To date we have discovered thousands of planets, but there remain regions of parameter space that are still bare: for example, the short period and intermediate mass/radius space known as the “Neptunian desert”. Planets in this region of parameter space should be easy to find but discoveries remain few. For there to be such a dearth of planets points towards unusual and rare formation and evolution processes that produce the few planets that do reside there. As part of the Nomads program to obtain precise masses of these planets, we present here the discovery of TOI-332b, an ultra-short period planet sitting firmly within the desert. It has a radius of 3.21 +0.16 -0.12 R?, smaller than that of Neptune, but an unusually large mass of 57.4 ± 3.8 M?, more than half that of Saturn, resulting in a density of 9.5 +1.2 -1.3 g cm-3. Internal structure modelling predicts a likely negligible H/He envelope mass fraction, and with a large mass and little envelope, TOI-332b presents a challenge to the core-accretion model of planetary formation. We find that photoevaporation cannot account for the mass loss required to strip this planet of the Jupiter-like envelope it would have been expected to accrete. We need to look towards other scenarios, such as giant impacts, high eccentricity migration and gap opening in the protoplanetary disk to explain this unusual discovery, and hope that future observations may be able to disentangle these scenarios.
[Poster PDF File]