The research activity is focused in two different aspects, namely 1) the modelling of stellar variations associated with magnetic activity and 2) the understanding of high-energy emissions of stars and their evolution over time.

• We have devised and constructed a sophisticated code to invert light and radial velocity curves simultaneously and reconstruct the stellar photosphere including spot and facular regions, dubbed StarSim (now in version 2.0). StarSim 2.0 considers surface inhomogeneities in the form of (dark) starspots and (bright) faculae, takes into account limb darkening (or brightening in the case of faculae), and includes time-variable effects such as differential rotation and active region evolution. In the case of radial velocities, it includes full modelling of line profile changes (bisector spatial variability and convective blueshift). StarSim 2.0 is able to reproduce to good precision the simultaneous measurements of photometry and radial velocity of active stars and we can reconstruct the surface map of a star, showing active regions. The applications of StarSim 2.0 are various: correction of radial velocity time series, correction of photometric time series, simultaneous use of visible and near-infrared radial velocities, investigation of the chromatic Rossiter-McLaughlin effect, etc. We are working on such applications.

This research bears directly on various instruments and space missions, which are aimed at discovering terrestrial (potentially habitable) planets and measuring their atmospheric properties. We use our precise stellar modelling to maximise the scientific output from the CARMENES instrument, the CHEOPS mission, the PLATO mission and the ARIEL mission.

• Over the past decade, we have been investigating the emissions of stars as the main source of energy shaping the structure, evolution, and even determining the mere existence of planetary atmospheres. “The Sun in Time” was a comprehensive programme to study solar proxies and trace the high-energy evolution of Sun-like stars during their main-sequence lifetime. We found that high-energy emissions were orders of magnitude stronger in the past, which should have had a strong influence on their planets.

To understand the distribution of life in the Universe in general, and to the design of terrestrial planet finding missions in particular, a detailed interdisciplinary study on the habitability of terrestrial planets in orbits around cool stars is crucial. The currently accepted definition of the habitable zone of a star is based on an Earth analogue with liquid water and a reservoir of CO2 to regulate the climate. Although this general climatological definition of the habitable zone can be applied to all stellar types, the evolution of the atmosphere and the planet's water inventory of terrestrial planets in the habitable zone of cool stars may differ from that of planets around solar-type stars. Planets around low-mass stars have closer orbits, which makes them more vulnerable to the radiation and particle emissions from their parent stars. Radiation in the XUV is absorbed high in the planetary atmosphere and thus induces exospheric heating, expansion of the upper layers and eventual mass loss, thus altering the volatile content of the planet.

We are opening a new arena in this context by studying stars less massive than the Sun. The strategy is based on characterising the emissions of a stellar sample and generalising the results to any low-mass star. Initial findings indicate that cool stars possibly have stronger and longer-lived XUV emissions than solar-like stars. Whether a terrestrial planet in the habitable zone around a low-mass star can keep an atmosphere and sustain a biosphere remains to be seen. UV radiation is absorbed deeper in the planet’s atmosphere leading to photodissociation of some key molecules thus giving rise to a rich photochemistry. Our present view of the stellar habitable zone is certainly incomplete. Only by characterising the emissions of the entire population of low-mass stars and by understanding the interactions of their radiation and particle fluxes with planet atmospheres will we be able to complete a true picture of the habitable zone around a star.