Kepler-408b

Lore & Background

Kepler-408b was initially detected in 2014 as a transiting sub-Earth planet on a tight 2.5-day orbit, reported with a relatively low confidence of 97.9%. It was confirmed in 2016. The planet's orbit is inclined to the equatorial plane of its host star by 41.7°, a strong spin-orbit misalignment that is unique for a sub-Earth transiting planet. This misalignment suggests either the presence of additional giant planets in the system or a history of close stellar encounters; the planet may also be a captured body originating from elsewhere. The host star, HD 176693, is an F8V star older than the Sun, with 75% of the solar abundance of heavy elements, and is chromospherically inactive, though there is weak evidence for tidal spin-up due to star-planet interaction.

Reader's Guide

Kepler-408b holds significance as a rare example of a sub-Earth exoplanet exhibiting a strong spin-orbit misalignment of 41.7°, a feature typically associated with larger planets or those in multi-planet systems. Its discovery and confirmation, initially at a 97.9% confidence level in 2014 and confirmed in 2016, highlight the challenges of detecting small transiting planets. The misalignment raises questions about the planet's formation and dynamical history, with proposed explanations including the influence of undetected giant planets or past close stellar encounters, as well as the possibility of capture from elsewhere. The host star's properties—an older, slightly metal-poor F8V star with weak evidence of tidal spin-up—provide context for the planet's evolution. As of 2016, no stellar companions have been detected, leaving the system's architecture open to further study. Kepler-408b thus serves as a key object for understanding the orbital dynamics and origins of small exoplanets in close-in orbits.

Did You Know?

• Kepler-408b orbits its star every 2.5 days.
• The planet's orbit is inclined 41.7° from the star's equatorial plane.
• The host star HD 176693 is 5% more massive and 25% larger than the Sun.
• The star is predicted to come as close as 7.0957 pc in 1.6 million years.