Kepler-91b

Lore & Background

Kepler-91b was first detected through a transit-like signal in data from the Kepler space telescope. Initially suspected to be a false positive caused by a self-luminous object, the signal was later attributed to the planet's gravitational effects on its host star. Because Kepler-91 has a low density, the star's shape is distorted into a slightly ellipsoidal form, producing ellipsoidal light variations that account for more than a third of the total light variation compared to the transit depth. These variations also allowed the planet's mass to be determined. The planet reflects some starlight from its host.

The planetary nature of the object was questioned again, with some analyses suggesting it might be a self-luminous object. However, further investigation using the radial velocity technique and re-analysis of the light curve modulations confirmed it as a planet. Kepler-91b orbits its star every 6.25 days. Despite having one of the least edge-on orbits relative to Earth, with an inclination of about 68.5 degrees, a transit was detected because the planet's semi-major axis is small compared to the host star's radius.

A possible trojan companion to Kepler-91b was suggested due to a small dim in the phase-folded light curve at phase 0.68. Subsequent study concluded that this transit signal was a false positive.

Reader's Guide

Kepler-91b is significant as a rare example of a planet orbiting a star that has left the main sequence and entered the red giant branch phase. Its detection and confirmation illustrate the challenges of identifying transiting planets around evolved stars, where stellar variability and ellipsoidal distortions can mimic or obscure planetary signals. The planet's low density—less than half that of water—and its large size relative to its mass highlight the diversity of giant planet structures. The use of ellipsoidal light variations to determine the planet's mass, rather than relying solely on radial velocity, demonstrates an alternative method for characterizing exoplanets. The planet's impending engulfment within about 55 million years provides a natural laboratory for studying the final stages of planetary systems as their host stars evolve. The investigation into a possible trojan companion, though ultimately ruled out, shows the level of scrutiny applied to subtle features in transit light curves. Kepler-91b remains a key object for understanding planet survival around post-main-sequence stars.

Did You Know?

• Kepler-91b is less than half the density of water.
• The planet's host star is a red giant branch star that has left the main sequence.
• Ellipsoidal light variations caused by Kepler-91b account for more than a third of the light variation compared to the transit depth.
• Kepler-91b is expected to be engulfed by its star within about 55 million years.