Optical Absorbers in the Atmosphere of WASP-98b

WASP-98b is a rare example of a transiting hot-Jupiter in orbit around a metal-poor main-sequence star. Since “metal” in this context refers to elements heavier than hydrogen and helium, how metal-rich or metal-poor a star is depends on its abundance of such elements. Based on the core-accretion model of planet formation, Jupiter-like planets are more common around metal-rich stars. The metallicity of the host star of WASP-98b is only one-third the metallicity of the Sun. It is very rare for a Jupiter-like planet to form around a star with such a low metallicity.

Figure 1: Artist’s impression of a gas giant planet.

WASP-98b orbits very close to its host star, with an orbital period of only 2.96 days. The planet is estimated to have ~0.8 times the mass and ~1.1 times the radius of Jupiter, and an equilibrium temperature of about 1180 K. New observations of two transit events of WASP-98b were done in four filters: g' (450.0 nm), r' (622.0 nm), i' (764.0 nm) and z' (898.9 nm). The observations show that WASP-98b appears larger in size when observed at g', r' and i' than at z'. The maximum difference is between r' and z', and it translates to about 5.5 pressure scale heights with a confidence level of roughly 6σ. The pressure scale height is the vertical distance through the planet’s atmosphere whereby the pressure decreases by a factor that is equal to the base of the natural logarithm (i.e. 2.71828…).

What this means is that there is an optical-absorbing molecular species in the upper atmosphere of WASP-98b that is making the planet’s atmosphere more opaque when observed in g', r' and i' than in z'. Gaseous titanium oxide (TiO) and vanadium oxide (VO) are optical-absorbing molecular species present in the atmospheres of the hottest hot-Jupiters where temperatures are too high for TiO and VO to condense out. However, the atmosphere of WASP-98b is not hot enough to support gaseous TiO and VO as they will simply condense out. Whatever that is absorbing optical radiation in the atmosphere of WASP-98b is currently unknown.

Figure 2: Variation of the ratio of the radii of WASP-98b and its host star (vertical axis) with wavelength (horizontal axis). The 4 black points with vertical and horizontal error bars indicate the observations in the 4 filters: g' (450.0 nm), r' (622.0 nm), i' (764.0 nm), z' (898.9 nm). The size of five atmospheric scale heights is shown on the right of the plot. Mancini et al. (2016)

Reference:
Mancini et al. (2016), “An optical transmission spectrum of the transiting hot Jupiter in the metal-poor WASP-98 planetary system”, arXiv:1606.00432 [astro-ph.EP]