INNATE Investigating the nature and origins of exoplanets in the Neptunian desert

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Our knowledge of exoplanets has undergone a step change since the discovery of 51 Peg b 25 years ago. Planets are now commonplace, with most stars found to be hosting planetary systems. In the set of over 4300 planets known, trends and gaps in the distribution are seen with respect to planet size, composition and host star properties, arising from the formation and evolution processes which sculpt these worlds.A striking signature in the planet distribution is the 'Neptunian desert', a dearth of Neptune-like planets orbiting close to their host stars. Planets which arrive so close to their stars are evaporated away, or are disrupted entirely by the strong gravitational forces of the star. Yet, recently a number of planets have been discovered inside the desert, surprising expectations and highlighting a gap in our understanding. The origin of these hot Neptunes is unknown, and they present a unique opportunity to study the extreme outcomes of planet formation. Such outliers of the normal processes allow us to benchmark planetary formation theories, with many of the typical degeneracies stripped away.With this proposal, we will combine photometric and spectroscopic observations from TESS, NGTS, CORALIE, HARPS and Gaia to carry out an ambitious and comprehensive research program investigating the nature and origins of planets in the desert. (1) We will uncover the unbiased demographic properties of the in-desert planets, finding the distributions of planet mass, radius, density, internal structure and host star properties. (2) We will establish the dynamical context of these systems by determining the presence of outer companions and local density of stars (3) We will connect the measured demographics, dynamics and internal structure to wider formation and evolution theory. The combined results will transform our understanding not only of the in-desert planets but of the wider formation, structure and evolution of planetary systems.