The nearest alien star system to Earth is the Alpha Centauri triple star system. The one nearest star to us besides our sun is Proxima Centauri 4.22 light-years away but it is the two sun-like stars of Alpha Centauri A and B 4.37 light-years away that seem promising as the closest possible stars to host planets with complex or intelligent life on them.
Alpha Centauri A is a G2 type main sequence yellow dwarf star slightly larger and a little more luminous than our own Sun and Alpha Centauri B is a K1 main sequence orange dwarf star slightly smaller and a little less luminous than our Sun. On average, Alpha Centaur A and B are 24 astronomical units (AU) apart from each other whole Proxima Centauri is 20,000 AU from both Alpha Centaur A and B. The two stars orbit each other once every 79.91 years and they are 11.2 AU from each other on closest approach and they are farthest apart when they are 35.6 AU from each other. Below is a size comparison of our Sum to the three Alpha Centauri Stars approximately to scale.
There have been high hopes that planets will be discovered in the Alpha Centauri Star System but so far only one planet has been discovered in that star system and it is way too close to its star to support life, Alpha Centauri Bb. In October 2012, astronomers have discovered the first known exoplanet orbiting a star in Alpha Centauri. This planet is roughly Earth-sized and just i little bit more massive than Earth. Sadly it does not orbit in Alpha Centauri B's habitable zone and is too hot to support life.
Now that Alpha Centauri Bb has been discovered, there is now more hope than ever that other planets; including Earth-like planets in the habitable zone, will be discovered in the near future. With a lack of gas giants and brown dwarfs coupled with a higher metallicity than our own sun, terrestrial planets are even more likely to exist. it is hoped that there are Earth-like planets in the habitable zone around either Alpha Centauri A or B or both.