Kepler-277b

Kepler-277b (also known by its Kepler Objects of Interest designation KOI-1215.01) is the second most massive and third-largest rocky planet ever discovered, with a mass close to that of Saturn. Discovered in 2014 by the Kepler Space Telescope, Kepler-277b is a sub-Neptune sized exoplanet with a very high mass and density for an object of its radius, suggesting a composition made mainly of rock and iron. Along with its sister planet, Kepler-277c, the planet's mass was determined using transit-timing variations (TTVs).

Kepler-277b was detected using the transit method and TTVs, allowing for both its mass and radius to be determined to some level. It is approximately 2.92 R_🜨, between the size of Earth and Neptune. At that radius, most planets should be gaseous Mini-Neptunes with no solid surface. However, the mass of Kepler-277b is extremely high for its size. Transit-timing variations indicate a planetary mass of about 87.3 M_E,^[1] comparable to Saturn's mass at 95.16 M_E. The planet has a density of approximately 19.3 g/cm³ and about 10.4 times the surface gravity of Earth. Such a high density for an object of this size implies that, like its sister planet, Kepler-277b is an enormous rock-based planet.^[2] It is currently the second most massive and third largest terrestrial planet ever discovered, behind Kepler-277c in radius^[4] and PSR J1719−1438 b in both radius and mass.^[5] Due to its proximity to its host star, Kepler-277b is quite hot with an equilibrium temperature of about 924 K (651 °C; 1,204 °F),^[3] hot enough to melt certain metals.

Models of Kepler-277b's internal structure suggest that it has a very large iron core with an estimated radius of 2.435 R_🜨. The core predominantly consists of an allotrope of iron with a face-centered cubic (FCC) crystalline structure. At the innermost region of Kepler-277b's core where pressures reach as high as 37.52 terapascals, iron exists in a body-centered-tetragonal (BCT) and body-centered cubic (BCC) crystalline structure.^[6]

Kepler-277b has a relatively thin silicate mantle in comparison to its core. The mantle of Kepler-277b is thought be predominantly composed of ultrahigh-pressure phases of magnesium silicates (MgSiO₃). The uppermost mantle of Kepler-277b is thought to consist of olivine, wadsleyite, and ringwoodite while the lower part of Kepler-277b's upper mantle consists of silicate perovskite and post-perovskite.^[6]

Kepler-277b orbits close to its host star, with one orbit lasting 17.324 days.^[1] Its semi-major axis, or average distance from the parent object, is about 0.136 AU. For comparison, the planet Mercury in the Solar System takes 88 days to orbit at a distance of 0.38 AU. At this distance, Kepler-277b is very hot and most likely tidally locked to its host star. It is close to a 2:1 resonance with Kepler-277c, which orbits at an average distance of about 0.209 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.

• Mega-Earth
• Kepler-277c