L. Doyle, D. J. Armstrong, L. Acuña, A. Osborn, S. G. Sousa, A. Castro-González, V. Bourrier, D. R. Alves, D. Barrado, S. C. C. Barros, D. Bayliss, K. Cui, O. Demangeon, R. F. Díaz, X. Dumusque, F. Eeles-Nolle, S. Gill, A. Hacker, J. S. Jenkins, M. A. F. Keniger, M. Lafarga, J. Lillo-Box, I. Lockley, L. D. Nielsen, L. Parc, J. Rodrigues, A. Santerne, N. C. Santos, P. J. Wheatley

Abstract
In this paper, we present a homogeneous analysis of close-in Neptune planets. To do this, we compile a sample of TESS-observed planets using a ranking criterion which takes into account the planet's period, radius, and the visual magnitude of its host star. We use archival and new HARPS data to ensure every target in this sample has precise radial velocities. This yields a total of 64 targets, 46 of which are confirmed planets and 18 of which show no significant radial velocity signal. We explore the mass–radius distribution, planetary density, stellar host metallicity, and stellar and planetary companions of our targets. We find 26 per cent of our sample are in multiplanet systems, which are typically seen for planets located near the lower edge of the Neptunian desert. We define a 'gold' subset of our sample consisting of 33 confirmed planets with planetary radii between 2$R_{oplus}$ and 10$R_{oplus}$. With these targets, we calculate envelope mass fractions (EMF) using the GAS gianT modeL for Interiors (GASTLI). We find a clear split in EMF between planets with equilibrium temperatures below and above 1300 K, equivalent to an orbital period of $sim$3.5 d. Below this period, EMFs are consistent with zero, while above they typically range from 20 per cent to 40 per cent, scaling linearly with the planetary mass. The orbital period separating these two populations coincides with the transition between the Neptunian desert and the recently identified Neptunian ridge, further suggesting that different formation and/or evolution mechanisms are at play for Neptune planets across different close-in orbital regions.

Keywords
techniques: radial velocities / surveys / planets and satellites: detection / planets and satellites: formation / planets and satellites: fundamental parameters / planets and satellites: interiors / Earth and Planetary Astrophysics

Monthly Notices of the Royal Astronomical Society
Volume 539, Page 19
2025 June

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