Not necessarily requirements, but they would be nice to have as equalizers against the hazards of life in this killer Universe:

Radiation:
* a medium, main sequence star

It is currently believed that GRBs are caused by the collapse of monster stars that fall immediately to black hole formation in a hypernova event, releasing a quantity of gamma radiation that would sterilize a sphere of space equal to several hundred light-years (as reference, it is believed that the Crab event, witnessed in July of 1054, sterilized a sphere of at least 50 light-years), so . . .

* no novas in the immediate vicinity of a life nursery
* a functioning magnetosphere (and a large moon to keep it charged)
* an atmosphere that includes a blocking agent similar to the ionosphere

Heavy Impactors:

Since the proposed planet would be perilously close to a system’s main gravity well (star or gas giant), thereby residing in a region of heavy bombardment:

* impactor collectors would be nice (i.e., Jupiter and our Moon)
* Heavy impactors spaced millions of years apart can occasionally clean a planet of “undesirables” (i.e., nonproductive nonsentients)

Is it possible that the impactors witnessed by very-early man -- those marmosetish creatures of our far-distant past -- spurred our minds to look beyond the horizon, and to wonder about the heavens? And more recently, are we curious now, because a mere 50,000 years ago a chunk of iron-nickel, hurtling in at approx. 40,000 mph, collided with a region of the planet Earth, later called Arizona, and quaked our Homo sapiens neandertalensis ancestors to their very cores? Is that why we look up today and wonder? Is challenging the nature of things a key factor for the advancement of species intellect and sentient awe?

If not, I’m sure it helped, therefore:

* medium impactors can force us to wonder, and to grow

Dan