A Hot Giant Planet as Black as Charcoal

Kepler-423b is a hot-Jupiter which zips around its host star every 2.7 days in a close-in orbit. On each orbit, Kepler-423b crosses in front of its host star, causing an observable dip in the star’s brightness, thereby allowing the planet’s size to be measured. Kepler-423b is estimated to have 60 percent the mass of Jupiter and 1.2 times its diameter. The host star of Kepler-423b is a very old Sun-like star with an estimated age of 11 ± 2 billion years.

The orbit of Kepler-423b also periodically brings it behind its host star in what is known as a secondary eclipse. During secondary eclipse, the star occults any emission from the planet, leading to a slight decrease in the combined star-planet flux. For Kepler-423b, the drop in the combined star-planet flux is remarkably small. This indicates that Kepler-423b is a very dark object (i.e. low reflectivity). Kepler-423b reflects less than 4 percent of the light it receives from its host star, making the planet as reflective as charcoal. If Kepler-423b were more reflective, the secondary eclipse signature would be stronger since the planet would contribute more to the combined star-planet flux.

Even though Kepler-423b is as black as charcoal, the planet’s dayside would still appear utterly glaring. The extreme closeness to its host star means that the insolation the planet receives is a few hundred times more intense than what Earth receives from the Sun. Reflecting just a few percent of that intense insolation would still create nothing short of a blazing glare. Temperatures on the planet’s dayside can get as high as ~2000K.

Reference:
Gandolfi et al. (2015), “Kepler-423b: a half-Jupiter mass planet transiting a very old solar-like star”, arXiv:1409.8245 [astro-ph.EP]