  Although atmospheric transmission spectroscopy of HD209458b with the Hubble
Space Telescope has been very successful, attempts to detect its atmospheric
absorption features using ground-based telescopes have so far been fruitless.
Here we present a new method for probing the atmospheres of transiting
exoplanets which may be more suitable for ground-based observations, making use
of the Rossiter effect. During a transit, an exoplanet sequentially blocks off
light from the approaching and receding parts of the rotating star, causing an
artificial radial velocity wobble. The amplitude of this signal is directly
proportional to the effective size of the transiting object, and the wavelength
dependence of this effect can reveal atmospheric absorption features, in a
similar way as with transmission spectroscopy. The advantage of this method
over conventional atmospheric transmission spectroscopy is that it does not
rely on accurate photometric comparisons of observations on and off transit,
but instead depends on the relative velocity shifts of individual stellar
absorption lines within the same on-transit spectra. We used an archival
VLT/UVES data set to apply this method to HD209458. The amplitude of the
Rossiter effect is shown to be 1.7+-1.2 m/sec higher in the Sodium D lines than
in the weighted average of all other absorption lines in the observed
wavelength range, corresponding to an increment of 4.3+-3% (1.4 sigma). The
uncertainty in this measurement compares to a photometric accuracy of 5e-4 for
conventional atmospheric transmission spectroscopy, more than an order of
magnitude higher than previous attempts using ground-based telescopes.
Observations specifically designed for this method could increase the accuracy
further by a factor 2-3.
