Our system of biological classification bases itself on similarities between species and that every species is related through speciation going back to the origins of life on the planet.
But if we became aware of alien life - it’s a big universe after all - this system falls flat on its face; what would be the choices of biologists in finding a nomenclature for an alien?
Um… they’re life. But they’re extraterrestrial life.
Our current basis is founded upon terran life, it would make more sense to just come up with a new classification system for them. I.e. class terra and class ex-terra.
I imagine that if we find intelligent life, and they have their own biological classification system we would likely adopt theirs for the purposes of their world.
No reason to re-invent the wheel when there’s The New Essential Guide To Alien Species:
starwars.wikia.com/.../The_New_Essential_Guide_to_Alien_Species
224 pages of cutting edge astrozoological research from a number of creative minds working on this very problem.
Simple. Make it a trinominal instead of a binominal system, with the first designator representing the planet of origin.
Terra Boa constrictor
Ceti Anguilla auditorymeatus
etc
The system that we have is useful because it reflects evolutionary relationships that actually exist on the planet. If and when we stumble across a new planet full of life, we’ll have to figure out a system that is useful for helping us understand the relevant information about life there. It’d take some time, but we know what we’re doing now, so we could manage it without too much trouble, assuming that the alien life is at all comprehensible to us.
All extant life on Earth belongs to a particular clade. That is, all life on Earth shares a common origin.
Now, if we find life on Mars, we could figure out if the Martian life forms are phylogenetically related to Terran life forms. This could happen if, early in the formation of the solar system, life forms from Mars were transferred to Earth, or vice versa. So it could be that the Martian life forms are related to Terran life forms, to the extent that they can fit into existing clades of Earth life.
Or it could be that Martian life is totally unrelated to Terran life, due to different instances of abiogenesis. In that case we’d invent a phylogenetic category higher than a kingdom or domain (Domain (biology) - Wikipedia), and stuff all Martian life into one group, and all Terran life into another.
If it turns out to be the case that each planet with life has only one super-domain that arose independently of every other planet, then we’d just create a new super-domain every time we discover a new planet with life. Or it might be that we’d find out that some planets have more than one super-domain. Earth has only one, but maybe on Mars we’d find two or more types of life that arose completely independently of each other.
Well, if the alien life is evolutionary (in the sense that there are discrete individuals with a limited lifespan, who reproduce with inherited variation) and have species (there are different groups that are substantially different genetically and do not interbreed) then the same kind of classification could be used. Assuming the alien and terrestrial life arose independently, then alien life would be a separate tree, but a tree with more closely related species on closer branches would still make sense.
I could imagine other forms of life that wouldn’t be classified this way, but how they’d be classified depends on what they’re like.
This is the most likely solution. We’d invent a new level, named “Genesis” or “Origen” or something like that, to designate what planet the lifeform evolved on.
Another point–if we could explore distant planets, we might very well find various species of life on Planet HD877356 b that are sessile autotrophs that use planar organs to capture light from the local star for conversion to chemical energy via photosynthesis. Heck, they might even use a chemical that’s recognizably similar to the chlorophyll used in Earth plants in the process. (Of course we don’t actually know how much convergent evolution there is likely to be between Earth life and hypothetical living organisms that may have evolved completely independently on planets orbiting other stars, at either the basic chemical level or at the level of overall gross anatomy.)
If so, I’ll bet everyone, even biologists, will refer to these things as “plants”, and if there are motile heterotrophic organisms that get energy by ingesting the sessile autotrophs (or by ingesting other motile heterotrophs that eat the sessile autotrophs) we’ll call those things “animals”. But of course, if we’re really talking about life on a planet orbiting another star, then the “plants” from HD877356 b are not going to be related at all to “trees” from Earth (even if they may happen to superficially resemble them). An Earth tree and a human or a whale are actually relatives, whereas the Earth tree and the “tree” from HD877356 b are presumably not related to each other at all (except insofar as they exist in the same universe, obey the same laws of physics and chemistry, and ultimately come from the same Big Bang).