Table 5.
Different Definitions of Habitable Zone Boundaries around Solar-Type Stars
| Habitable zone | Inner edge (AU) | Outer edge (AU) |
|---|---|---|
| Empirical | 0.95 | 1.77–2.4 |
| “Standard” model | 0.95 | 1.37 |
| “Standard” model CHZ | 0.95 | 1.15 |
| “Kopparapu” model | 0.99 | 1.70 |
| “Dry” model | 0.72–0.95 | 1.37–1.5 |
| H2 greenhouse | ∼2 | ∼10 |
| “Leconte” greenhouse | 0.95 | no value |
| “Wolf and Toon” greenhouse | 0.93 | no value |
The inner edge provided for the empirical habitable zone (Selsis et al., 2007) and the standard model (Kasting et al., 1993; Kopparapu et al., 2013) is based on the location of significant water loss. The outer edge of the standard model is where CO2 cloud condensation occurs. The empirical habitable zone's outer edge is defined to include Mars at 1.5 AU (4 billion years ago) because there are geological signs of past water. The inner edge of the “dry” habitable zone is between Earth and Venus; its outer edge is between Earth and Mars (Abe et al., 2011). The H2 greenhouse habitable zone is for a 3-Earth-mass planet with a 40 bar H2 atmosphere (Pierrehumbert and Gaidos, 2011).