The answer to this question can go as far as anybody's imaginations. So I'll just tell why the answer is what it is, and try to include scientific facts (possibly with references) wherever possible.
Simple Answer: Designing alien life forms on the basis of current theoretical and practical knowledge of science (if this is what you mean) is still, and seems to remain, a field of science fiction only.
Why? Because, simply, the proposed organisms can be as many as you can count. The point is that, even if this field comes under the category of science, it will remain as large as the universe itself, because there are literally no boundations and limitations to speculation and imagination. Only recently have researchers agreed that discovering organisms similar to humans is much much less than any layman thinks. Actually, there is even a formula for it, known as the Drake equation after Frank Drake who proposed it. The equation is:
$N = R^* \times f_p \times n_e \times f_l \times f_i \times f_c \times L$
- $R^* =$ the average rate of star formation in our galaxy
- $f_p =$ the fraction of formed stars that have planets
- $n_e =$ the average number of planets per star that can potentially support life
- $f_l =$ the fraction of those planets, fl, that actually develop life
- $f_i =$ the fraction of planets bearing life on which intelligent, civilized life has developed
- $f_c =$ the fraction of these civilizations that have developed communications i.e. technologies that release detectable signs into space
- $L =$ the length of time over which such civilizations release detectable signals
Obviously, this answer comes out to be very small, meaning that intelligent life is very difficult to find. But that doesn't mean finding life is difficult. And this claim gets even more support when scientists find water on mars and even mercury! Now, to give a bit of knowledge, this field is not really limitless; there are (at least) two limits (i.e. I'm not including more). The first limit is source of liquid water$^*$. Now, knowing that water is easy to be found almost anywhere, this does not seem quite a difficult one. Another one is source of energy. Now this is a difficult one. The problem with discovering life (in near future at least) is the area in which we are searching. Most of the searches (by telescopes of course) are being conducted near red dwarf stars. There are quite some reasons for it:
- red dwarf stars are quite stable
- they have very long life
- they are pretty less shiny (making observations easier)
- they may be home for many super-Earths
But being less shiny also poses a problem: too less energy for life. Red dwarf stars emit most of their radiation in the infrared portion of spectrum, which is pretty useless for plants (on Earth at least). Now, to survive in a red dwarf system, plants need to capture infrared light. Thus, the speculated plants in red dwarf systems will be pitch black just to absorb more energy. Also, red dwarf stars are pretty lightweight, as less as 7.5% of sun. This means they have less gravitational pull i.e. they will have much lighter planets in their Goldilocks zone. Now, much lighter planets means much less gravity on planet's surface. Thus, the speculated plants in red dwarf systems will be a lot higher than plants on Earth. But, less gravity also means much less density of air even on surface. Now, you can interpret it yourself. And, not just to surprise you, there would be absolutely no surprise if in future a satellite discovers silicon-based life, because it is really possible for life to emerge from silicon instead of carbon!
$^*$why I am continuously saying liquid water has a reason. Water is actually one of the very few candidates that can serve the purpose of maintaining life forms. Its one of a handful of liquids which are liquids at desired temperature. As one might think, life cannot survive below a certain temperature, simply because below that temperature, the liquid itself would absorb all the metabolic heat from organism and evaporate. So, it is quite relevant to say liquid water here.
What I meant to say is that there are technically no boundations to designing alien life forms, as long as you talk about the whole universe. So, this does not really count as science. After all, if you search
science definition on Google, you get this:
the intellectual and practical activity encompassing the systematic study of the structure and behaviour of the physical and natural world through observation and experiment.
This does not include imagining alien life forms in any way. So, I do not think this subject would ever be considered as scientific by the scientific community. Mostly, scientists are not indulged in this kind of work because they like to observe and interpret natural phenomena rather than imagining them. Scientists make laws from nature rather than applying laws on nature. Still, there are some scientists who take this matter seriously and publish scientific papers about it. I could find only one (PDF) related to this point. You might get some more on the internet.
In the end, if you think about how plants would look on a planet in a red dwarf system, you might end up with this:
Bonus: Dawn space probe from NASA has detected organic materials on Ceres(!) Now, what kind of organisms would you expect to see there based on our current knowledge about Ceres?