Kepler-11e

Lore & Background

Kepler-11e was announced on February 2, 2011, along with its five sister planets, after confirmation by multiple observatories including the Keck 1 telescope, Hale and Shane telescopes, Harlan J. Smith and Hobby–Eberly telescopes, Nordic Optical Telescope, and telescopes at WIYN and Whipple observatories, as well as the Spitzer Space Telescope. The findings were published in Nature on February 3, 2011. The planet orbits its star every 31 days in an ellipse that would fit within the orbit of Mercury.

Kepler-11e has a mass 8.4 times that of Earth and a radius 4.52 times that of Earth, giving it a density of 0.5 g/cm³—less than that of Saturn, the least dense planet in the Solar System. Its low density is likely due to a large hydrogen and helium atmosphere. Formation models indicate the planet has a gaseous envelope less massive than 20% of its total mass, which accounts for about 60% of its radius (or about 90% of its volume). Because it is not as close to its star as Kepler-11b and Kepler-11c, the Kepler team suggests its low density may result from a large atmosphere that has not been blown away by stellar wind.

The host star Kepler-11 is located in the constellation Cygnus, with a mass of 0.95 solar masses and a radius of 1.1 solar radii. It has an effective temperature of 5680 K, a metallicity of 0, and an estimated age of eight billion years—about 1.73 times older than the Sun. The star is 613 parsecs away with an apparent magnitude of 14.2, invisible to the naked eye. The inner five planets, including Kepler-11e, orbit in a tightly knit configuration within the orbit of Mercury, while Kepler-11g orbits much farther out.

Reader's Guide

Kepler-11e is significant as part of the first discovered extrasolar system with more than three transiting planets, demonstrating the Kepler space telescope's ability to detect multiple planets in a single system. Its extremely low density—less than that of Saturn—highlights the diversity of exoplanet compositions, particularly the existence of gas giants with extensive hydrogen-helium envelopes that have not been stripped by stellar radiation. The planet's orbital period of 31 days within a compact system that fits inside Mercury's orbit illustrates how planetary systems can be far more tightly packed than our own Solar System. The confirmation of Kepler-11e and its sister planets through a coordinated network of ground-based telescopes and the Spitzer Space Telescope set a precedent for follow-up observations of transiting exoplanets. The system's flat, compact architecture also provides insights into planetary formation and migration, suggesting that gas giants can form and migrate inward early in a star's lifetime. Kepler-11e's equilibrium temperature of 617 K places it in a regime where its atmosphere is heated but not fully evaporated, offering a natural laboratory for studying atmospheric escape and retention in close-in exoplanets.

Did You Know?

• The planet orbits its star every 31.995990 days at a mean distance of 0.194 AU, well within Mercury's orbit.
• Kepler-11e's gaseous envelope accounts for about 60% of its radius and 90% of its volume.