Kepler-277c

Kepler-277c was detected using the transit method and TTVs, allowing for both its mass and radius to be determined to some level. It is approximately 3.36 R_🜨, close to the size of Neptune. At that radius, most planets should be gaseous Mini-Neptunes with no solid surface. However, the mass of Kepler-277c is extremely high for its size. Transit-timing variations indicate a planetary mass of about 64.2 M_🜨,[1] close to Saturn's mass at 95.16 M_🜨. The planet has a density of approximately 9.33 g/cm³ and about 5.7 times the surface gravity of Earth. Such a high density for an object of this size implies that, like its sister planet, Kepler-277c is an enormous rock-based planet with a small portion of its mass as water.[2] It is currently the third most massive and second largest terrestrial planet ever discovered, behind Kepler-277b in mass[4] and PSR J1719-1438 b in both radius and mass.[5] Due to its proximity to its host star, Kepler-277c is quite hot with an equilibrium temperature of about 745 K (472 °C; 881 °F),[3] hot enough to melt certain metals.

Kepler-277c orbits close to its host star, with one orbit lasting 33.006 days.[1] Its semi-major axis, or average distance from the parent object, is about 0.209 AU. For comparison, the planet Mercury takes 88 days to orbit the Sun at a distance of 0.38 AU. At this distance, Kepler-277c is very hot and most likely tidally locked to its host star. It is close to a 1:2 resonance with Kepler-277b, which orbits at an average distance of about 0.136 AU.

The parent star Kepler-277 is a large yellow star. It is 1.69 R_☉ and 1.12 M_☉, with a temperature of 5946 K, a metallicity of -0.315 [Fe/H], and an unknown age.[1] For comparison, the Sun has a temperature of 5778 K, a metallicity of 0.00 [Fe/H], and an age of about 4.5 billion years. The large radius in comparison to its mass and temperature suggest that Kepler-277 could be a Subgiant star.