The main reason behind expecting aliens to be similar to Earth life, is that we know carbon-based life works. Given how little we know about the universe at large, it's reasonable to imagine "strange" forms of life could evolve, completely different than life as we know it. But without examples, any assumptions are mostly speculative. How do you look for something when you're not exactly sure what you're looking for?
At first glance, it seems easy to distinguish what is alive from what is not. Most definitions of life consist of a list of qualities -- mobility, growth, reproduction, etc. -- and anything possessing these qualities is alive. Loose interpretation of qualitative lists can potentially identify non-living processes as alive; after all, fire consumes resources and reproduces itself. Borderline cases confuse matters further. Viruses do not possess a metabolism of their own and must hijack the cellular machinery of a host in order to replicate. Depending on who you ask, viruses can be considered alive or not; there is no consensus on this basic question. Lacking a realistic definition of life makes it difficult to predict what other forms of life may or may not be possible.
Silicon-based life is sometimes mentioned as an alternative to carbon. Chemically, this substitution is entertained as possible because silicon falls directly beneath carbon on the periodic table, so the two elements have similar reactivities. Carbon and silicon both have four electrons in their outermost shell, and tend to form bonds with four neighboring atoms. It seems doubtful silicon life could exist as a direct analogue of carbon-based life however. A primary waste product of our metabolism is carbon dioxide, which normally exists as a gas. The equivalent, silicon dioxide, or sand, is a little more difficult to excrete at standard Earth temperature and pressure. If it exists, any silicon life would probably exist in conditions alien to anything we could reasonably survive; perhaps a permanently molten surface too close to it's host star.
Earth life depends on water, such that a cell can be roughly described as a bag of water. At a basic level, more important for life than water is the existence of a liquid solvent. Alternate solvents for the chemistry of life have been considered, which would be necessary on worlds where water does not exist as a liquid. Ammonia is normally a gas on Earth, but is a liquid at temperatures where water is a solid.
In my mind, the most exciting astrobiological target within our solar system is Titan, as it posses the most potential for strange biochemistry. Titan is the largest moon of Saturn, and is very cold far from the Sun. All the water is locked up as ice, hard as granite. But there is a liquid cycle, with lakes of methane and ethane -- which we burn as natural gas on Earth. The Cassini probe recently spotted seasonal rain on Titan. Theoretical models have described the potential for life utilizing acetylene as a primary food source, and some calculations suggest we should see more acetylene at the surface of Titan. That doesn't automatically imply strange alien life forms exist on Titan... but it's one possible answer out of the four or five easiest explanations.
Lacking clear examples of alternative forms of life, we primarily strive to find worlds that could harbor the one kind of life we know for sure exists in the universe. Carbon based life in a water solvent, like us.
Earth life depends on water, such that a cell can be roughly described as a bag of water. At a basic level, more important for life than water is the existence of a liquid solvent. Alternate solvents for the chemistry of life have been considered, which would be necessary on worlds where water does not exist as a liquid. Ammonia is normally a gas on Earth, but is a liquid at temperatures where water is a solid.
In my mind, the most exciting astrobiological target within our solar system is Titan, as it posses the most potential for strange biochemistry. Titan is the largest moon of Saturn, and is very cold far from the Sun. All the water is locked up as ice, hard as granite. But there is a liquid cycle, with lakes of methane and ethane -- which we burn as natural gas on Earth. The Cassini probe recently spotted seasonal rain on Titan. Theoretical models have described the potential for life utilizing acetylene as a primary food source, and some calculations suggest we should see more acetylene at the surface of Titan. That doesn't automatically imply strange alien life forms exist on Titan... but it's one possible answer out of the four or five easiest explanations.
Lacking clear examples of alternative forms of life, we primarily strive to find worlds that could harbor the one kind of life we know for sure exists in the universe. Carbon based life in a water solvent, like us.
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