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).

Size and temperature
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, 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, comparable to Saturn's mass at 95.16. 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. It is currently the second most massive and third largest terrestrial planet ever discovered, behind Kepler-277c in radius and PSR J1719−1438 b in both radius and mass. Due to its proximity to its host star, Kepler-277b is quite hot with an equilibrium temperature of about 924 K, hot enough to melt certain metals.

Internal structure and composition
Models of Kepler-277b's internal structure suggest that it has a very large iron core with an estimated radius of 2.435. 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.

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.

Orbit
Kepler-277b orbits close to its host star, with one orbit lasting 17.324 days. 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.

Host star
The parent star Kepler-277 is a large yellow star. It is 1.69 and 1.12, with a temperature of 5946 K, a metallicity of -0.315 [Fe/H], and an unknown age. 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.