Sub-Neptunes, planets with radii between 1.7 and 3.5 times that of Earth, are among the most frequent exoplanet populations. However, their internal nature remains an enigma: some could be 'gas-dwarfs' with rocky cores and hydrogen-helium envelopes, while others might be water worlds formed beyond the snow line and subsequently migrated inwards. Distinguishing between these scenarios is challenging, as vastly different compositions can yield similar masses and radii.
The THIRSTEE project, initiated at the Andalusian Institute of Astrophysics (IAA-CSIC), seeks to address this question. The research is based on the premise that, in contrast to individual analyses yielding contradictory results, using high-quality data on atmospheres and densities will enable the creation of a sub-Neptune demographic map. This map will be compared with planetary formation and evolution models to determine if the water world theory, plausible for red dwarf stars, also applies to sub-Neptunes around Sun-like stars.
The project is structured into three working groups. The first focuses on the study and characterization of sub-Neptune atmospheres using data from the James Webb Space Telescope. The second group is dedicated to measuring the densities of these planets via the radial velocity method, employing instruments like ESPRESSO and MAROON X, and aims to triple the existing mass measurement sample. The third group, to be launched later, will develop the mathematical and statistical framework to jointly analyze transit, radial velocity, and atmospheric data, correcting for the biases inherent in each technique.
The initial results from the James Webb are promising, with two papers in preparation on the atmospheric characterization of sub-Neptunes orbiting young Sun-like stars. The high precision in measuring chemical composition and the low magnetic activity of the chosen stars facilitate these studies. Furthermore, a significant observation program has been secured at the ESO to precisely measure the mass of these planets, which could confirm the existence of smaller water worlds.
The project also includes a large JWST program with international collaboration to study the atmospheres of sub-Neptunes in resonance chain systems. While the search for biomarkers on terrestrial planets is still decades away, research on sub-Neptunes is at a crucial juncture. Current and future tools, such as PLATO and Earth 2.0, will provide extensive data to indirectly infer the potential presence of life.
In essence, the THIRSTEE project aims to answer fundamental questions about the proportion of sub-Neptunes that are water worlds and their prevalence in the galaxy. If these objectives are met, the project will serve as a key link in 21st-century astrophysics, paving the way for astrobiology and the search for life beyond our solar system.
