Creating Belevable Aliens

so you want to be able to Create believable Alien life that Could really exist. ok Lets get too it

Any alien must follow the basic rules of biology. A believable Alien Would most likely not look any thing like a human or really Any sort of earth life were familiar with to much. they very well may share traits like limbs eyes and hair, but they will likely be different in their placement. There mouths could be on there backs or under belly. senses could be any where. teeth could be on there tongue or on the head a true alien would be strange, unlike anything we have seen. There are many possible body plans and the life of earth only follows a few of them. an alien would be more like the Creatures found in the bottom of the ocean. Angler fish, bioluminescent jellies, leeches or worms. Or prehistoric animals
 * 1) It must have a way of taking in Nutrients and energy from its surroundings. such as mouths, leaves or something similar.
 * 2) the ability to sense its environment. like eyes, ears, nose, touch receptors excreta.
 * 3) the ability to Defend its self and stay alive. think legs so it can escape. Claws or teeth to bite. poison or odour.    Deep sea 3.jpg

Location
Before we can create an alien we need to know where it lives.

What is the planet like? thick or thin Atmosphere. hot or Cold. Sand or rocks. Wet or Dry.

Knowing this we can work out what sort of adaptations will be required to live there. if its a hot dry planet a creature will need a way of storing water on a low gravity world creatures will be able to be taller and fly easier. look at real animal's as a guide for what sort of adaptations they have. Camels humps, polar bear fat.

ok so the look of the creature is only part of it. another is how does it behave. what does it eat. does it get energy some other way. what other creatures does it have to defend against. does it live in a groupe or go solo.

intelligent aliens
when it comes to intelligence things get a bit more complicated. the rules are mostly the with again emphasis on them not being human like. an intelligent alien would need a appendage of some sort which will allow the ability to make and use tools as well as being able to use technology. now dependent on how advanced the aliens are they may have replaced parts of them selves with machinery or use some sort of clothing or something similar.