Kepler-11c

Kepler-11c's host star, Kepler-11, is a G-type star 2,000 light-years away^[4] in the Cygnus constellation. With a mass of .95 M_sun, a radius of 1.1 R_sun, a metallicity of [Fe/H] = 0, and an effective temperature of 5680 (± 100) K, Kepler-11 is almost identical to the Sun in terms of radius, mass, and temperature. However, Kepler-11 is much older than the Sun, with an estimated age of 8 (± 2) billion years^[6] (the Sun is approximately 4.6 billion years old^[7]). Along with Kepler-11c, Kepler-11 is host to the planets Kepler-11b, Kepler-11d, Kepler-11e, Kepler-11f, and Kepler-11g. The inner five planets' orbits would fit within the orbit of planet Mercury, while Kepler-11g orbits Kepler-11 at a much further distance in comparison to the inner components.^[2]

With an apparent magnitude of 14.2, Kepler-11 cannot be seen from Earth with the naked eye.^[3]

Kepler-11c has a mass of 13.5 M_E and a radius of 3.15 R_E, making it over 13.5 times the mass of earth, but approximately 3.15 times its radius. Neptune, in comparison, has a radius approximately 3.9 times that of Earth.^[8] With a density of 2.3 grams/cm³, Kepler-11c has a mass over double of that of pure water at 0 °C; it is also denser than all the Sun's gas giants, but less dense than any of its rocky planets. Its density is closest to the dwarf planet Pluto.^[9] Due in part to its proximity to its star, the planet's equilibrium temperature is 833 K, about three times hotter than Earth's average temperature. It orbits Kepler-11 every 13.02502 days at a distance of .106 AU; it is Kepler-11's second-closest planet. Mercury, in comparison, orbits every 87.97 days at a distance of .387 AU.^[10] The orbit's inclination of Kepler-11c is 89°, and is thus almost edge-on as seen from Earth.^[3]

The Kepler team has said that Kepler-11b and Kepler-11c are probably composed mostly of water with a thin hydrogen and helium atmosphere. In comparison to the outer planets of the system, which probably have large hydrogen and helium atmospheres, Kepler-11c's proximity to its star has blown off most of its atmosphere. In fact, formation and evolution calculations indicate that Kepler-11c lost over 50% (by mass) of the hydrogen envelope it had accreted after formation.^[11] Support to the idea that the planets of this system formed ex situ and contain large amounts of H₂O is provided by the radius of Kepler-11b, which can only be explained by the presence of a thin gaseous envelope. However, since hydrogen and helium cannot survive for the age of the system bound to the planet because of photo-evaporation blow-off, the most likely alternative is the presence of a vapor atmosphere generating from its surface.^[11]

Kepler-11 and its six-planet system form what NASA considers to be the most compact and flattest planetary system yet discovered.^[2] Kepler-11b and Kepler-11c orbit Kepler-11 with a phenomenon called orbital resonance, a gravitational tugging that keeps their orbits stable at a ratio of the orbital periods close to 5:4 .^[1] Orbital resonances can arise from orbital migration during formation.^[11] Kepler-11c may be at risk of eventually colliding with b due to a precarious orbital configuration in which a secular resonance could emerge and cause planet b's orbit to become dangerously eccentric.^[12]

• Denise Chow (4 February 2011). "A tourist's guide to the new Kepler-11 planet system". Space section. NBC News. Retrieved 13 March 2011.^{[dead link]}