During the early stages of planetary formation, planets undergo strong changes in their physical and orbital properties due to internal and external forces, which also affect their primordial atmospheres composed mainly of H and He. The study of planets in these early stages and their comparison with the more mature planet population is crucial for a better understanding of different processes such as: planet formation, boil-off, accretion or inflation in gas giant planets, and evaporation of the primordial atmospheres of small planets with gas envelopes.

As part of a large project investigating the evolution, outflow, and evaporation processes of exoplanet atmospheres across different planet types, we present the first results of the MOPYS (Measuring Outflows in Planets around Young Stars) survey. Our focus was on young planets, specifically the sub-Neptune and mini-Neptune population, with well-constrained ages of less than 1 Gyr. We conducted high-resolution transmission spectroscopy observations of over 20 young transiting planets, examining their atmospheres and targeting the two primary evaporation tracers accessible from the ground: Hα in the visible and He I triplet in the near-infrared. However, because the differences between young and old planetary populations is a crucial aspect to study, we have conducted an exhaustive literature review of H and He observations of young and old exoplanets. Our survey was complemented by the available literature results, with a total set of 30 young planets. We compared the observational results between young and old planet populations, using a total sample of 70 planets that covered a wide range of the Mass-Radius-Period parameter space. This comprehensive study of planets enabled us to gain a general understanding of atmospheres across different ages, radii, and temperatures, while minimizing observational biases in the sample. We also discovered new relationships that explain the evaporation processes. This large sample of observations can serve as a benchmark to test various evaporation models and theories of planet formation.

In this talk I will review the latest CARMENES and GIARPS observations in search of H/He absorption features and evaporation. Additionally, preliminary results of He observations from the new high-resolution spectrograph NIRPS will be presented. These include the first transmission spectra for over 20 young exoplanets, as well as two new He detections.