DIX Planetary Science Seminar

Neptune-sized planets on close-in orbits are exceptionally rare, leaving a prominent gap in radius-period space known as the 'Neptune desert.' Yet, this desert is not entirely empty. Published studies have revealed that planets located in this region tend to be denser than their Neptune-sized counterparts on more distant orbits, and that they also orbit more metal-rich stars. This can potentially be explained by scenarios in which the desert is populated by the stripped cores of Jovian-mass progenitors. However, with only 26 published desert dwellers, we cannot definitively confirm or disprove this hypothesis using statistical arguments. Here, we present 21 newly validated transiting planets located within the Neptune desert. These planets were initially identified by the TESS mission and validated using a combination of multi-color ground-based photometry, high-resolution imaging, and stellar spectroscopy. We use this expanded sample to explore trends in host star metallicities, kinematic ages, and rotational velocities and compare the resulting distributions to those of stars hosting either hot Jupiters or Neptune-sized planets on more distant orbits. We find that stars hosting desert dwellers exhibit even higher metallicities, appear kinematically older, and rotate more slowly after accounting for age effects, than hot Jupiter hosts. This combination favors the stripped core hypothesis and disfavors alternative models such as collisional mergers. We conclude by discussing the implications of this result for our understanding of the interior structures of gas giant exoplanets.

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JWST provides a unique opportunity to spectroscopically characterize the surface compositions of terrestrial exoplanets for the first time. Close-in, rocky planets orbiting M dwarfs are ideal targets for these studies, as recent Spitzer and JWST measurements have shown that many have little to no atmosphere. However, all published JWST observations of their emission spectra lack the sensitivity required to detect surface spectral features. LHS 3844 b is an ultra short period (P = 11 hours) bare-rock super-Earth (Rp = 1.3 Rearth) orbiting a 3000 K M dwarf, and is one of the most favorable targets available for these studies. We present a first look at its combined NIRSpec G395H and MIRI LRS emission spectrum, which we constructed from a total of 13 (9 of which were recently taken) separate eclipse observations. This is the highest SNR emission spectrum for any rocky exoplanet thus far, with a median SNR of 29 across 12 MIRI wavelength bins. We leverage this improved sensitivity to search for evidence of the Christiansen feature (an indication of the presence of silicate rocks) and the transparency feature (whose presence or absence reveals information about the surface's grain size independent of composition). We discuss our preliminary constraints on both of these features and the corresponding implications for the surface properties of LHS 3844 b.