Why do we assume aliens will need (roughly) the same conditions as us?

[Djsnowboy267]

Just wondering, why do we look for planets that are the same as ours (besides future colonization), and when it comes to looking for aliens, why do we assume they need to have the same temperature range or atmosphere? What would stop some strange alien racing from living on Venus, if they are accustomed to the temperatures and can breath the atmosphere of the planet? Actually, just thought about this while typing the last sentence: Does it have anything to do with how it is usually assumed we evolved from small organisms from space/ meteors?

[WestAir]

DNA.

DNA can only survive feasibly given certain circumstances the habitat zone provides. Very few elements have the properties of DNA that allow for protein construction, sequencing, and what not. As you know, DNA is the code that makes us... us. All life, even on-life like viruses have DNA. We've yet to observe a life form on Earth without DNA, and Earth has had several billion years to work on it.

Outside of that, it may be theoretically possible for life to exist without DNA - I've read some concepts of plasma or hydrogen gas based life-forms being possible, but I digress.

[Djsnowboy267]

Totally didn't think about DNA! I am guessing that would cover the aspect of an alien race on a planet that was very hot. But what about atmosphere composition, would that affect DNA? What if there were no atmosphere at all?

I probably sound like I am interviewing or something, apologies, but this is interesting!

[Custard Donut (In Space)]

You make an interesting point, i once heard someone say that if organic beings had fully turned themselves to robotic beings (perhaps living in a virtual paradise inside their own minds) they may be found hanging around giant stars, from where they'd get their power source.

[softweir]

The problem is that we know of only one life-supporting planet. It is easy to argue that very different forms of life might evolve on other worlds, but we have no way of working out which ones might be hospitable to xeno-life. For all we know, every gas giant might be inhabited by enormous gas=bag lifeforms that derive energy from electric potentials in the atmosphere, and every ultra-hot world might be inhabited by beings composed of molten minerals - but we have no way to assess or predict the likelihood of these scenarios. Most importantly, we can't detect these creatures, because we don't have any idea what mark they might leave on their worlds. Life on Earth generates molecular oxygen - what would electricity-feeding life on a gas-giant generate that we could spot with telescopes?

Earth-like worlds are another matter. If we spotted a planet of approximately the right size and temperature that had an oxygen-rich atmosphere then we would be quite right to say "there's got to be life there!"

So we are starting with a search for Earth-like worlds, though I am sure other worlds are getting a look at, just in case somebody spots something as unusual as molecular oxygen, such as molecular chlorine or other highly reactive compounds.

[Pyre]

There are other possibilities. Project Rho has a whole page on alien life, and what it would be made of.

[Nibb31]

Only Star Trek (and Defiance) figures that Aliens are pretty much like us, except for weird hair colors and shapes on their forehead.

[K^2]

Life doesn't have to be exactly like ours, but there are several key elements we can pin down. Most importantly, energy flow is critical for maintaining low entropy, which you can't have life without. So we only need to look for life either on planets/moons that orbit a hot enough star, or these that give off its own thermal energy. We also know that some form of phase transitions will be required. You cannot manipulate entropy otherwise. This puts upper limits on temperatures. Id est, surface of a star itself is not a good place to look for life. Finally, and this is more of an extrapolation, we assume that evolution takes its time. That means the stable environment should exist for a long period of time. This effectively leaves us with planets orbiting stars or their moons as the only places to look for life. As that, in itself, places constraints on types of environments we deal with, we can make further statements on the types of life possible.

Long story short, while the habitable zones, as currently defined, are somewhat anthropocentric, reasonable limits for where we should look for life aren't that far out from there.

Of course, all of this assumes life that has naturally evolved in that environment. If we consider pan spermia or artificial life, a lot of these restrictions disappear. We are suddenly looking at any number of asteroids, comets, gas giants, rogue planets, and maybe even some dwarf stars as potential safe harbors for some kind of life. But that life got there from somewhere else, so looking for potential cradles of life is still a very good strategy.

The problem is that we know of only one life-supporting planet.

We might know two. If we confirm methanophiles on Titan, it'd be a huge breakthrough in our understanding of life in this universe. It'd take us from total uncertainty about life elsewhere and bring us to an almost absolute certainty that universe is filled with life.

