Why do we presume that alien life requires oxygen and water?

Question

When we look for other life we always look for planets with water and oxygen, what says that other life needs them, yet for all we know we could know they could use a compleatly different element to breathe, drink and even eat, because we belive the elements we have found are the only ones, Why do we presume that alien life requires oxygen and water?

Answer 1

we dont assume that it requires oxygen. we assume that it requires water, which is a pretty safe assumption. the cell needs some type of fluid for transport. water is pretty much the only option there. would you expect a cell to have liquid nitrogen, or liquid iron. there are very limited options when it comes to liquids in the universe. i think you would agree that life wouldnt form in a place that is -200 C or in a place that is thousands of degrees.

Answer 2

Oxygen is not that important for life in fact there is life on Earth that does not require oxygen.

Water and carbon however are a different story. Water has some properties that makes it pretty unique as a medium to support and start life. Carbon is the only element capable of forming any complex molecules ( DNA, proteins... ) that are the key for life as we know it.

Answer 3

we presume that alien life requires oxygen as it is the only gas which helps in respiration unless your required alien does not respire he will need oxygen to live we presume that water is required as the first life form on earth started with water and bacteria or other microrganisms may be present in any type of water so we presume that where there is water theres got to be life

Answer 4

Actually, there are a few primitive life forms on Earth that do not require oxygen.  They are called anaerobic life forms.  However, in order to develop higher forms of life (including intelligence), other things are required.  Those are, primarily a solvent for chemical reactions and carbon because it combines readily with many other elements in those reactions.  Oxygen is needed to provide oxidation in those same reactions. Theoretically, helium can be used instead of oxygen, but in nature, it only occurs in abundance in stars.  It is possible to hypothesize about life based on silicon instead of carbon,  but if we met it, we probably wouldn't recognize it, as it would be incredibly slow moving.  A being based on silicon would make the average snail look like an F16 in action.

Answer 5

In order to find something, you need to know what to look for.  Since the only life we have experience with requires oxygen and water, that's all we can know to look for.

Anything otherwise, it would be like me asking you to go search a junkyard for a sminthalic.  How would you know when you've found one?

Answer 6

bcuz we have to start somewhere. hydrogen and oxygen are common in the universe, so it's not a bad place to start.

Answer 7

Like others are also saying, it's our only example so far. Scientists are well aware that there are other possible biologies than those found on Earth. You can even read the Wikipedia article on alternate biology. It's pretty interesting.

However, we want to be able to say with some reasonable amount of certainty that life is possible when observing another world. If we say "it's possible for methane breathing life to exist," then find a methane world, it's just a complete and total c**p shoot whether there is life there. We've yet to encounter methane breathing complex life. Sure, there could be methane breathing microbes on Titan, for instance, but to declare it as "possible" is a whole lot less exciting than it would be to find a world that could support life 'as we know it.'

Since we know that Earth's conditions support life, if we find those conditions elsewhere, then it's a much higher probability that we've also found somewhere that life exists.

Answer 8

i think it's because we breath with oxegen and we think that the aliens will probably breath... the astrologists don't know what they breath yet so that's what we think. i don't know why we think they need water... for all we know they could be drinking liquid mercury.

Answer 9

Because all organic life requires water (and just to make sure we're clear, all life on earth is organic).  Most organic life needs oxygen, although plants need CO2, of which they use the carbon and send the oxygen back out into the air.  But all life, plants and animals, requires water.

Since organic life is the only life that we have ever encountered, that's the kind of life we're looking for.  Scientists don't exactly presume that alien life is organic like terrestrial life, but we can say "if that planet has liquid water, and carbon, and oxygen, then it can support organic life" but if there is any other kind of life in the universe, we don't know what it needs to live, so we can't determine whether or not a place is hospitable to any other kind of life.