Spectral-Type Dependence of Habitable Zone Classification Differences in Confirmed Exoplanets: A Systematic Comparison of Temperature-Dependent and Constant-Flux Models

Abstract

Habitable zone (HZ) boundaries are commonly estimated using either the temperature-dependent formulation of Kopparapu et al. (2013, 2014) or simplified constant-flux models that neglect stellar effective temperature. Although both approaches are widely used in exoplanet studies, their influence on habitable-zone classification across the confirmed exoplanet population has not been systematically quantified. In this study, we compare these two HZ formulations using a dataset of 1,347 confirmed exoplanets with available stellar effective temperature, stellar luminosity, and orbital semi-major axis obtained from the NASA Exoplanet Archive. The conservative Kopparapu model classifies 71 planets (5.27%) as residing within the HZ, whereas the constant-flux model identifies 46 planets (3.41%). The two approaches disagree for 39 planets, corresponding to an overall disagreement rate of 2.90% (95% CI: 2.13–3.93%). This disagreement exhibits a strong spectral-type dependence, increasing from negligible levels for A- and F-type stars to 8.33% for M-dwarf systems (95% CI: 5.40–12.65%), with a statistically significant association confirmed by a chi-square test (χ² = 31.62, p < 0.0001). Model-level analysis demonstrates that the observed trend originates from the temperature-dependent stellar flux correction incorporated in the Kopparapu formulation, which accounts for differences in stellar spectral energy distributions and produces the greatest divergence from the constant-flux approximation for cool stars. These findings demonstrate that simplified constant-flux HZ criteria can systematically underestimate habitable-zone candidates around M dwarfs, potentially influencing habitable-zone catalogues, target prioritisation for atmospheric characterisation, and future exoplanet habitability surveys.