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ATREIDES I. Embarking on a trek across the exo-Neptunian landscape with the TOI-421 system

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Main Authors:	Bourrier, V., Steiner, M., Castro-González, A., Armstrong, D. J., Attia, M., Gill, S., Timmermans, M., Fernandez, J., Hawthorn, F., Triaud, A. H. M. J., Murgas, F., Palle, E., Chakraborty, H., Poppenhaeger, K., Lendl, M., Anderson, D. R., Bryant, E. M., Friden, E., Seidel, J. V., Osorio, M. R. Zapatero, Eeles-Nolle, F., Lafarga, M., Lockley, I. S., Bell, J. Serrano, Allart, R., Meech, A., Osborn, A., Díaz, R. F., Keniger, M. A. Fetzner, Frame, G., Heitzmann, A., Ringham, A., Eggenberger, P., Alibert, Y., Almenara, J. M., Leleu, A., Sousa, S. G., Mercier, S. J., Adibekyan, V., Battley, M. P., Mena, E. Delgado, Dethier, W., Egger, J. A., Barkaoui, K., Bayliss, D., Burdanov, A. Y., Ducrot, E., Ghachoui, M., Gillon, M., Chew, Y. Gómez Maqueo, Jehin, E., Pedersen, P. P., Pozuelos, F. J., Wheatley, P. J., Zúniga-Fernández, S., Carteret, Y., Cegla, H. M., Correia, A. C. M., Davis, Y. T., Doyle, L., Ehrenreich, D., Hara, N. C., Lavie, B., Lillo-Box, J., Lovis, C., Petit, A. C., Santos, N. C., Scott, M. G., Venturini, J., Ahrer, E. -M., Aigrain, S., Barros, S. C. C., Gillen, E., Luo, X., Mordasini, C., Moulla, K. Al, Pepe, F., Pietrow, A. G. M.
Format:	Preprint
Published:	2025
Subjects:	Earth and Planetary Astrophysics Solar and Stellar Astrophysics
Online Access:	Acceder al recurso
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_version_	1866908547098345472
author	Bourrier, V. Steiner, M. Castro-González, A. Armstrong, D. J. Attia, M. Gill, S. Timmermans, M. Fernandez, J. Hawthorn, F. Triaud, A. H. M. J. Murgas, F. Palle, E. Chakraborty, H. Poppenhaeger, K. Lendl, M. Anderson, D. R. Bryant, E. M. Friden, E. Seidel, J. V. Osorio, M. R. Zapatero Eeles-Nolle, F. Lafarga, M. Lockley, I. S. Bell, J. Serrano Allart, R. Meech, A. Osborn, A. Díaz, R. F. Keniger, M. A. Fetzner Frame, G. Heitzmann, A. Ringham, A. Eggenberger, P. Alibert, Y. Almenara, J. M. Leleu, A. Sousa, S. G. Mercier, S. J. Adibekyan, V. Battley, M. P. Mena, E. Delgado Dethier, W. Egger, J. A. Barkaoui, K. Bayliss, D. Burdanov, A. Y. Ducrot, E. Ghachoui, M. Gillon, M. Chew, Y. Gómez Maqueo Jehin, E. Pedersen, P. P. Pozuelos, F. J. Wheatley, P. J. Zúniga-Fernández, S. Carteret, Y. Cegla, H. M. Correia, A. C. M. Davis, Y. T. Doyle, L. Ehrenreich, D. Hara, N. C. Lavie, B. Lillo-Box, J. Lovis, C. Petit, A. C. Santos, N. C. Scott, M. G. Venturini, J. Ahrer, E. -M. Aigrain, S. Barros, S. C. C. Gillen, E. Luo, X. Mordasini, C. Moulla, K. Al Pepe, F. Pietrow, A. G. M.
author_facet	Bourrier, V. Steiner, M. Castro-González, A. Armstrong, D. J. Attia, M. Gill, S. Timmermans, M. Fernandez, J. Hawthorn, F. Triaud, A. H. M. J. Murgas, F. Palle, E. Chakraborty, H. Poppenhaeger, K. Lendl, M. Anderson, D. R. Bryant, E. M. Friden, E. Seidel, J. V. Osorio, M. R. Zapatero Eeles-Nolle, F. Lafarga, M. Lockley, I. S. Bell, J. Serrano Allart, R. Meech, A. Osborn, A. Díaz, R. F. Keniger, M. A. Fetzner Frame, G. Heitzmann, A. Ringham, A. Eggenberger, P. Alibert, Y. Almenara, J. M. Leleu, A. Sousa, S. G. Mercier, S. J. Adibekyan, V. Battley, M. P. Mena, E. Delgado Dethier, W. Egger, J. A. Barkaoui, K. Bayliss, D. Burdanov, A. Y. Ducrot, E. Ghachoui, M. Gillon, M. Chew, Y. Gómez Maqueo Jehin, E. Pedersen, P. P. Pozuelos, F. J. Wheatley, P. J. Zúniga-Fernández, S. Carteret, Y. Cegla, H. M. Correia, A. C. M. Davis, Y. T. Doyle, L. Ehrenreich, D. Hara, N. C. Lavie, B. Lillo-Box, J. Lovis, C. Petit, A. C. Santos, N. C. Scott, M. G. Venturini, J. Ahrer, E. -M. Aigrain, S. Barros, S. C. C. Gillen, E. Luo, X. Mordasini, C. Moulla, K. Al Pepe, F. Pietrow, A. G. M.
