Library‎ > ‎LabNotes‎ > ‎

Current Number of Habitable Exoplanets: 16

posted Sep 23, 2011, 5:20 PM by Abel Mendez   [ updated Sep 24, 2011, 7:46 PM ]
There are now two potential habitable exoplanets out of the confirmed 687, and 14 out of the published 1235 Kepler candidates. The original Kepler paper by Borucki et al. (2011) overestimated the number of planets in the habitable zone (HZ) as 54 and suggested 6 potential habitable candidates (within the habitable zone and radius less than twice Earth). Our analysis shows that there are only four habitable candidates in their dataset and that their best candidates, KOI 314.02 and KOI 326.01, are instead outside the HZ. However, new stellar measurements of the Kepler stars by Muirhead et al. (2011) revised some of the planets radius estimates with lower values and added 10 new habitable candidates and corrected two (KOI 854.01 and KOI 1026.01).

    Three candidates from the Muirhead revised Kepler dataset, KOI 494.01, KOI 736.01, and KOI 947.01, are particularly interesting because they have a similar Earth radius (±25%) and are also positioned equivalently to Earth with respect to their star HZ (Table 1). These bodies might be very Earth-like under a similar terrestrial greenhouse effect. There are other candidates with sizes similar to Earth but they are positioned closer to the edges of the HZ. It is interesting to note that we don't have both the mass and radius of any of these 16 potential habitable exoplanets. This is a priority for future observations. Current observations are biased toward large exoplanets (Figure 1). New observations and Kepler results will modify and expand this list in the near future, and that is something that will be presented as part of the Habitable Exoplanet Catalog.

Table 1. Potential habitable exoplanets from confirmed and Kepler exoplanets candidates. Only those within the star habitable zone and with a radius less than 2 Earth Radii or less than 10 Earth masses are considered potentially habitable. The table shows mass (M), radius (R), period (P), distance from the star (d), equilibrium temperature (Teq), and habitable zone distance (HZD). The equilibrium temperature of Solar System planets was calculated using the same 0.3 albedo used for the exoplanets calculations for a better comparison with them. Those very Earth-like are highlighted with red. Updated Muirhead values to the original Borucki values are highlighted with blue.
                                  Name   M(EU)*  R(EU)* P(days)   d(AU)  Teq(K)     HZD

Confirmed Exoplanets        HD 85512 b    3.50    1.52      58.    0.26    299.   -0.96
                              Gl 581 d    6.04    1.81      67.    0.21    183.    0.62

Kepler Candidates               268.01    5.98    1.80     110.    0.41    296.   -0.94
Borucki et al. (2011)           701.03    4.99    1.70     122.    0.45    263.   -0.65
                                854.01    7.10    1.90      56.    0.22    248.   -0.55
                               1026.01    5.98    1.80      94.    0.33    242.   -0.48

Revised Kepler Candidates       227.01    3.08    1.46      18.    0.11    290.   -0.93
Muirhead et al. (2011)          610.01    0.60    0.87      14.    0.10    287.   -0.91
                                817.01    2.08    1.29      24.    0.13    276.   -0.82
                                784.01    1.41    1.14      19.    0.12    275.   -0.81
                                255.01    5.98    1.80      28.    0.15    258.   -0.66
                                736.01    1.15    1.07      19.    0.12    251.   -0.59
                                947.01    1.84    1.24      29.    0.15    233.   -0.36
                                494.01    1.09    1.05      26.    0.16    231.   -0.33
                               1361.01    3.96    1.58      60.    0.24    228.   -0.27
                                463.01    0.74    0.93      18.    0.11    194.    0.34
                                854.01    1.45    1.15      56.    0.22    186.    0.54
                               1026.01    0.85    0.97      94.    0.32    178.    0.80

Solar System Comparison          Venus    0.82    0.95     225.    0.72    300.   -0.94
                                 Earth    1.00    1.00     365.    1.00    255.   -0.50
                                  Mars    0.11    0.54     687.    1.52    207.    0.35

Notes: *The radius of confirmed exoplanets and the mass of Kepler candidates was estimated with a mass-radius relationship. EU = Earth Units.


Figure 1.
 Combined mass vs habitable zone distance (HZD) for confirmed, Kepler candidates (Borucki et al., 2011), and revised Kepler exoplanets candidates (Muirhead et al., 2011). There are a combined total of nearly 130 in the habitable zone (HZ) (green shade) but only 16 potentially habitable (within the HZ and the terrans groups). Of those, only two are confirmed exoplanets but they are superterrans at the edges of the HZ. New more interesting terran candidates need confirmation. A Solar System based mass-radius relationship was used to illustrate Kepler candidates in this plot.