Kepler-277c
{{Infobox planet
| name = Kepler-277c
| discovery_site = Kepler Space Observatory
| discovered = 2014
| discovery_method = Transit
| apsis = astron
| semimajor = ~0.209 AU
| star = Kepler-277
| mean_radius = 3.36 {{±|0.83|0.72}} {{Earth radius|link=y}}
| mass = 64.2 {{±|18.1|15.7}}{{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|sym=y|link=y}}
| density = {{val|9.33|+15.33|-5.69}} g cm−3
| surface_grav = {{val|5.69|+6.12|-2.93}} g
| single_temperature = {{convert|745|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-277c (also known by its Kepler Objects of Interest designation KOI-1215.02) is the third most massive and second-largest rocky planet ever discovered, with a mass about 64 times that of Earth. Discovered in 2014 by the Kepler Space Telescope, Kepler-277c is a Neptune-sized exoplanet with a very high mass and density for an object of its radius, suggesting a composition made mainly of rock with some amounts of water. Along with its sister planet, Kepler-277b, the planet's mass was determined using transit-timing variations (TTVs).
Characteristics
=Size and temperature=
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 {{Earth_radius}}, 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 {{Earth mass|sym=y}}, close to Saturn's mass at 95.16 {{Earth mass|sym=y}}. 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. It is currently the third most massive and second largest terrestrial planet ever discovered, behind Kepler-277b in mass{{cite web | url=https://exoplanetarchive.ipac.caltech.edu/cgi-bin/DisplayOverview/nph-DisplayOverview?objname=Kepler-277+b&type=CONFIRMED_PLANET | title=Kepler-277 }} and PSR J1719-1438 b in both radius and mass.{{cite news | 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-277c is quite hot with an equilibrium temperature of about {{convert|745|K|C F}}, hot enough to melt certain metals.
=Orbit=
Kepler-277c orbits close to its host star, with one orbit lasting 33.006 days. 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.
=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.