As with other criteria, stability is the critical consideration in evaluating the effect of orbital and rotational characteristics on planetary habitability. Orbital eccentricity is the difference between a planet's farthest and closest approach to its parent star divided by the sum of said distances. It is a ratio describing the shape of the elliptical orbit. The greater the eccentricity the greater the temperature fluctuation on a planet's surface. Although they are adaptive, living organisms can stand only so much variation, particularly if the fluctuations overlap both the freezing point and boiling point of the planet's main biotic solvent (e.g., water on Earth). If, for example, Earth's oceans were alternately boiling and freezing solid, it is difficult to imagine life as we know it having evolved. The more complex the organism, the greater the temperature sensitivity. The Earth's orbit is almost perfectly circular, with an eccentricity of less than 0.02; other planets in the Solar System (with the exception of Mercury) have eccentricities that are similarly benign. Nonetheless, there may be scientific support, based on studies reported in March 2020, for considering that parts of the planet Mercury may have been habitable, and perhaps that actual life forms, albeit likely primitive microorganisms, may have existed on the planet after all.