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arXiv · 2605.12324

An Ultra-Short Period Super-Earth and a Sub-Neptune Orbiting the K dwarf TOI-4311

Yoshi Nike Emilia Eschen·Thomas G. Wilson·Andrew Collier Cameron·Alexander James Mustill·Jo Ann Egger·Solène Ulmer-Moll·Davide Gandolfi·Alexis M. S. Smith·Olivier D. S. Demangeon·Sérgio G. Sousa·Andrea Bonfanti·Alexandre C. M. Correia·Vardan Adibekyan·Gaia Lacedelli·Alexis Brandeker·Camilla Pezzotti·Babatunde Akinsanmi·Yann Alibert·Roi Alonso·Tamas Bárczy·David Barrado·Susana C. C. Barros·Wolfgang Baumjohann·Willy Benz·Nicolas Billot·Luca Borsato·Christopher Broeg·Amadeo Castro-González·Szilard Csizmadia·Patricio E. Cubillos·Melvyn B. Davies·Magali Deleuil·Adrien Deline·Brice-Olivier Demory·Aliz Derekas·Giuseppe Di Persio·Billy Edwards·David Ehrenreich·Anders Erikson·Andrea Fortier·Luca Fossati·Malcolm Fridlund·Kosmas Gazeas·Michaël Gillon·Manuel Güdel·Maximilian N. Günther·Alexis Heitzmann·Ch. Helling·Kate G. Isaak·Jon M. Jenkins·Tatiana Keller·Laszlo L. Kiss·Daniel Kitzmann·Judith Korth·Kristine W. F. Lam·Jacques Laskar·Alain Lecavelier des Etangs·Adrien Leleu·Monika Lendl·Demetrio Magrin·Pierre F. L. Maxted·Bruno Merín·Christoph Mordasini·Valerio Nascimbeni·Göran Olofsson·Hugh P. Osborn·Roland Ottensamer·Isabella Pagano·Enric Pallé·Gisbert Peter·Glen Petitpas·Daniele Piazza·Giampaolo Piotto·Don Pollacco·Didier Queloz·Roberto Ragazzoni·Nicola Rando·Heike Rauer·Ignasi Ribas·Nuno C. Santos·Gaetano Scandariato·Damien Ségransan·Avi Shporer·André M. Silva·Attila E. Simon·Richard Southworth·Manu Stalport·Sophia Sulis·Gyula M. Szabó·Stéphane Udry·Bernd Ulmer·Valérie Van Grootel·Julia Venturini·Eva Villaver·Nicholas A. Walton·Sebastian Wolf·Carl Ziegler·Tiziano Zingales
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Abstract

We report the discovery and characterisation of the multi-planetary system around TOI-4311, a K dwarf kinematically between the Galactic thick disk and Hercules stream. TOI-4311 hosts an ultra-short-period super-Earth (P$\sim$0.99 d, $1.376\substack{+0.077\\-0.080}$ R$_\oplus$) and a longer period sub-Neptune (P$\sim$15 d, $2.47\substack{+0.12\\-0.11}$ R$_\oplus$) that was first detected in the TESS photometry. Using follow-up observations with CHEOPS and HARPS, we refine the planetary radius of both planets, derive the mass of planet b ($4.5\substack{+1.5\\-1.4}$ M$_\oplus$) and confirm the planetary nature of planet c. Intriguingly, a third periodic signal is clearly detected in our HARPS RVs that we cannot link to stellar activity. This signal could be attributed to a third planet (P$\sim$38 d, Msin(i)=$26.4\substack{+6.3\\-6.8}$ M$_\oplus$) in the system, however with the current photometric dataset we do not find a transit. Our dynamical analysis highlights that this potential outer planet would remain stable. Using the precise radius and mass for TOI-4311 b we model its interior structure and find that it is very dense given the host star's galactic kinematics and chemistry. Hence this system could challenge current formation theories and provide insights into planet formation across the galaxy.

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Yoshi Nike Emilia Eschen, Thomas G. Wilson, Andrew Collier Cameron, Alexander James Mustill, Jo Ann Egger, Solène Ulmer-Moll, Davide Gandolfi, Alexis M. S. Smith, Olivier D. S. Demangeon, Sérgio G. Sousa, Andrea Bonfanti, Alexandre C. M. Correia, Vardan Adibekyan, Gaia Lacedelli, Alexis Brandeker, Camilla Pezzotti, Babatunde Akinsanmi, Yann Alibert, Roi Alonso, Tamas Bárczy, David Barrado, Susana C. C. Barros, Wolfgang Baumjohann, Willy Benz, Nicolas Billot, Luca Borsato, Christopher Broeg, Amadeo Castro-González, Szilard Csizmadia, Patricio E. Cubillos, Melvyn B. Davies, Magali Deleuil, Adrien Deline, Brice-Olivier Demory, Aliz Derekas, Giuseppe Di Persio, Billy Edwards, David Ehrenreich, Anders Erikson, Andrea Fortier, Luca Fossati, Malcolm Fridlund, Kosmas Gazeas, Michaël Gillon, Manuel Güdel, Maximilian N. Günther, Alexis Heitzmann, Ch. Helling, Kate G. Isaak, Jon M. Jenkins, Tatiana Keller, Laszlo L. Kiss, Daniel Kitzmann, Judith Korth, Kristine W. F. Lam, Jacques Laskar, Alain Lecavelier des Etangs, Adrien Leleu, Monika Lendl, Demetrio Magrin, Pierre F. L. Maxted, Bruno Merín, Christoph Mordasini, Valerio Nascimbeni, Göran Olofsson, Hugh P. Osborn, Roland Ottensamer, Isabella Pagano, Enric Pallé, Gisbert Peter, Glen Petitpas, Daniele Piazza, Giampaolo Piotto, Don Pollacco, Didier Queloz, Roberto Ragazzoni, Nicola Rando, Heike Rauer, Ignasi Ribas, Nuno C. Santos, Gaetano Scandariato, Damien Ségransan, Avi Shporer, André M. Silva, Attila E. Simon, Richard Southworth, Manu Stalport, Sophia Sulis, Gyula M. Szabó, Stéphane Udry, Bernd Ulmer, Valérie Van Grootel, Julia Venturini, Eva Villaver, Nicholas A. Walton, Sebastian Wolf, Carl Ziegler, Tiziano Zingales. 2026-05-12. An Ultra-Short Period Super-Earth and a Sub-Neptune Orbiting the K dwarf TOI-4311. https://arxiv.org/abs/2605.12324

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