Forget Mars. This is where we should be looking for life. Even if we find evidence that life existed on Mars, big whoop. It could have come from Earth. Or Terrestrial life could have found its origin on Mars. We have chunks of rock filled with organic materials being bounced between these two planets all the time, and possibility of one of these bringing in some archaea bacteria is far from remote. Titan is in different category all together, however. Life there would be unlike anything on Earth. We find life there and we have two planets in one solar system with very diverse environments given rise to life. That would be the end of any kind of discussion about whether or not Earth is special in the universe. We'd know with certainty that it is not.

Edited May 10, 2013 by K^2

[falofonos]

Proportions of elements in the observed universe... we are pretty much average for the proportions of elements available.

There may be other forms of data storage than DNA but it's unlikely to use radically different elemental composition.

There are life forms of earthly origins that live in some crazy extremes.

http://www.cosmonline.co.uk/extremophiles-meet-indestructibles?size=_original

None are likely to be sending us a first contact message soon though.

[magnemoe]

Only Star Trek (and Defiance) figures that Aliens are pretty much like us, except for weird hair colors and shapes on their forehead.

On the other hand, nature copy itself, fish, dolphins and sea living dinosaurs looked pretty similar, yes water put down some restrictions, however the Tasmanian wolf was an marsupial and looked like an wolf.

Many of the wild alien ideas does not work well, they would lose out against animals with more conversational designs. Yes you could get land living octopus, but they would not be dominating if you had animals with skeleton.

Yes its lots of things we don't know, why has we four limbs and not six, fish with six fins would work just as well, this would have the benefit that the front legs could easy have other use like gathering food or fighting. However I guess its an decent chance an intelligent alien would look humanoid. Walk on two legs, two arms, two eyes.

One other body plan who could work would be the dinosaur ones, think an raptor with better arms. huge tail who counterbalance the forward body, same body plan but not like the human one.

Follow the symmetry and no unneeded parts however not humanoid.

How I guess first contact will work out, we used 100 billion$ and 50 years going where and they try to eat our probe.

[drakesdoom]

I find it more likely we try to eat their probe but whose to know?

[magnemoe]

I find it more likely we try to eat their probe but whose to know?

Why should we, we would know it was an machine or at least something alien.

Some stone age people would not know, anything who moves around would be an animal as only animals move.

Add the issue that if we had alien civilizations going interstellar they should have run into us long ago, one option is that they already did but somebody ate the rover so they moved on.

Or perhaps this: http://freefall.purrsia.com/ff1400/fv01349.htm

[drakesdoom]

Who says their probe moves? Who says it is mechanical? What if it was a solid state crystal? What if it was an energy pattern with no physical form?

[Kerbface]

I find it more likely we try to eat their probe but whose to know?

Um... why would we try to eat an alien probe?

Modern humans don't tend to put machines, energy signatures, mysterious crystals or anything that we can't identify into our mouths intentionally.

[Nibb31]

The G'Gugvuntt were enemies of the Vl'hurgs, and these strange and warlike beings are on the brink of an interstellar war, because of an insult uttered by the G'Gugvuntt leader to the mother of the Vl'hurg leader. Resplendent in their black-jeweled battle shorts, they were meeting for the last time, and a dreadful silence filled the air as the Vl'hurg leader was challenging the G'Gugvuntt leader to retract the insult. At the precise moment, the phrase "I seem to be having tremendous difficulty with my lifestyle" (muttered by Arthur Dent to himself, which for some strange reason was carried by a freak wormhole in space back in time to the farthest regions of the universe where the G'Gugvuntts and the Vl'hurgs lived) filled the air over the conference table, which in the Vl'hurg tongue was the most dreadful insult imaginable. It left them no choice but to declare war on the G'Gugvuntts, which went on for a few thousand years and decimated their entire galaxy.

After millennia of battle the surviving G'Gugvuntt and Vl'hurg realised what had actually happened, and joined forces to attack the Milky Way in retaliation. They crossed vast reaches of space in a journey lasting thousands of years before reaching their target where they attacked the first planet they encountered, Earth. Due to a terrible miscalculation of scale the entire battle fleet was swallowed by a small dog. The Hitchhiker's Guide to the Galaxy states that this sort of thing happens all the time.

