Optical transmission spectroscopy provides crucial constraints on the reference pressure levels and scattering properties for hot Jupiter atmospheres .
For certain planets , where alkali atoms are detected in the atmosphere , their line profiles could serve as a good probe to link upper and lower atmospheric layers .
WASP-21b is a Saturn-mass hot Jupiter orbiting a thick disc star , with a low density and an equilibrium temperature of 1333 K , which makes it a good target for transmission spectroscopy .
Here , we present a low-resolution transmission spectrum for WASP-21b based in one transit observed by the OSIRIS spectrograph at the 10.4 m Gran Telescopio Canarias ( GTC ) , and a high-resolution transmission spectrum based in three transits observed by HARPS-N at Telescopio Nazinale Galileo ( TNG ) and HARPS at the ESO 3.6 m telescope .
We performed spectral retrieval analysis on GTC ’ s low-resolution transmission spectrum and report the detection of Na at a confidence level of > 3.5- \sigma .
The Na line exhibits a broad line profile that can be attributed to pressure broadening , indicating a mostly clear planetary atmosphere .
The spectrum shows a tentative excess absorption at the K D _ { 1 } line .
Using HARPS-N and HARPS , we spectrally resolved the Na doublet transmission spectrum .
An excess absorption at the Na doublet is detected during the transit , and shows a radial velocity shift consistent with the planet orbital motion .
We proposed a metric to quantitatively distinguish hot Jupiters with relatively clear atmospheres from others , and WASP-21b has the largest metric value among all the characterized hot Jupiters .
The detection of Na at both lower and upper atmosphere of WASP-21b reveals that it is an ideal target for future follow-up observations , providing the opportunity to understand the nature of its atmosphere across a wide range of pressure levels .