what do aliens look like

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What do Aliens look like?

Prof. Dr. Ulrich WalterInstitute of AstronauticsTU Mnchen, Garching

2004

Published in

StarObserver Spezial with the title "Sind wir allein im All?", March 1999, p.71-79

Stories happen in your head. When youread a novel, the story builds up in pictures

right before you eyes. We all do that, andthis is also why somebody might be bitterlydisappointed by a movie based on a novelhe or she had already read. Because yourown phantasy is very often streets aheadof movie versions. There is a legendarynumber of different images in our headsabout what aliens could look like, I daresaythat there are as many different images aspeople on the Earth. To put it in a nutshell:Because nobody knows anything specific,everybody knows something different. We

want to put an end to this situation, andfrom a scientific-biological perspective,we'll have a look as to what might be

possible in principle.

An intelligent alien being is characterisedby a large information storage, from abiological point of view it is called a brain,and a body, which interacts with the envi-ronment, i.e. it takes up outside informati-on (hearing, seeing, feeling ), transmitsit to the brain, and on the other hand trans-lates brain signals into outside information(talking, writing, drawing, gesticulating ),and probably, but not necessarily, alsomoves. If you just have a look at the infor-mation storage, there have been quite

unusual suggestions: the order of particlesbecause of the interaction between char-ged particles and magnetic fields; the

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order between ortho and para hydrogen insolid hydrogen at lowest temperatures; oralso polymeric atoms in neutron stars,which, just like nucleic acid, form longchains, and can store information. Butthere is no possible way that reproducingintelligent life could have developed natu-rally from these very exotic foundations.This is why I would exclude these approa-ches as being highly improbable.

There are many basic considerationswhether life could e.g. be based on siliconchemistry or silicon circuits, maybe withammonia as a solvent. Life in the form ofhighly developed circuits, or a combinationof organic structures and inorganic elect-

ronic structures could be possible. Theprecondition for the reproduction of this lifeform however would be a kind of self-organisation and self-repair of the circuits,and we have not even been able to obser-ve any attempts of this kind of behaviour.So it could only be an artificially generatedform of life, not an original form, which onlyin evolutionary stages higher than theknown ones would have acquired intrinsicreproduction mechanisms. No matter fromwhich angle you approach the problem,

and many scientist have already rackedtheir brains over it, the only realistic ap-proach seems to be the variant realised bythe human race: The basis of evolutionarylife has to be carbon chemistry, or organicchemistry. According to the experience ofcarbon chemistry, the most diverse orga-nic structures were formed out of the fourbasic substances carbon, hydrogen, oxy-gen, and nitrogen. This is why it is veryprobable that every form of life is basicallycomposed of H, C, N and O. So potentially

existing extraterrestrial life cannot be thatmuch different from earthly life with regardto its basic substances. How life is indeedformed right down to the vital complexmolecules, whether e.g. the 24 elementsnecessary on the Earth (H, C, N, O, F, Na,Mg, Si, P, S, Cl, K, Ca, V, Cr, Mn, Fe, Co,Cu, Zn, Se, Mo, Sn, I) are also indispen-sable elsewhere, or whether there aredifferent atoms or more than 24, might

vary very much.

And: organic chemistry requires liquidwater as a solvent - as a neutral mediumwhere the chemical reactions take place.Water is an indispensable elixir of life, notonly on the Earth, but the liquid state ofwater predetermines the consistence of allforms of life. They are inevitably soft andvulnerable, as they need to contain a lot ofwater (about 60% of the human bodyconsists of water). The firmness of theouter skin is no longer sufficient when thebody has reached a certain size; then abody-bearing skeleton is necessary.

So the question is what aliens could look

like with a soft carbon-based body. Howbig or how small can an intelligent beingbe? Are there any bodily features we canderive from basic considerations? Well,that is indeed possible. The scientific fieldof allometry (allo... = other, metry = mea-sure) deals with the issue of how the cha-racteristics of objects with substantiallydifferent dimensions behave. One typicalexample are model aeroplanes. Althoughthey have been reduced according toscale, their behaviour in the air is totally

different from that of their originals. Most ofthem are not able to fly at all. So modelaeroplanes have to be constructed totallydifferently from their originals, and of cour-se, on the other hand no large aeroplaneconstructed according to the well-knownpaper models used at school during thebreaks, would be able to fly at all. Thereason is that the relationship between thesize of the flying object and the air viscosi-ty, the so-called Reynolds' number, whichdetermines the flight behaviour, drastically

changes with different aeroplane scales.Smaller aeroplane models model theirlarge originals realistically in special windtunnels only because there, the air viscosi-ty can be cooled down to the temperaturenear liquid nitrogen, about -200C, and soyou receive the same Reynolds' number,and with it the same flight behaviour, asthe original aeroplane at room temperatu-re.

