Kepler-91b

Kepler-91b is a giant planet orbiting Kepler-91, a star slightly more massive than the Sun. Kepler-91 has left the main sequence and is now a red giant branch star.

Kepler-91b

Discovery
Discovery date	2013
Detection method	Transit (Kepler Mission)
Orbital characteristics
Semi-major axis	0.072+0.007 −0.002 AU
Orbital period (sidereal)	6.24658 d
Inclination	68.5+1.6 −1
Star	Kepler-91
Physical characteristics
Mean radius	1.322+0.094 −0.086 RJ
Mass	0.88+0.33 −0.17 MJ
Mean density	0.44 g/cm3 (0.016 lb/cu in)
Albedo	0.39 [1]
Temperature	2132 K

Discovery and further confirmation

Kepler-91b was detected by analyzing the data of Kepler space telescope where a transit-like signal was found. Initially thought to be a false positive due to light curve variations by a self-luminous object, it was later revealed that due to Kepler-91's low density, its shape is distorted to a slightly ellipsoidal shape due to gravitational effects of the planet. Ellipsoidal light variations caused by Kepler-91b constitute more than the third of the light variations compared to transit depth. Ellipsoidal light variations also allowed to determine the planet's mass. It was also found that Kepler-91b reflects some of the starlight from its star.^[2]

Further analysis managed to question the planetary nature of the object, suspecting that it is a self-luminous object.^[3] However, the planetary nature was eventually confirmed again through both the radial velocity technique^[4] and re-analysis of the light curve modulations.^[5]

Characteristics

Kepler-91b is about 14% less massive than Jupiter while being more than 35% larger, making it less than half of the density of water. Kepler-91b orbits around the host star in about 6.25 days. Despite being one of the least edge-on orbits relative to Earth with inclination being about 68.5 degrees, transit was detected due to low semi-major axis to host star radius ratio.

Kepler-91b is expected to be engulfed by the parent star within about 55 million years.^[2][1]

Possible trojan companion

The possibility of a trojan planet to Kepler-91b was suggested due to the presence of a small dim in the phase-folded light curve at phase 0.68.^[2] This was subsequently studied but the conclusion was that the transit-signal was a false-positive.^[6]