Planets Beyond the Sun
Up until recently, we did not know whether there were any planets further out than Neptune, only those in the imaginations of science fiction writers such as H. G. Wells. It was only in 1992 that mankind first discovered a planet beyond our Solar System - an exoplanet (extrasolar planet). Since 1992 we have discovered thousands more, several of which are believed to be capable of hosting life.
Life out there?
For all we know, there may well be intelligent life looking upon right now. In fact, it's reasonably probable that there is intelligent life in the universe, even if it does not know that we exist. Therefore it's worthwhile to search the skies for planets which may host life, however it helps to know what we're looking for. That's why we have this set of criteria for habitability:
Some other less major factors also have to be considered such as gravitational pull of host star and neighbouring planets, though these are the three most important criteria.
The criteria above are only those for hosting life, not necessarily intelligent life. The criteria for the possibility of intelligent life which can communicate humanity is set out in the Drake Equation (named after American astronomer Frank Drake):
N = R* x fp x ne x f1 x fi x fc x L
BUT WHAT DOES ALL THAT MEAN?!
N is the number of civilizations of intelligent life which could communicate with humanity.
R* is the average rate at which stars are formed in the Milky Way.
fp is the proportion of stars which have planets.
ne is the average number of planets which can host life.
f1 is the proportion of habitable planets which develop life.
fi is the proportion of planets with life where intelligent life develops.
fc is the proportion of intelligent civilisations can communicate with outside civilisations.
L is the time over which the civilisations try to communicate.