Kepler-11b

Lore & Background

Kepler-11b was discovered by the Kepler space telescope, a NASA-led mission to find Earth-like planets. Its host star, Kepler-11, is located in the constellation Cygnus and has a mass 0.95 times that of the Sun and a radius 1.1 times the Sun's. The star's metallicity is nearly identical to the Sun's. The planet's discovery was announced on February 2, 2011, after follow-up observations using ground-based telescopes in California, Hawaii, the Canary Islands, Arizona, and Texas, as well as the Spitzer Space Telescope. The detection of an orbital resonance effect between Kepler-11b and Kepler-11c confirmed the find.

Kepler-11b is estimated to be 2.78 Earth masses and 1.8 Earth radii, with a density of 4.5 g/cm³—denser than the solar system's gas giants but slightly less dense than the terrestrial planets. Early estimates suggest it is not of Earth-like composition, yet it is composed mostly of elements heavier than helium. The planet orbits its host star every 10.30375 days at a distance of 0.091 AU, making it the closest planet in the system. Its effective temperature is 900 K, the hottest among the Kepler-11 planets. The planet's orbit has an inclination of 88.5°, deviating slightly from the orbital plane more than the other five planets.

Kepler-11b's close proximity to its star implies strong insolation, which caused it to lose its light-element envelope acquired during formation. However, its observed low density requires the presence of a gaseous envelope, likely produced via outgassing of hydrogen or evaporation of H₂O from the condensed core. The mass of its atmosphere is not strongly constrained but may represent 0.04% of planetary mass, close to that of Venus. Kepler-11b and Kepler-11c orbit with a strong 5:4 orbital resonance, which gravitationally tugs them into stable orbits. The planet's orbit is in a regime where secular resonance may abruptly destabilize it, potentially causing a collision with Kepler-11c.

Reader's Guide

Kepler-11b is significant as part of the first discovered planetary system with more than three transiting planets, marking a milestone in exoplanet astronomy. The Kepler-11 system is the most compact and flattest known planetary system, with the first five planets all able to fit within the orbit of Mercury. This discovery demonstrated that tightly packed, multi-planet systems exist and can be detected via transit timing variations, a technique used to confirm Kepler-11b's mass. The planet's density and composition challenge Earth-like models, showing that super-Earths can have diverse compositions. Its atmosphere, possibly derived from outgassing or water evaporation, provides insight into planetary evolution under intense stellar radiation. The orbital resonance between Kepler-11b and Kepler-11c highlights the dynamical stability possible in such crowded systems, while the potential for secular resonance destabilization underscores the delicate balance in these architectures. Kepler-11b's legacy lies in expanding the understanding of planetary system formation and the variety of worlds beyond the solar system.

Did You Know?

• Kepler-11b is the hottest of the six planets in the Kepler-11 system.
• The planet's density is 4.5 g/cm³, less dense than Venus but denser than gas giants.
• Kepler-11b and Kepler-11c orbit in a 5:4 orbital resonance.
• The Kepler-11 system was the first discovered with more than three transiting planets.