A Planet on the Verge of Engulfment

Figure 1: Artist’s impression of a hot-Jupiter transiting its host star. Credit: Mark A. Garlick.

The exoplanet Kepler-91 b orbits around an evolved K3 host star that is in the process of transforming into a red giant. Observations show that Kepler-91 b is a gas-giant planet measuring 0.88 times the mass and 1.38 times the radius of Jupiter. Its host star has 1.3 times the mass and 6.3 times the radius of the Sun. Kepler-91 b circles around its host star in a slightly eccentric, close-in orbit with a period of 6.25 days. Given the planetary mass and radius, the mean density of Kepler-91 b works out to be 0.33 times the density of Jupiter. This low density suggests that Kepler-91 b is somewhat inflated due to the strong stellar irradiation from its host star.

Although the orbit of Kepler-91 b is nowhere near the shortest for exoplanets, the sheer size of its host star means that Kepler-91 b is a mere 1.32 stellar radii from the surface of its host star at closest approach. As the host star continues to expand into a red giant, estimates show that Kepler-91 b is expected to be swallowed in less than 55 million years - a mere blink of the eye on astronomical scales. Even that is considered as an upper limit to the planet’s life. The equilibrium temperature of Kepler-91 b is estimated to be over 2000 K.

Figure 2: Best-fit solutions for the transit of Kepler-91 b in front of its host star. Source: Lillo-Box et al. (2013).

 Figure 3: Diagram illustrating the irradiation of Kepler-91 b by its host star. The red lines represent the boundaries of the stellar irradiation that hits the planet’s surface. The yellow part represents the dayside of the planet. The black part represents the night side and the red part is the extra region illuminated due to the close-in orbit and the large stellar radii of the host star. Source: Lillo-Box et al. (2013).

The close-in orbit of Kepler-91 b and the sheer size of its host star result in more than half of the planet being illuminated by the host star (Figure 3). In fact, around 70 percent of the planet is illuminated by the host star. When Kepler-91 b is at closest approach, its host star would appear to subtend a remarkable 48 degrees, covering around 10 percent of the sky as seen from the planet. In comparison, the Sun covers only 0.0005 percent of the sky as seen from Earth. Kepler-91 b is indeed on the verge of being swallowed by its host star.

Reference:

Lillo-Box et al. (2013), “Kepler-91b: a giant planet at the end of its life”, arXiv:1312.3943 [astro-ph.EP]