The life on Earth has existed for about 3.7 billion years. The primary types were molecules which can reproduce themselves. The main molecules such as RNA can reproduce itself, but the definition of life is not so simple.
Hearing the world life what is the first thing that you imagine? Human, a certain plant, a dog, bacteria, which one?
At a glance everybody would say that Life on the Earth is a common and usual phenomena but defining it is so hard. Living thing can do transformation by doing chemical reactions. It can reproduce, grow, adapt or communicate. Not all the species can do all of the above. In other words life can be define as a phenomena which is open and continues system able to decrease their internal entropy at the expense of substances or energy taken from environment and reject degraded form.[life] This last definition doesn't contain viruses, but viruses are formed by molecules similar to cells like bacteria and also they can reproduce themselves.
The early Earth had chemically reducing atmosphere, which combining with the lightning the first amino-acid molecules created. After that the more transformations had been take place and the more complex molecules such as organic polymers accrued, so it's the origin of life on the earth. This hypothesis is called Primordial soup theory. [Origin of life]
The main source of energy on the Earth is the sun. the only exception is the internal volcanic activities. Both of this could be the source of energy to produce early life on the earth.So life could be any where which there is a kind of energy source, suitable matters such as carbon and semi stable environment.
Life on The Earth is carbon-water based. life on the Earth is based on the water(H2O) and carbon. The carbon atoms have the characteristics to support life. I mean the organic molecules, Molecules such as DNA or lipids or proteins. Carbon is the element which can carry life. It's a fact because it's had been tested on the Earth for more than 3.7 billions years. [Periodic Table]
Solar system has an important role in creation and protection of life on the earth. But how? Do you remember Comet Shoemaker-Levy 9? it collided with Jupiter in 1994.[Comet] Jupiter like an gigantic vacuum cleaner attract such meteors and it doesn't let them to crash on internal planets in the solar system such as the Earth. So life on the Earth is protected from huge comets and meteors by the means of it's bigger sister Jupiter. But this is recent functionality what about the early periods?
The distance of our planet form it's sun is so suitable in which water can be in liquid shape. If the Earth was a little bit far away from sun the water would freeze or if it was a little bit closer the water would boil and escape from it's atmosphere. So we are at the right distance from our sun ;). Another good news, the size and mass of the Earth is so suitable, if it has less mass it couldn't keep it's atmosphere so there wouldn't be protecting layer and the life couldn't survive form disturbing rays such as UV. Also in this distance form sun the bigger planet can't stand on orbit. We are at right place with right size and at right time. Also Moon has it's own positive role in creation and survival of life on Earth.[solar System]
So let answer the original question, what could be living thing?
Creation of Life needs energy source, semi stable environment and life carrying chemical element. Foreign life can have similar or different bases from what we know as life on the Earth. The chemical elements which can carry life are bounded. It depends on the chemical characteristics of that element, for example carbon. We know all possible natural elements from periodic table. Also a semi stable place could be a moon or a planet, a rocky planet or even a gas made planet. In a system, in a galaxy.
