Context :

Aims : The super-earth planet GJ 1214b has recently been the focus of several studies , which use the transit spectroscopy technique to determine the nature of its atmosphere .
Here , we focus on the H \alpha line as a tool to further restrict the nature of GJ 1214b ’ s atmosphere .

Methods : We used the Gran Telescopio Canarias ( GTC ) OSIRIS instrument to acquire narrow-band photometry with tunable filters .
We were able to observe the primary transit of the super-Earth GJ 1214 in three bandpasses : two centered in the continuum around H \alpha ( 653.5 nm and 662.0 nm ) and one centered at the line core ( 656.3 nm ) .
We measure the depth of the planetary transit at each wavelength interval .

Results : By fitting analytic models to the measured light curves , we were able to compute the depth of the transit at the three bandpasses .
Taking the difference in the computed planet-to-star radius ratio between the line and the comparison continuum filters , we find \Delta ( R _ { p } / R _ { \star } ) _ { H \alpha - 653.5 } = ( 6.60 \pm 3.54 ) \times 10 ^ { -3 } and \Delta ( R _ { p } / R _ { \star } ) _ { H \alpha - 662.0 } = ( 3.30 \pm 3.61 ) \times 10 ^ { -3 } .
Although the planet radius is found to be larger in the H \alpha line than in the surrounding continuum , the quality of our observations and the sigma level of the differences ( 1.8 and 1.0 , respectively ) do not allow us to claim an H \alpha excess in GJ 1214 ’ s atmosphere .
Further observations will be needed to resolve this issue .

Conclusions :