[Awaras]

We might know two. If we confirm methanophiles on Titan, it'd be a huge breakthrough in our understanding of life in this universe. It'd take us from total uncertainty about life elsewhere and bring us to an almost absolute certainty that universe is filled with life.

Or maybe even three.

[PIRATEONTHERUN]

I'm also thinking, although I'm no bio-chemist, that the system most of our life forms use (unless there're a lot more extremophiles than I thought) of cellular respiration works pretty well. And it's entirely based on consuming oxygen and glucose and producing carbon dioxide. Not to mention, carbon is a good atom to base life off of, and I assume Earth is the ideal condition for carbon lifeforms, so looking for places similar to Earth seems the best idea to me.

[nyrath]

We might know two. If we confirm methanophiles on Titan, it'd be a huge breakthrough in our understanding of life in this universe. It'd take us from total uncertainty about life elsewhere and bring us to an almost absolute certainty that universe is filled with life.

There might be five. Exobiologists think there is a chance of life on Europa, Enceladus, Ganymede and Titan.

Which has implications. If our solar system has one hot-life planet (Earth), and four cold-life planet, it could mean that on the average cold-life outnumbers hot-life four to one throughout the galaxy.

[magnemoe]

There might be five. Exobiologists think there is a chance of life on Europa, Enceladus, Ganymede and Titan.

Which has implications. If our solar system has one hot-life planet (Earth), and four cold-life planet, it could mean that on the average cold-life outnumbers hot-life four to one throughout the galaxy.

On the ice moons we assume they use the same biochemistry as on earth. the water below the ice will be pretty similar to the deep ocean on earth. Tidal forces keep the water from freezing and might generate volcanic activity on the rocky core similar to the black smokers on earth. The other ideas is life based on other building blocks.

Downside of many of them is that they are far more energy restricted, you have carbon based life without oxygen, its also contains far less energy so its hard to get large or fast animals, I guess it will be hard to get intelligent life without being warm blooded as brains use lots of energy all the time.

Back to ice moons, the ice moons will be far more common than earth like conditions, where you have limits to who orbits will work and size requirements.

For an ice moon with water you only need multiple large enough moons to get tidal stress.

Now another interesting possibility, some of the bacteria will be inert in the ice as they froze in. Lots of asteroid impacts in an gas giant system, some will hit the moon who has low gravity and it will shatter ice with bacteria protected inside in orbit, some will reach solar orbit or even go interstellar.

[falofonos]

Who says their probe moves? Who says it is mechanical? What if it was a solid state crystal? What if it was an energy pattern with no physical form?

What if it were an adaptive self replicating probe relying on complex encoding of data within a double helix????!!!!

[3_bit]

I actually argued this point with some people. What we SHOULD look for is a planet roughly as old/older as ours with similar conditions. WHY? Well, then we could go down there and say Hi with our own vocal chords and not a speaker. Either that, or some politician will send a generation ship to take it over. Freon-cold wars, anyone? (meaning really, really conflictless war)

[K^2]

Yeah... Until we figure out superluminal travel, interstellar conflicts are pointless. Either one civilization outguns the other by so much that it'd be like declaring a war on one of the newly discovered tribes in the Amazon, or warships one side sends will become outdated by the time they reach the other.

[lobsterbark]

DNA.

DNA can only survive feasibly given certain circumstances the habitat zone provides. Very few elements have the properties of DNA that allow for protein construction, sequencing, and what not. As you know, DNA is the code that makes us... us. All life, even on-life like viruses have DNA. We've yet to observe a life form on Earth without DNA, and Earth has had several billion years to work on it.

Outside of that, it may be theoretically possible for life to exist without DNA - I've read some concepts of plasma or hydrogen gas based life-forms being possible, but I digress.

Viruses, and some bacteria, use RNA, which is similar to DNA, but more durable, and less likely to mutate.

[drakesdoom]

What if it were an adaptive self replicating probe relying on complex encoding of data within a double helix????!!!!

It's the SQUIRRELS, I knew they were up too something!

[magnemoe]

Viruses, and some bacteria, use RNA, which is similar to DNA, but more durable, and less likely to mutate.

RNA is single chain not dual like DNA so it should mutate more. Does not matter for most as if you have DNA the RNA is mostly used as an working copy and are replaced regulary.