So far more than 800 planets have been discovered and their
characterization is becoming more important. Transiting planets offer
the unique opportunity of detecting planetary atmospheres, helping to
improve the theoretical models of atmospheric composition under
different physical parameters (densities, irradiation, etc). However,
the precision needed to detect atoms and molecules, requires big
telescopes and stable instruments in order to obtain a good
signal-to-noise. In this talk I'll review the efforts, technical
challenges and current results that our group of Planets and Low Mass
stars is obtaining using GTC to study extra-solar planets.