Kepler-11d

Kepler-11 is a G-type star in the Cygnus constellation. With a mass of .95 M_sun, a radius of 1.1 R_sun, a measured metallicity of 0, and an effective temperature of 5680 K, Kepler-11 is near in mass (95% of Sun), radius (110% of Sun), and iron content as the Sun.^[3] Metallicity has been observed to play a major role determining the type of planet a star forms. As metal-rich gas clouds tend to cause planetary cores to aggregate to a gravitationally prominent size while primordial gases still exist in the system, gas giants tend to form under such conditions.^[5] It is also slightly cooler than the Sun. However, it is estimated to be eight (± two) billion years old, far older than the Sun is.^[6] Kepler-11 is host to five other planets other than Kepler-11d: Kepler-11b, Kepler-11c, Kepler-11e, Kepler-11f, and Kepler-11g. The first five planets in the system have orbits that would collectively fit inside the orbit of planet Mercury, while Kepler-11g orbits at a considerably further distance in relation to the orbits of its inner counterparts.^[2]

At a distance of approximately 659 parsecs,

Kepler-11d has a mass of 6.1 M_E and a radius of 3.43 R_E, making it some six times more massive than Earth with nearly three and a half times the radius. With a density of .9 grams/cm³, Kepler-11d's density is less than that of water and comparable to that of the gas giant Saturn. This suggests that, unlike its sister planets Kepler-11b and Kepler-11c, which are closer to their host star, Kepler-11d has maintained a large (in volume) atmosphere that is most likely composed of hydrogen and helium.^[2] Nonetheless, formation models indicate that most of the mass is accounted for by heavy elements and that the mass fraction of hydrogen/helium is less than 10%.^[7] Kepler-11d has an equilibrium temperature of 692 K, nearly 2.7 times the equilibrium temperature of Earth. Kepler-11d orbits at a mean distance of .159 AU from its host star and completes an orbit every 22.68719 days, making it the third planet from Kepler-11. In comparison, planet Mercury orbits the Sun every 87.97 days at a distance of .387 AU.^[8] Kepler-11d has an orbital inclination of 89.3°. Thus, Kepler-11d is almost exactly edge on with respect to Earth.^[3]

The presence of large quantities of hydrogen and helium on Kepler-11d, Kepler-11e, and Kepler-11f suggests that these planets formed within the first few million years of the system's existence, when gas could still be captured from the forming protoplanetary disc.^[1] In fact, according to detailed formation calculations, all planets of the system require some amount of hydrogen/helium in order to match their observed radii. The innermost planet, Kepler-11b, which likely lost all of its primordial light-element envelope, still requires a tenuous atmosphere (either made of steam or of gases sequestered in the heavy-element core) to account for its observed radius.^[7]

• 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]}