Actually, they don't assume this. Some exobiologists have looked into many other types of life there could be.
Some of these might even exist in our own solar system, here are a few (not trying to be comprehensive):
SULFUR BASED LIFE ON IO
There might be life on Io, sulfur based, in underground pools of liquid SO2, with the life chemistry probably also using H2S.
POLYSYLANE BASED LIFE ON TRITON
There might be life on triton using polysylanes instead of polycarbonates.
LIFE ON TITAN WITH NITROGEN IN PLACE OF OXYGEN
Life on Titan is a challenge for us to model because of the cold, and the novel chemistry - no oxygen to speak of, no water, no CO2. But - nitrogen might take the place of oxygen - or it might use reactions involving the energy in double and triple bonded carbon.
SILICON BASED LIFE IN LIQUID LAVA
One of the most challenging forms of life would be silicon based life living in liquid lava.
If this was possible, we might find it on Venus - which has entire rivers of liquid lava flowing in lava tubes - for huge distances across its surface.
These could use Silicones - organosilicon polymers with a silicon-oxygen backbone.
These are stable at temperatures so high they would destroy any organics. But - would it remain stable at the temperatures of even cooling magma pools on Venus?
If life is possible in magma pools - then why hasn't it evolved on the Earth also? Why have we never found silicon based life fossils in lava flows on the Earth?
So - silicon life seems unlikely. But is it just that it is a low probability life form - after all - for all we know organics based life in oceans might also be low probability. Maybe on other planets there are silicon based life forms who are theorizing about the impossiblity of water based life because though there is water on their planet, is no life in it - and it seems too cold to sustain life :).
HARD TO JUDGE IF OTHER FORMS OF LIFE ARE LIKELY
You get people arguing that some of these forms of life are impossible because organic chemistry is so much more varied than any of these other forms of chemistry. But how much of that is because organic chemistry is much easier for us to study?
We know a huge amount about how Carbon, Hydrogen, Oxygen and other atoms combine together at Earth pressures and temperatures - but is only rarely that we try e.g. experimental conditions resembling those of Titan, or Triton or Io.
And is no way we could invent organic based life processes ourselves - "inventing" something like a living cell from scratch is still way beyond us, only copying the way life does it and changing it slightly.
So how much are we biased by our knowledge of how our own type of life works? And our natural tendencies to do experiments in conditions we find comfortable and using substances that are common where we live?
HARD TO DETECT LIFE AT A DISTANCE - JAMES LOVELOCK'S ATMOSPHERE IN EQUILIBRIUM ARGUMENT IS NO LONGER SO SIMPLE AND STRAIGHTFORWARD TO APPLY AS IT WAS IN THE 1970S
Also - it's likely to be really hard to detect life on an exoplanet from the distance.
Is true that back in the days of Viking it seemed simple - just look for an atmosphere that is out of balance.
But - several things have turned that around.
- Discovery of life on Earth not connected in any way to its surface (e.g. deep underground kms below the surface discovered in mines and living off - ultimately -radioactivity of the rocks as its energy source - through chemical effects of that radioactivity - and the H2S using lifeforms at bottom of sea etc
- Discovery that Earth itself went through snowball phases when life went nearly extinct and would surely be undetectable - but present - on the surface. And - habitats on Earth with very low levels of life - but still there - just barely detectable with lifetimes of thousands of years for a single lifeform. We couldn't detect the effect of life like that on Mars if it is present there right now as it might be.
- Discovery of situations that can make a planet's atmosphere go out of equilibrium without life. E.g. the oxygen rich oceans of Europa predicted in some models - and quite possibly also oxygen rich early Mars - in both cases due to ionization effects dissociating the H2O so the H2 disappears into space leaving an oxygen rich out of balance environment.
And if its atmosphere is in equilibrium - it might be that all the life is deep underground, underwater - or that it is in a snowball phase with some life on its surface but not enough to have much influence on its atmosphere.
However with organic life - at least we know what to look for - at least - one possibility anyway.
LOOKING FOR YOUR KEYS BENEATH A STREET LIGHT
It's a case, basically - of looking for your keys beneath the street lights first - because you haven't got a torch - but is at least a chance that you might have dropped them there.
“Did You Lose the Keys Here?” “No, But the Light Is Much Better Here”
If life is easy to find, and at least some of it is organic based - then perhaps we will find organic based life and maybe it won't take too long to find.
If it is rare, or most is not organic based, - then who knows how long it will take to find it or if we will recognize it easily when we find it.
See also: Robert Walker's answer to What planets/moons do scientists expect to find life in our solar system? I only know of Europa?
For more about the alternative life chemistries mentioned here, see Cosmic Biology
Some extracts available online here:
How Life Could Evolve on Other Worlds (Google eBook)
Louis Neal Irwin, Dirk Schulze-Makuch
See also Hypothetical types of biochemistry (wikipedia article)
About the Author
Robert Walker
Writer of articles on Mars and Space issues - Software Developer of Tune Smithy, Bounce Metronome etc.Studied at Wolfson College, Oxford
Lives in Isle of Mull
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