form of integrated circuits, batteries, electroplating, photovoltaics, motors, etc. > > How such a system would actually be put together, I have no idea. (..) Neither do I, nor if it is possible in the first place. But if we ever find something like that, I’m sure we’ll have another discussion forthcoming: whether we’ve found first or second generation life. ---
**Author:** ![wheresmymind](https://avatars.discourse-cdn.com/v4/letter/w/91b2a8/32.png) [@wheresmymind](https://boards.straightdope.com/u/wheresmymind)\ **Post date:** [November 16, 2010, 2:39am UTC](https://boards.straightdope.com/t/sagans-list-and-life-in-the-universe/560372/39 "2010-11-16T02:39:06Z")
“1. lie on the outlying regions of a galaxy in order to have reduced impinging radiation levels from the galaxy center” - We have organisms here on earth that can survive radiation levels thousands or even millions of times higher than any place on earth (google deinococcus radiodurans, tardigrades). They evolved this ability basically by accident, it just turns out that some adaptations that protect and repair organisms from the effects of dessication work for radiation exposure as well. “3. must have an iron core to provide shielding from its sun’s radiation” - see above. “4. must be a larger than “a” to ensure it can hold an atmosphere” - I don’t see why an atmosphere is necessary. Life on earth certainly first evolved in water. “5. must be a smaller than “b” in order to ensure that gravitational effects won’t squash life forms” - Gravity would have little effect on organisms living in water (or another liquid), or very small terrestrial organisms. The force of gravity on a bacterium is so small it can be ignored - even if it was hundreds or thousands of times higher. “6. must have a moon larger than “c” to shield the planet it from space debris” - perhaps life would have evolved a bit later without the protection of the moon, but Jupiter and the inner planets have done a good job in the past 4 billion years of clearing the inner solar system. If the moon disappeared tomorrow we wouldn’t be beset by planet-sterilizing impacts. “7. the moon must be smaller than “d’ in order to avoid destructive gravitational effect” - As long as the planet isn’t literally ripped apart, I don’t think tidal forces would have much of an effect on aquatic life. It could even help life along, by keeping the planet’s interior molten and providing energy through volcanism. “8. must be a certain distance from its sun in order to provide sufficient energy to warm the planet, but not too close, or not too far.” - Volcanism, radioactive decay, and tidal forces could also warm a planet (or moon) just fine. Examples in our own solar system include Io, Europa, Ganymede and Titan. The sun must: 1. must be of a certain age and size in order to provide the radiation spectrum conducive to life - Solar radiation isn’t necessary for life (see 8 above). On our own planet we have many chemoautotrophic organisms that get their energy from inorganic compounds. Hydrothermal vent communities are one obvious example. They’ve even found photopigments that can harvest photons of blackbody radiation coming from the vents. 2. must be a larger than “X” in order to have the required energy output. - See above. 3. must be smaller than “Y” in order to minimize gravitational effects - See 7 above. I think the only thing in this regard that is necessary is that the orbit of the planet is stable and the planet is far enough away for water or some other liquid solvent to exist. I’m with Mr. Dribble in that all I think is really necessary is some sort of energy source, and some sort of “stuff” that can react. ---
**Author:** ![Blake](https://avatars.discourse-cdn.com/v4/letter/b/a9a28c/32.png) [@Blake](https://boards.straightdope.com/u/Blake)\ **Post date:** [November 16, 2010, 3:00am UTC](https://boards.straightdope.com/t/sagans-list-and-life-in-the-universe/560372/40 "2010-11-16T03:00:28Z")
> [@wheresmymind](#): > > “4. must be a larger than “a” to ensure it can hold an atmosphere” - I don’t see why an atmosphere is necessary. Life on earth certainly first evolved in water. Unless you are talking about molten metal, any liquid is going to have an appreciable vapor phase. So surface liquid=atmosphere. This is most definitely rue of water. If a planet can’t hold an atmosphere it sure can’t hold liquid water. [Previous page](https://boards.straightdope.com/t/sagans-list-and-life-in-the-universe/560372.md?page=1) [Next page](https://boards.straightdope.com/t/sagans-list-and-life-in-the-universe/560372.md?page=3)