:Kepler-277b
{{Short description|Rocky planet orbiting the star Kepler-277}}
{{Infobox planet
| name = Kepler-277b
| discovery_site = Kepler Space Observatory
| discovered = 2014
| discovery_method = Transit
| apsis = astron
| semimajor = ~0.136 AU
| eccentricity = null{{cite web
| url=https://exoplanetarchive.ipac.caltech.edu/cgi-bin/DisplayOverview/nph-DisplayOverview?objname=Kepler-277+b&type=CONFIRMED_PLANET
| title=Kepler-277 b CONFIRMED PLANET OVERVIEW PAGE
| work=NASA Exoplanet Archive
| access-date=2018-01-20
}}
| star = Kepler-277
| mean_radius = 2.92 {{±|0.73|0.63}} {{Earth radius|link=y}}
| mass = 87.3 {{±|41.7|39.9}}{{cite journal |title=Transit Timing Variation of Near-Resonance Planetary Pairs. Ii. Confirmation of 30 Planets in 15 Multiple-Planet Systems|arxiv=1309.2329 |first1=Ji-Wei |last1=Xie |journal=The Astrophysical Journal Supplement Series |date=7 January 2014|volume=210 |issue=2 |page=25 |doi=10.1088/0067-0049/210/2/25 |bibcode=2014ApJS..210...25X |s2cid=119105092 }} {{Earth mass|link=y}}
| density = {{val|19.33|+39.9|-13.96}} g cm−3
| surface_grav = {{val|10.24|+14.36|-6.68}} g
| single_temperature = {{convert|924|K|C F}}{{Cite web |url=http://phl.upr.edu/projects/habitable-exoplanets-catalog/calculators |title=PHL's calculators |access-date=2018-01-21 |archive-date=2019-06-02 |archive-url=https://web.archive.org/web/20190602080028/http://phl.upr.edu/projects/habitable-exoplanets-catalog/calculators |url-status=dead }}
}}
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).
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 {{Earth_radius}}, 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 {{Earth_mass}}, comparable to Saturn's mass at 95.16 {{Earth_mass}}. 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{{cite web | url=https://exoplanetarchive.ipac.caltech.edu/cgi-bin/DisplayOverview/nph-DisplayOverview?objname=Kepler-277+c&type=CONFIRMED_PLANET | title=Kepler-277 }} and PSR J1719−1438 b in both radius and mass.{{cite web | url=http://www.manchester.ac.uk/discover/news/a-planet-made-of-diamond/ | title=A planet made of diamond }} Due to its proximity to its host star, Kepler-277b is quite hot with an equilibrium temperature of about {{convert|924|K|C F}}, 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 {{Earth radius}}. 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.{{cite conference|title=Kepler-277 b: A Supermassive Terrestrial Exoplanet in the Kepler-277 Planetary System|url=https://www.hou.usra.edu/meetings/lpsc2020/pdf/1055.pdf|first1=P.|last1=Futó|display-authors=etal|conference=51st Lunar and Planetary Science Conference|publisher=Lunar and Planetary Institute|date=March 2020|bibcode=2020LPI....51.1055F}}
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 =
{{main|Kepler-277}}
The parent star Kepler-277 is a large yellow star. It is 1.69 {{Solar_radius}} and 1.12 {{Solar_mass}}, 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.