During the early stages of planetary formation, planets suffer severe changes in their physical and orbital properties due to internal and external forces, which also affect their primary atmospheres. The study of planets at these early stages and its comparison with the already mature planet population is crucial for a better comprehension of different processes, such as: planet formation, gas accretion or inflation in giant planets, and evaporation of primary atmospheres of rocky-core planets with gas envelopes.

As part of a large project to investigate the evolution, out-flow, and evaporation of atmospheres across different planet types, but focusing on young and sub-Neptunes planets, we performed observations of GJ 1214b. Although many planets with similar properties have been found, GJ1214b was one of the first planets to be discovered with a mass and radius between those of Earth and Neptune. This planet offered an exceptional opportunity to explore the composition of this type of objects with no analogues in the Solar System, and has been the subject of intense observing campaigns from space- and ground-based observatories. So far, the results have revealed a featureless transmission spectrum linked to the presence of clouds and hazes in its atmosphere. In particular, there have been several attempts to detect or place upper limits on the presence of He in the atmosphere of this planet, although so far a detection has not been achieved.

In this talk we will review the latest CARMENES observations in search for H/He absorption features and evaporation. This includes the observations of one transit of GJ1214b, reporting the first tentative detection of a chemical species in the atmosphere of this benchmark sub-Neptune planet.