Kepler-277b (also known by its Kepler Objects of Interest designation KOI-1215.01) is the second most massive and third-largest

rocky ''Rocky'' is a 1976 American independent film, independent sports drama film directed by John G. Avildsen and written by and starring Sylvester Stallone. It is the first installment in the Rocky (film series), ''Rocky'' franchise and also star ...

planet ever discovered, with a mass close to that of

Saturn Saturn is the sixth planet from the Sun and the second largest in the Solar System, after Jupiter. It is a gas giant, with an average radius of about 9 times that of Earth. It has an eighth the average density of Earth, but is over 95 tim ...

. Discovered in 2014 by the

Kepler Space Telescope The Kepler space telescope is a defunct space telescope launched by NASA in 2009 to discover Earth-sized planets orbiting other stars. Named after astronomer Johannes Kepler, the spacecraft was launched into an Earth-trailing heliocentric orb ...

, Kepler-277b is a sub-Neptune sized

exoplanet An exoplanet or extrasolar planet is a planet outside the Solar System. The first confirmed detection of an exoplanet was in 1992 around a pulsar, and the first detection around a main-sequence star was in 1995. A different planet, first det ...

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).

Characteristics

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 Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...

and

Neptune Neptune is the eighth and farthest known planet from the Sun. It is the List of Solar System objects by size, fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 t ...

. 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 , 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 Iron is a chemical element; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...

core with an estimated radius of 2.435 . The core predominantly consists of an allotrope of iron with a

face-centered cubic In crystallography, the cubic (or isometric) crystal system is a crystal system where the unit cell is in the shape of a cube. This is one of the most common and simplest shapes found in crystals and minerals. There are three main varieties o ...

(FCC)

crystalline structure In crystallography, crystal structure is a description of ordered arrangement of atoms, ions, or molecules in a crystalline material. Ordered structures occur from intrinsic nature of constituent particles to form symmetric patterns that repeat ...

. 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 In crystallography, the cubic (or isometric) crystal system is a crystal system where the Crystal structure#Unit cell, unit cell is in the shape of a cube. This is one of the most common and simplest shapes found in crystals and minerals. There ...

(BCC) crystalline structure. Kepler-277b has a relatively thin

silicate A silicate is any member of a family of polyatomic anions consisting of silicon and oxygen, usually with the general formula , where . The family includes orthosilicate (), metasilicate (), and pyrosilicate (, ). The name is also used ...

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 The mineral olivine () is a magnesium iron Silicate minerals, silicate with the chemical formula . It is a type of Nesosilicates, nesosilicate or orthosilicate. The primary component of the Earth's upper mantle (Earth), upper mantle, it is a com ...

wadsleyite Wadsleyite is an orthorhombic mineral with the formula β-(Mg,Fe)2SiO4. It was first found in nature in the Peace River meteorite from Alberta, Canada. It is formed by a phase transformation from olivine (α-(Mg,Fe)2SiO4) under increasing press ...

, 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 e/H and an unknown age. For comparison, the Sun has a temperature of 5778 K, a metallicity of 0.00 e/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 A subgiant is a star that is brighter than a normal main-sequence star of the same spectral class, but not as bright as giant stars. The term subgiant is applied both to a particular spectral luminosity class and to a stage in the evolution ...

star.

References