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The detection of increased sodium absorption during primary transit implies the presence of an atmosphere around an extrasolar planet, and enables us to infer the structure of this atmosphere. Sodium has only been detected in the atmospheres of two planets to date – HD 189733b and HD 209458b. WASP‐17b is the least dense planet currently known. It has a radius approximately twice that of Jupiter and orbits an F6‐type star. The transit signal is expected to be about five times larger than that observed in HD 209458b. We obtained 24 spectra with the GIRAFFE spectrograph on the VLT, eight during transit. The integrated flux in the sodium doublet at wavelengths 5889.95 and 5895.92 Å was measured at bandwidths 0.75, 1.5, 3.0, 4.0, 5.0 and 6.0 Å. We find a transit depth of  0.55 ± 0.13  per cent at 1.5 Å. This suggests that, like HD 209458b, WASP‐17b has an atmosphere depleted in sodium compared to models for a cloud‐free atmosphere with solar sodium abundance. We observe a sharp cut‐off in sodium absorption between 3.0 and 4.0 Å which may indicate a layer of clouds high in the atmosphere.

Transmission spectroscopy of the sodium ‘D’ doublet in WASP‐17b with the VLT ★