K2-26b and K2-9b are two small, temperate planets orbiting relatively nearby, low-mass stars. Both planets are larger than Earth but smaller than Neptune. Their sizes place them near the transition between Earth-like rocky planets and Neptune-like planets with thick gaseous envelopes. Both K2-26b and K2-9b transit their host stars, and were detected by K2, the re-purposed Kepler mission.
Figure 1: Artist’s impression of a rocky planet.
Figure 2: Transit light curve indicating the presence of K2-26b. Schlieder et al. (2016)
K2-26b is estimated to have 2.67 times the radius of Earth and its orbital period around its host star is 14.57 days. The host star of K2-26b is located ~300 light years from Earth. It is a low-mass star with ~56 percent the Sun’s mass, ~52 percent the Sun’s radius, ~49 percent the Sun’s luminosity, and an effective temperature of around 3785 K.
The time it takes for K2-26b to transit its host star is too long to be consistent with the planet having a circular orbit. Instead, the orbit of K2-26b around its host star is somewhat elongated, with an eccentricity of at least 0.14 with a 95 percent confidence. Due to its eccentric orbit, K2-26b is predicted to experience significant tidal heating, with a tidal heat flux of at least ~600 W/m². This level of tidal heating amounts to ~8 percent of the incident flux K2-26b receives from its host star.
Based on the planet’s distance from its host star, the level of incident flux K2-16b gets from its host star is at least ~5.4 times what Earth gets from the Sun. When tidal heating is factored in, this value becomes ~5.8 times. As a result, the estimated equilibrium temperature of K2-26b is ~430 K. This means that K2-26 b is somewhat too hot to be in the habitable zone of its host star. The composition of K2-16b is unknown as its mass has not yet been measured. Nevertheless, K2-26b has a likelihood of less than ~3 percent of having a bulk composition that is denser than silicate rock. This means that K2-26b has a good chance of having a thick gaseous envelope.
Figure 3: Transit light curve indicating the presence of K2-9b. Schlieder et al. (2016)
K2-9b is another small temperature planet with 2.25 times the radius of Earth. Its orbital period around its host star is 18.45 days. The host star of K2-9b is located ~350 light years from Earth. It is a low-mass star with ~30 percent the Sun’s mass, ~31 percent the Sun’s radius, ~1.2 percent the Sun’s luminosity, and an effective temperature of around 3390 K.
The level of incident flux K2-9b gets from its host star is estimated to be ~1.36 times what Earth gets from the Sun. This gives K2-9b an equilibrium temperature of roughly 315 K, or 42°C. K2-9b has a ~21 percent probability of being a rocky planet with an Earth-like composition. Given these conditions, K2-9b is a potentially habitable planet in the habitable zone of its host star, albeit on the warm side. However, K2-9b receives strong ultraviolet flux from its host star and this may affect the planet’s atmosphere.
Schlieder et al. (2016), “Two Small Temperate Planets Transiting Nearby M Dwarfs in K2 Campaigns 0 and 1”, arXiv:1601.02706 [astro-ph.EP]