Kepler-11d

Lore & Background

Kepler-11d was announced on February 2, 2011, alongside five other planets in the Kepler-11 system, following extensive follow-up observations using telescopes at the Hale, Shane, Keck I, WIYN, Whipple, MMT, Hobby–Eberly, and Harlan J. Smith observatories, as well as the Spitzer Space Telescope. Its discovery paper was published in Nature the next day. The planet is named for its host star, Kepler-11, which itself was named after the Kepler space telescope, a NASA Earth-trailing satellite designed to detect Earth-like planets via the transit method.

Kepler-11d has a mass of 6.1 Earth masses and a radius of 3.43 Earth radii, giving it a density of 0.9 g/cm³—less than water and comparable to Saturn. This low density suggests a large atmosphere composed primarily of hydrogen and helium, though formation models indicate that most of its mass is from heavy elements, with the hydrogen/helium fraction less than 10%. Its equilibrium temperature is 692 K, far hotter than Earth. The planet orbits its star every 22.68719 days at a mean distance of 0.159 AU, with an orbital inclination of 89.3°, meaning it is almost exactly edge-on as seen from Earth.

The presence of hydrogen and helium on Kepler-11d, along with planets e and f, indicates that these planets formed within the first few million years of the system's existence, while gas from the protoplanetary disc was still available. All planets in the system require some hydrogen/helium to match their observed radii, though the innermost planet, Kepler-11b, likely lost its primordial envelope and retains only a tenuous atmosphere.

Reader's Guide

Kepler-11d is significant as part of the first system discovered by the Kepler space telescope to host multiple transiting planets, demonstrating the power of the transit method to reveal compact planetary systems. Its low density, comparable to Saturn, provides evidence that some planets can retain thick hydrogen-helium envelopes even at close orbital distances, challenging earlier assumptions about atmospheric escape. The planet's equilibrium temperature of 692 K and its orbit well within Mercury's distance highlight the diversity of exoplanetary environments. The Kepler-11 system, with five planets orbiting inside Mercury's orbit, offers a laboratory for studying planetary formation and migration, as the presence of hydrogen and helium on the outer planets suggests they formed early while gas was still present. The discovery of Kepler-11d and its sister planets, announced in 2011, marked a milestone in exoplanet science, showing that systems with multiple small planets are common and that detailed follow-up observations from ground-based telescopes and the Spitzer Space Telescope are essential for confirming and characterizing such worlds.

Did You Know?

• Kepler-11d is the third planet from its host star, Kepler-11.
• Its density of 0.9 g/cm³ is less than that of water and comparable to Saturn.
• The planet orbits its star every 22.68719 days, well within the orbit of Mercury.
• Kepler-11d was announced on February 2, 2011, along with five other planets in the system.