I introduce a simple diagram that combines semi-major axis, eccentricity, and stellar properties to quantify the fraction of orbital period spent within habitable zone. The diagram identifies whether an orbit lies fully within, fully outside, or partly overlaps the habitable zone. The diagram reveals that exoplanets sometimes classified as outside the HZ actually spend more than half of their orbit within it. Climate modeling further suggests that planetary systems may retain habitable conditions even outside formal HZ boundaries. These cases should be considered when selecting targets for biosignature searches.
Arthur Mary (Mon,) studied this question.