Thousands of known exoplanets cluster into distinct size and orbit categories, but one region—the Neptunian desert—is nearly empty: planets the size of Neptune should not exist in scorching orbits close to their stars, yet a handful have been found there anyway. These desert-dwelling planets defy current formation models. Standard theory says they should either be stripped of their atmospheres by stellar radiation or torn apart by tidal forces before they can settle into such orbits. Their mere existence reveals a gap in our understanding of how planetary systems form and evolve. This project will combine data from multiple telescopes—TESS, NGTS, HARPS, and others—to measure the masses, radii, densities, and internal structures of these outliers, and to search for outer companion planets that might have pushed them inward. If successful, the work will transform theories of planet formation by providing a clean test case where normal degeneracies are stripped away. This is fundamental science with no immediate practical application. But understanding how planets form and survive in extreme environments sharpens the models used to interpret all exoplanet data—including searches for potentially habitable worlds. Past fundamental studies of planetary dynamics have underpinned everything from satellite navigation corrections to the discovery of thousands of exoplanets themselves.
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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.
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