The hunt for life sustaining planets elsewhere in the Milky Way. There are two opposed schools of thought :
1. That there are so many stars and galaxies, that space is teeming with life
2. That conditions for life are so rare, that earth is unique
For life to develop, not only is the distance of a planet from its sun important but also the relative location of that sun within its galaxy. It should be situated in the "Galactic Habitable Zone", far from the galactic core with its deadly gamma radiation.
Planets have been located by detecting very small changes in motion (wobble) of their parent sun. New planet finding techniques are being developed with the latest utilising variations in sun brightness.
Attention has focused on sun like stars, seeking planets in the habitable zone, where liquid water exists. However, it appears that stars like our sun are particularly hostile to life and attention has now switched to Red Dwarfs or M Stars, which make up 76% of the stars in the Milky Way. However, as these stars give off very low levels of energy compared to Earth's sun, life sustaining planets would have to orbit very close, which most likely means that the planet would be tidally locked to the sun. Even planets that are tidally locked to their sun with one side always facing may be candidates for life if wind circulation can redistribute the heat to the cold dark side.
There is a lot of excitement concerning the Red Dwarf star Gliese 581 in the Libra constellation, which has four orbiting planets. Planet D, the furthest from the star, is 7x the mass of Earth and is a possible life sustaining candidate.