Last April 2011 the Kepler Mission Team published a paper describing 1235 new planet candidates (Borucki et al., 2011). They argue that 56 of those where in the habitable zone (HZ). However, after some analysis with the data for our Habitable Exoplanet Catalog we only found that only 39 are in fact in the HZ. A closer inspections of the paper shows that they intended to use the correct equilibrium temperature range of 183 K to 307 K as a thermal range for the HZ (although these also depend on the star's temperature), but they used a different interpretation for their Table 5 where they listed 56 candidates in the HZ. It appears from Table 5 that it was assumed that the equilibrium temperature of their candidates had to be in the range of 239 K (273 K - 34 K, greenhouse?) to 373 K (boiling point of water) for a planet to be in the HZ, but this is a mix of surface and equilibrium temperatures.
Planets closer to their star will reach a point where the evaporation of water will cause further warming and a runaway greenhouse effect. This mainly depends on the flux of the star but a temperature of 307 K can be used as a reference limit for an Earth-like planet on a Sun-like star. This value is much lower than the boiling point of water because after this limit a runaway effect will increase it to such and not allowing a gradual process. Planets farther from the star will experience at some point extensive ice regions that will cause a runaway glaciation due to an increase of albedo. This will generally happen for equilibrium temperatures close to 183 K. The boundaries of the HZ does not happen at exactly the point where the equilibrium temperature matches the liquid water temperature range. Using temperatures to define the limits of the HZ is simpler for calculations, but it can be confused with surface temperatures or the temperature range for liquid water. In any case, the HZ is not simply calculated as the region where the equilibrium temperature is in the liquid water range.
Here we calculated the HZ for the published 1235 Kepler candidates (Figure 1). We used the Habitable Zone Distance (HZD) metric to visualize how those rank with the HZ. We also created a mass vs HZD plot of the Kepler candidates (using a mass-radius relationship) so they can be compared with the current list of confirmed exoplanets from the EPE (Figure 2 and 3). Only 39 Kepler planet candidates fall within the HZ defined for planets with maximum greenhouse and 50% cloud cover. This is a 30% reduction to the original 56 candidates, but an insignificant change with many more candidates that will be announced. Also, an interesting study by Muirhead et al. (2011) suggests a re-calibration of radii and effective temperatures of several Kepler candidates and yields six new Earth-sized candidates within the HZ. We will check on this issue in a later post.
Figure 1. Radius vs Habitable Zone Distance (HZD) for the 1235 Kepler planet candidates in 997 stellar systems of Borucki et. al., 2011. There are 1193 in the Hot Zone, 39 in the Habitable Zone, and only 2 in the Cold Zone. Not all planets shown within the scale.
Figure 2. Mass vs Habitable Zone Distance (HZD) for the 1235 Kepler planet candidates in 997 stellar systems of Borucki et. al., 2011. Mass was estimated from a mass-radius relationship for the Solar System. There are 1193 in the Hot Zone, 39 in the Habitable Zone, and only 2 in the Cold Zone. Not all planets shown within the scale.
Figure 3. Mass vs Habitable Zone Distance (HZD) for 684 confirmed exoplanets in 562 stellar systems. Minimum mass was used. There are 294 in the Hot Zone, 82 in the Habitable Zone, and 52 in the Cold Zone. Not all planets shown within the scale.