Heat Redistribution on WASP-18b

WASP-18b is a massive extrasolar planet with a mass equal to 10 Jupiter masses and it is in a close-in 0.94-day orbit around an F6V parent star - a star that is somewhat hotter and larger than the Sun. The planet is believed to be tidally-locked with the same side permanently facing its parent star. As a result, temperatures on the day-side of WASP-18b can get as high as ~3000 K. Like Jupiter, WASP-18b is a gas giant planet comprised primarily of hydrogen and helium.

Figure 1: Artist’s impression of HAT-P-7b. Like WASP-18b, HAT-P-7b is gas giant planet in a close-in orbit around its parent star. Credit: NASA, ESA, and G. Bacon (STScI).

Observations of WASP-18b together with atmospheric models show nearly no day-side to night-side redistribution of heat. Any winds transporting heat away from the planet’s day-side is expected to be very weak. In an atmospheric model of WASP-18b that is consistent with almost no heat redistribution, the planet’s day-to-night temperature difference is around 2000 K to 3000 K (Figure 2). In another atmospheric model of WASP-18b, this time with 6.5 km/s superrotating winds racing around the planet from day-side to night-side, the day-to-night temperature difference drops to ~800 K (Figure 3). The effect of the superrotating winds is more efficient heat redistribution, resulting in a lower day-to-night temperature contrast.

Figure 2: Modelled atmospheric thermal profiles for selected longitudes for the case with almost no redistribution of heat. (N. Iro, P.F.L. Maxted, 2013)

Figure 3: Modelled atmospheric thermal profiles for selected longitudes for the case with 6.5 km/s superrotating winds redistributing heat from day-side to night-side. (N. Iro, P.F.L. Maxted, 2013)

Using the Spitzer space telescope, observations of the secondary eclipses of WASP-18b (i.e. when the planet passes behind its parent star and is blocked) show a relatively high planet-to-star flux ratio that is consistent with a very hot day-side, indicating nearly no redistribution of heat (Figure 4). This is because, with efficient heat redistribution, like in the atmospheric model with superrotating winds, the planet’s day-side would not be as hot, resulting in a lower planet-to-star flux ratio (Figure 5).

Figure 4: Modelled planet-to-star flux ratio as a function of wavelength for the case with almost no redistribution of heat. The red circles with error bars represent actual data points from observations by the Spitzer space telescope. (N. Iro, P.F.L. Maxted, 2013)

Figure 5: Modelled planet-to-star flux ratio as a function of wavelength for the case with 6.5 km/s superrotating winds redistributing heat from day-side to night-side. The red circles with error bars represent actual data points from observations by the Spitzer space telescope. (N. Iro, P.F.L. Maxted, 2013)

References:

- N. Iro, P.F.L. Maxted, “On the heat redistribution of the hot transiting exoplanet WASP-18b”, Icarus 226 (2013) 1719-1723

- Nymeyer et al. (2011), “Spitzer Secondary Eclipses of WASP-18b”, arXiv:1005.1017 [astro-ph.EP]