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View Full Version : How Would You Define Life on Another Planet?



Mr. Milton Banana
2004-Jun-05, 06:35 PM
On the Europa/radiation thread I started, someone brought up the fact that radiation might actally be part of a possible life cycle inside the icy moon. I thought about this some more, and I came up with the idea for this particular thread.

And this subject most likely has been discussed here, but just in case...

How would one go about defining what life is on another planet? Keep in mind that there is the danger of allowing ourselves to define life based solely on what we've been exposed to on Earth. It is a vast Universe, and to use the Earth as the definitive example of what to look for in regards to life might be rather limiting.

What are the suitable criteria to start off with, and where should we see about thinking beyond the box?

:-k

The Supreme Canuck
2004-Jun-05, 07:21 PM
Well, one criteron should be that it must be self-propogating.

eburacum45
2004-Jun-05, 08:13 PM
Fire is self propagating; is fire life?

It is going to be very difficult to define life, especially as we might encounter some of the prebiotic progenitors of life such as proteins and nucleic acids on some worlds...
are prions alive? viruses? cell-like structures without genetic material?

another problem I can imagine is that some distant civilisation might have developed self replicating Von Neumann machines and sent them out into the galaxy to spread out; if they foolishly allowed these machines to have the capacity to evolve, they might spread into every available ecological niche in the deeps of space...

and it may even be the case that these artificial lifeforms are the predominant form of life on many worlds.

The Supreme Canuck
2004-Jun-05, 09:27 PM
Fire isn't life, no. One of the criteria for life is self-propogation; there are others that fire doesn't possess.

Cougar
2004-Jun-05, 11:49 PM
Metabolism. (Utilization of external energy.)

Reproduction.

Homeostasis. (Maintaining relatively constant internal conditions, different from external environment.)

PhantomWolf
2004-Jun-06, 08:58 AM
Fire isn't life, no. One of the criteria for life is self-propogation; there are others that fire doesn't possess.

Though it isn't, it does fit the basic criteria.

Respiration and external fuel use (It breathes and consumes food for energy)

Self-replication (It can create more of its own kind)

Excretion (It leaves a waste product)

What it doesn't have, that all living things do, is an internal blueprint that can be passed on to offspring. That I think is probably the most important factor in determining life forms.

Of course as a Chemist, I'd have to point out that the likelihood is that other life would to be close to what we already know. Once I would have argued otherwise, but now…. The reason for this is that chemicals generally react the same way all the time. Since Carbon is the only element that is capable of creating the long polymers required for life the odds are that life forms must be carbon-based (silicon won't do it well enough, it only forms short polymers without Oxygen in it, and Si-O bonds are to rigid to create life like chemicals.)

There is a little more room to manoeuvre with the atmosphere or "blood types" though not much. Inert gases such as Helium are no use for life, as they don't react. Halogens such as Chlorine and Fluorine tend to be too oxidative and would destroy any organics. Nitrogen tends to be too un-reactive to oxide sugars etc. This leaves Oxygen or Hydrogen atmospheres as the most likely candidates and Hydrogen is most often a reducing agent so from that, the majority would be most likely be oxygen based.

Blood types would be far greater with a choice of metals to transport gases about the blood, though again Iron and Copper are the best two, Nickel, Cobalt, and Chromium are also highly possible.

The most probable difference we are likely to see in any alien life is in the polarity of the organics. All life on Earth is left-hand polarised. Alien organics could very easily be right-hand polarised instead, it'd be a 50-50 call. ;)

Ilya
2004-Jun-06, 08:04 PM
Self-organization.

dvb
2004-Jun-06, 09:13 PM
I was thinking about this very subject lastnight. We all know that animals evolved from the amoeba and that all plant life has evolved from paramecium. We also know that these 2 one celled creators of life depended on each other to evolve into more complex multi cellular organisms. For instance animals produce carbon dioxide from oxygen that plants generate, and we also provide highly nutrient soils for plants to grow in with our urine and feces.

Now if the paramecium didn't exist, then the amoeba may have evolved with completely different requirements, and became a lifeform uncomprehensible to us. The amoeba also may not have evolved at all had it not been for its counterpart. Or, for all we know, there could be another type of 1 celled organism that never evolved here on earth, and thus could have different requirements to evolve and mutate into more complex organisms.

Let me know if i'm not making sense here! :D

ness012345
2004-Jun-06, 09:27 PM
I would probably be say anything that can think, no matter how basic, would be life to me.

Even if its only smart enough to think about feeinding it's self, or breeding, if the object can make a thought, no matter how basic it is, it's life.

eburacum45
2004-Jun-06, 09:56 PM
So no trees then.

The Supreme Canuck
2004-Jun-06, 11:16 PM
I would probably be say anything that can think, no matter how basic, would be life to me.

Even if its only smart enough to think about feeinding it's self, or breeding, if the object can make a thought, no matter how basic it is, it's life.

What about jellyfish? No central nervous system. What about bacteria, too?

Spacedog
2004-Jun-06, 11:43 PM
i have dna, therefore, i think i am somebody special