At least thirty stars lie within a radius of approximately 12 light years from Earth. Seven of them are multiple star systems, of which our closest neighbour - a triple star system called Alpha and Proxima Centauri - is located a mere 4.22 to 4.3 light years away. Perhaps a few more very faint stars may exist within this 12-light-years range, but our instruments are not sensitive enough to detect their presence at the moment.
A brief analysis of all the known stars within this specified range clearly shows Red Dwarfs are the most common. These are small red stars believed to possess the minimum mass required to initiate thermonuclear reactions (ie. the source of all heat and light from stars) in their central core. Because red dwarfs have a low mass compared to other stars, they glow a dim red colour and burn their hydrogen fuel very slowly and so have the longest lifespan of any star. For example, Barnard's Star is a red dwarf lying at a distance of 6 light years from Earth; its estimated age of 1000 billion years is considered very old.
Of all the stars lying within 12 light years of Earth that may support alien life, Tau Ceti and Epsilon Eridani are sufficiently similar to our Sun to be thought of as a potential source for ETs. Two other stars are also worth noting in this regard - Alpha Centauri A and B, and Epsilon Indi.
