Potassium in the Atmosphere of a Hot Jupiter

Observations have shown that there is a lot of diversity in the atmospheres of hot Jupiters. Some hot Jupiters have atmospheres that are obscured by high altitude hazes, while others have relatively clear skies. When a transiting hot Jupiter passes in front of its host star, the amount of starlight it obscures depends on the composition of its atmosphere. This is because the presence of atmospheric species such as sodium and potassium let less light through at certain wavelengths. Since the inferred size of a hot Jupiter is based on the amount of starlight it obscures, the planet’s size can appear slightly larger at wavelengths where the planet’s atmosphere is more opaque. 

Figure 1: Artist’s impression of a planet transiting a star. Image credit: ESA/ATG medialab.

HAT-P-1b is a hot Jupiter in a close-in 4.47 day orbit around a Sun-like star. Being so close to its host star, its dayside is heated to temperatures well over 1,000 °C. HAT-P-1b has 1.319 times the radius and 0.525 times the mass of Jupiter, giving it an average density of only 0.345 g/cm³. A study by Wilson et al. (2015) reports the detection of potassium in the atmosphere of HAT-P-1b. Four transits of HAT-P-1b were observed - two at wavelengths that are not affected by the presence of potassium (6792 Å and 8844 Å) and two at wavelengths where potassium is more opaque (7582.0 Å and 7664.9 Å).

The measured planet-to-star size ratio is 0.1176 ± 0.0013 at 6792 Å and 0.1168 ± 0.0022 at 8844 Å. At the other two wavelengths, the planet’s atmosphere becomes more opaque, causing the planet to appear larger with a planet-to-star size ratio of 0.1248 ± 0.0014 at 7582.0 Å and 0.1268 ± 0.0012 at 7664.9 Å. Basically, the detection of potassium is based on the observed increase in the planet-to-star size ratio, Δ0.0073 ± 0.0017 at 7582.0 Å and Δ0.0094 ± 0.0016 at 7664.9 Å. The strong detection of potassium is due to the high temperatures on HAT-P-1b which keep the upper atmosphere of the planet puffed up. When a transit of HAT-P-1b is observed at a wavelength where potassium is opaque, the planet’s puffed up upper atmosphere allows less light of that particular wavelength through, resulting in a larger planet-to-star size ratio.

Figure 2: Transit lightcurve of HAT-P-1b observed in the 6792 Å and 8844 Å wavelengths. Wilson et al. (2015).

Figure 3: Transit lightcurve of HAT-P-1b observed in the 7582.0 Å and 7664.9 Å wavelengths where potassium is more opaque. The half transits were obtained on 19 November 2010 whilst the full transits were obtained on 26 November 2013. Wilson et al. (2015).

Reference:
Wilson et al. (2015), “GTC OSIRIS transiting exoplanet atmospheric survey: detection of potassium in HAT-P-1b from narrowband spectrophotometry”, arXiv:1503.07165 [astro-ph.EP]