Using 3 years (Q1 to Q12) of data from NASA’s Kepler space telescope, Rowe et al. (2015) report the discovery of 855 additional planetary candidates, bringing the current total to 3697. Of these planetary candidates, about 130 receive less than twice the flux Earth gets from the Sun (Sᴇ) and about 1100 are less than 1.5 times the Earth’s radius (Rᴇ). Potentially habitable planets are those that meet both criteria (< 2 Sᴇ and < 1.5 Rᴇ). This latest addition brings the number of candidate potentially habitable planets to over a dozen. The reason they are termed planetary candidates is because additional observations are required to confirm if they are bona fide planets. Nevertheless, most of the planetary candidates are expected to be true planets.
Figure 1: Artist’s impression of a terrestrial planet.
In this latest addition of planetary candidates, all of the candidate potentially habitable planets orbit cool K or M-dwarf stars. KOI-3138.01 is a Mars-sized (Rᴘ = 0.57 Rᴇ) planetary candidate that receives a Mars-like amount of flux (Sᴘ = 0.47 Sᴇ). It has an 8.7 day orbit around a very cool M-dwarf star which has an estimated temperature of 2703 K. KOI-3284.01, confirmed as a bona fide planet and designated Kepler-438b, is an Earth-sized (Rᴘ = 0.98 Rᴇ) planet which receives 31 percent more flux than Earth gets from the Sun (Sᴘ = 1.31 Sᴇ). It orbits a cool M-dwarf star every 35.2 days.
KOI-2418.01 (Rᴘ = 1.12 Rᴇ) and KOI-2626.01 (Rᴘ = 1.12 Rᴇ) are two Earth-sized planetary candidates that respectively receive 35 percent and 65 percent the flux Earth receives. Both planetary candidates orbit cool M-dwarf stars with orbital periods of 86 days (KOI-2418.01) and 38 days (KOI-2626.01). At the hotter inner edge of the habitable zone, KOI-2124.01 (Rᴘ = 1.00 Rᴇ) and KOI-3255.01 (Rᴘ = 1.37 Rᴇ) are Earth-sized planetary candidates that receive ~80 percent for flux than Earth. They are both more likely to resemble Venus than Earth. With more data from Kepler yet to be analysed and with improving data analysis methods, a large number of interesting planetary candidates still await discovery.
Figure 2: A plot of planet radius versus incident flux for all planet candidates known in the Q1-Q12 catalogue. (Note that some planet candidates lie outside the chosen axis limits for the plot, and thus are not shown.) The temperature of the host star is indicated via the colour of each point, and the signal-to-noise of the detection is indicated via the size of each point. Planet candidates that were newly designated in Q1-Q12 are indicated with black circles around the point. The two vertical dashed lines indicate the incident flux received by Mars and Venus, as a broad guide to the potential habitable zone. The horizontal dotted line is set at 1.5 Rᴇ as a suggested upper size limit to terrestrial-type planets (i.e. Earth-like planets). Rowe et al. (2015).
Rowe et al. (2015), “Planetary Candidates Observed by Kepler V: Planet Sample from Q1-Q12 (36 Months)”, arXiv:1501.07286 [astro-ph.EP]