contents	The distribution of close-in exoplanets is shaped by the interplay between atmospheric and dynamical processes. The Neptunian Desert, Ridge, and Savanna illustrate the sensitivity of these worlds to such processes, making them ideal to disentangle their roles. Determining how many Neptunes were brought close-in by early disk-driven migration (DDM; maintaining primordial spin-orbit alignment) or late high-eccentricity migration (HEM; generating large misalignments) is essential to understand how much atmosphere they lost. We propose a unified view of the Neptunian landscape to guide its exploration, speculating that the Ridge is a hot spot for evolutionary processes. Low-density Neptunes would mainly undergo DDM, getting fully eroded at shorter periods than the Ridge, while denser Neptunes would be brought to the Ridge and Desert by HEM. We embark on this exploration via ATREIDES, which relies on spectroscopy and photometry of 60 close-in Neptunes, their reduction with robust pipelines, and their interpretation through internal structure, atmospheric, and evolutionary models. We carried out a systematic RM census with VLT/ESPRESSO to measure the distribution of 3D spin-orbit angles, correlate its shape with system properties and thus relate the fraction of aligned-misaligned systems to DDM, HEM, and atmospheric erosion. Our first target, TOI-421c, lies in the Savanna with a neighboring sub-Neptune TOI-421b. We measured their 3D spin-orbit angles (Psib = 57+11-15 deg; Psic = 44.9+4.4-4.1 deg). Together with the eccentricity and possibly large mutual inclination of their orbits, this hints at a chaotic dynamical origin that could result from DDM followed by HEM. ATREIDES will provide the community with a wealth of constraints for formation and evolution models. We welcome collaborations that will contribute to pushing our understanding of the Neptunian landscape forward.
format	Preprint
id	arxiv_https___arxiv_org_abs_2509_15746
institution	arXiv
publishDate	2025
record_format	arxiv
spellingShingle	ATREIDES I. Embarking on a trek across the exo-Neptunian landscape with the TOI-421 system Bourrier, V. Steiner, M. Castro-González, A. Armstrong, D. J. Attia, M. Gill, S. Timmermans, M. Fernandez, J. Hawthorn, F. Triaud, A. H. M. J. Murgas, F. Palle, E. Chakraborty, H. Poppenhaeger, K. Lendl, M. Anderson, D. R. Bryant, E. M. Friden, E. Seidel, J. V. Osorio, M. R. Zapatero Eeles-Nolle, F. Lafarga, M. Lockley, I. S. Bell, J. Serrano Allart, R. Meech, A. Osborn, A. Díaz, R. F. Keniger, M. A. Fetzner Frame, G. Heitzmann, A. Ringham, A. Eggenberger, P. Alibert, Y. Almenara, J. M. Leleu, A. Sousa, S. G. Mercier, S. J. Adibekyan, V. Battley, M. P. Mena, E. Delgado Dethier, W. Egger, J. A. Barkaoui, K. Bayliss, D. Burdanov, A. Y. Ducrot, E. Ghachoui, M. Gillon, M. Chew, Y. Gómez Maqueo Jehin, E. Pedersen, P. P. Pozuelos, F. J. Wheatley, P. J. Zúniga-Fernández, S. Carteret, Y. Cegla, H. M. Correia, A. C. M. Davis, Y. T. Doyle, L. Ehrenreich, D. Hara, N. C. Lavie, B. Lillo-Box, J. Lovis, C. Petit, A. C. Santos, N. C. Scott, M. G. Venturini, J. Ahrer, E. -M. Aigrain, S. Barros, S. C. C. Gillen, E. Luo, X. Mordasini, C. Moulla, K. Al Pepe, F. Pietrow, A. G. M. Earth and Planetary Astrophysics Solar and Stellar Astrophysics The distribution of close-in exoplanets is shaped by the interplay between atmospheric and dynamical processes. The Neptunian Desert, Ridge, and Savanna illustrate the sensitivity of these worlds to such processes, making them ideal to disentangle their roles. Determining how many Neptunes were brought close-in by early disk-driven migration (DDM; maintaining primordial spin-orbit alignment) or late high-eccentricity migration (HEM; generating large misalignments) is essential to understand how much atmosphere they lost. We propose a unified view of the Neptunian landscape to guide its exploration, speculating that the Ridge is a hot spot for evolutionary processes. Low-density Neptunes would mainly undergo DDM, getting fully eroded at shorter periods than the Ridge, while denser Neptunes would be brought to the Ridge and Desert by HEM. We embark on this exploration via ATREIDES, which relies on spectroscopy and photometry of 60 close-in Neptunes, their reduction with robust pipelines, and their interpretation through internal structure, atmospheric, and evolutionary models. We carried out a systematic RM census with VLT/ESPRESSO to measure the distribution of 3D spin-orbit angles, correlate its shape with system properties and thus relate the fraction of aligned-misaligned systems to DDM, HEM, and atmospheric erosion. Our first target, TOI-421c, lies in the Savanna with a neighboring sub-Neptune TOI-421b. We measured their 3D spin-orbit angles (Psib = 57+11-15 deg; Psic = 44.9+4.4-4.1 deg). Together with the eccentricity and possibly large mutual inclination of their orbits, this hints at a chaotic dynamical origin that could result from DDM followed by HEM. ATREIDES will provide the community with a wealth of constraints for formation and evolution models. We welcome collaborations that will contribute to pushing our understanding of the Neptunian landscape forward.
title	ATREIDES I. Embarking on a trek across the exo-Neptunian landscape with the TOI-421 system
topic	Earth and Planetary Astrophysics Solar and Stellar Astrophysics
url	https://arxiv.org/abs/2509.15746