The conditions best suited to life as we understand it.
A good article in "The Conversation" by three astronomers from the University of Southern Queensland: "For Life To Form On A Planet, It Needs To Orbit The Right Kind Of Star".
It lists the attributes a star needs to possess for us to consider investigating if life has arisen there. They are:
- Not too young.
- Not too big.
- Not too small.
- Not too violent.
- Not too rich (metallicity).
- Not too poor.
- Not too crowded.
In practice this apparently means eliminating, for a start, all Type O, B & A stars and concentrating on Type F, G & K stars, whilst not eliminating Type M stars.
In my mind it would probably in theory also eliminate stars near the galactic nucleus, stars in globular clusters, stars in stellar nurseries, Population II stars & Population III stars (if there are any).
This eliminates a huge number of stars in the galaxy before even beginning to consider the planetary systems of the remainder.
It also eliminates stars which have been recently exposed to nearby supernovae or gamma ray bursts. Fortunately that does not apply to our galactic neighbourhood, otherwise we would not be here.
Of course, the biggest eliminating factor is that of distance, because our ability to examine planetary systems is currently limited to those in our galactic neighbourhood. According to the authors of the article, one of whom - Jonti Horner - is well known to us at Macarthur Astronomical Society, the number of stars worth investigating is whittled down to "tens, if not hundreds, of promising worlds".
They are not talking about intelligent life either, they are only searching for suitable stars which may have planets with atmospheres which contain any signs of any kind of life at all.
Hundreds of stars is not a lot to work on really.