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Every body usually has special and adap-ted characteristics with regard to its envi-ronment. And it is the same with beingsand their different sizes. For the dimensi-ons of highly developed beings the gravi-tational force they are submitted to, has afundamental influence. In 1979, M.H.Hartfound out with computer simulations, that aplanet may have a mass of no less than

85% and no more than 133% of the Earthto be able to have moderate surface tem-peratures over a period of two billionyears, which seems to be necessary forthe evolution of an intelligent being. Withthese very Earth-like gravitational conditi-ons intelligent beings cannot have consi-derably different characteristics comparedto the human being. Just an example to

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illustrate this for bodily size. Let's assumean alien is a huge being, ten times biggerthan humans, but with an identical compo-sition; in that case its weight would be onethousand times more, about 80t. The crosssection of its bones, because it is two-dimensional, would only be one hundredtimes bigger, i.e. the strain on the boneswould be ten times higher compared to ourbones. Such a strain on the bones of ourgiant alien would inevitably break its thigh-bones already after the first step. A giantcould form superproportionally large bonessuch as an elephant or the dinosaurs, butthis is also limited, as at a certain stage, abody would be only composed of bones,and they would just be able support them-

selves.

On the other hand, it is also not possiblethat beings have an infinitely small size.The smallest conceivable storage andinformation unit is the base as such, a

complex organic molecule with its cor-responding "infrastructure". A highly deve-loped intelligent being such as the humanbeing needs a system of about 100 billionbasic elements (i.e. the human nervecells), which are linked with each otherand constitute the "intelligence". Thisseems to be a minimum size. Skilful mon-keys with the same stereoscopic visionand the same hand-eye coordination ashumans despite their long evolution time ofabout 25 million years have not managedto produce a higher intelligence. Theirabsolute brain mass of less than 100grams with just some kg bodily weight, andthe considerably smaller amount of links isapparently not sufficient. Only with its 400

grams of brain mass, the human beingseems to have overcome the critical thres-hold to higher intelligence. As moleculeshave a naturally determined size, thecontrol centre (brain) of an intelligent beingalso needs to have a certain minimum

size. Information on bodily functi-ons and intelligence could theore-tically be optimised to a volume ofmaybe some cubic centimetres,and then the complete beingwould have a size of at least

10cm.

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The complete being would alsoinclude an energy supply system.If our alien is a being which liveson land, and metabolises bymeans of oxygen in the air, justlike we do, then it will need a lung.Because only small insects with adiameter of less than 1 cm areable to directly transfer oxygen

into their cells by means of diffu-sion on their body surface. Withlarger animals, whose majority ofcells is more than some milli-metres below the surface, thissimply does not work anymore, orelse, you would have a being witha thickness of just 1cm, but a veryextensive size, which is obviouslynot very practical. Only a lung likeour lung with an internal surfaceof 100 m2, and a very branchedblood vessel system is able toprovide enough oxygen for all thecells of a being with a size of


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more than 10 cm. Insects may be verysuccessful in the geological evolution asthe most numerous species, but they forma kind of physiological dead end in thedevelopment of intelligent life. Without theslightest signs of a lung or bone systemthey just do not have the chance to increa-se their size sufficiently to build up a brainsize that would be able to produce intelli-gence.

And the same is true for food intake (me-tabolism). This is why larger beings cannotsimply metabolise via their body surface,just as the small beings do, but they haveto "eat"; in order to do that, they havesome kind of food inlet (mouth), an exten-

sive digestive tract (the intestine with manyvilli), which transmits the nutrients into thebody, and last but not least, they also havean outlet (...). We can summarise thataliens would, just like us, not be largerthan the more primitive beings becausethey are more complicated, but they wouldbe more complicated, because they had tobe larger.

The eyes are the most important organ fororientation in the environment when vital

sunlight is around. So in the geologicalcourse of history, the eye has been rein-vented about 40 times (wings only three ormaybe four times), and it seems obviousthat organic aliens would also have eyes(although not necessarily wings). As theeyes are so important for orientation, mosthigher beings have two eyes. The opti-mum eyes however would need to have a

size similar to the human eye. With theresolution and visual field necessary forour bodily size, our light-sensitive retinalrods have a certain minimum number anddimensions which correspond more or lessexactly with the wavelength of light. Thevision of smaller eyes is much more blur-red, and larger eyes do not make anysense for these reasons. And indeed thereis no other being on the Earth with consi-derably larger eyes. Even an elephant hasrelatively small eyes, which looks a bitstrange compared to the huge size of itsbody. There are mammals with smallereyes such as mice, but because of the badresolution they are not able to distinguish ahuman face from another at a distance of

2 metres, although for these animals thiswould not be relevant at all.

If you sum up all the different characte-ristics, aliens who originated from naturalevolution, would, just like us, be composedof "flesh" (agglomerate of reproducingaqueous cells) and "blood" (a liquid whichtransports oxygen and nutrients to all thebody cells), their size would be between0.1 and 10m, they would probably have asize of about one metre, and they would

have a properly dimensioned skeleton tobear the body, probably they would alsohave eyes, a lung, a mouth and its functio-nal counterpart. The only question thatremains is how these parts are to be loca-ted. And this is something, the only thing,we leave to the imagination of ScienceFiction authors and fantasy painters.

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what do aliens look like

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