TESS and CHEOPS Discover Two Warm Sub-Neptunes Transiting the Bright K-dwarf HD 15906
Creators: name: Tuson, Amy
name: Queloz, Didier
name: Osborn, Hugh P.
name: Wilson, Thomas G.
name: Hooton, Matthew J.
name: Beck, Mathias
name: Lendl, Monika
name: Olofsson, Göran
name: Fortier, Andrea
name: Bonfanti, Andrea
name: Brandeker, Alexis
name: Buchhave, Lars A.
name: Cameron, Andrew Collier
name: Ciardi, David R.
name: Collins, Karen A.
name: Gandolfi, Davide
name: Garai, Zoltan
name: Giacalone, Steven
name: da Silva, João Gomes
name: Howell, Steve B.
name: Patel, Jayshil A.
name: Persson, Carina M.
name: Serrano, Luisa M.
name: Sousa, Sérgio G.
name: Ulmer-Moll, Solène
name: Vanderburg, Andrew
name: Ziegler, Carl
name: Alibert, Yann
name: Alonso, Roi
name: Anglada, Guillem
name: Bárczy, Tamas
name: Navascues, David Barrado
name: Barros, Susana C. C.
name: Baumjohann, Wolfgang
name: Beck, Thomas
name: Benz, Willy
name: Billot, Nicolas
name: Bonfils, Xavier
name: Borsato, Luca
name: Broeg, Christopher
name: Cabrera, Juan
name: Charnoz, Sébastien
name: Conti, Dennis M.
name: Csizmadia, Szilard
name: Cubillos, Patricio E.
name: Davies, Melvyn B.
name: Deleuil, Magali
name: Delrez, Laetitia
name: Demangeon, Olivier D. S.
name: Demory, Brice-Olivier
name: Dragomir, Diana
name: Dressing, Courtney D.
name: Ehrenreich, David
name: Erikson, Anders
name: Essack, Zahra
name: Farinato, Jacopo
name: Fossati, Luca
name: Fridlund, Malcolm
name: Furlan, Elise
name: Gill, Holden
name: Gillon, Michaël
name: Gnilka, Crystal L.
name: Gonzales, Erica
name: Güdel, Manuel
name: Günther, Maximilian N.
name: Hoyer, Sergio
name: Isaak, Kate G.
name: Jenkins, Jon M.
name: Kiss, Laszlo L.
name: Laskar, Jacques
name: Latham, David W.
name: Law, Nicholas
name: Etangs, Alain Lecavelier des
name: Curto, Gaspare Lo
name: Lovis, Christophe
name: Luque, Rafael
name: Magrin, Demetrio
name: Mann, Andrew W.
name: Maxted, Pierre F. L.
name: Mayor, Michel
name: McDermott, Scott
name: Mecina, Marko
name: Mordasini, Christoph
name: Mortier, Annelies
name: Nascimbeni, Valerio
name: Ottensamer, Roland
name: Pagano, Isabella
name: Pallé, Enric
name: Peter, Gisbert
name: Piotto, Giampaolo
name: Pollacco, Don
name: Pritchard, Tyler
name: Ragazzoni, Roberto
name: Rando, Nicola
name: Ratti, Francesco
name: Rauer, Heike
name: Ribas, Ignasi
name: Ricker, George R.
name: Rieder, Martin
name: Santos, Nuno C.
name: Savel, Arjun B.
name: Scandariato, Gaetano
name: Schwarz, Richard P.
name: Seager, Sara
name: Ségransan, Damien
name: Shporer, Avi
name: Simon, Attila E.
name: Smith, Alexis M. S.
name: Steller, Manfred
name: Stockdale, Chris
name: Szabó, Gyula M.
name: Thomas, Nicolas
name: Torres, Guillermo
name: Tronsgaard, René
name: Udry, Stéphane
name: Ulmer, Bernd
name: Van Grootel, Valérie
name: Vanderspek, Roland
name: Venturini, Julia
name: Walton, Nicholas A.
name: Winn, Joshua N.
name: Wohler, Bill
Publisher: name: arXiv
Year: 2023
Subjects: subject: Earth and Planetary Astrophysics (astro-ph.EP)
subject: FOS: Physical sciences
subject: FOS: Physical sciences

description: We report the discovery of two warm sub-Neptunes transiting the bright (G = 9.5 mag) K-dwarf HD 15906 (TOI 461, TIC 4646810). This star was observed by the Transiting Exoplanet Survey Satellite (TESS) in sectors 4 and 31, revealing two small transiting planets. The inner planet, HD 15906 b, was detected with an unambiguous period but the outer planet, HD 15906 c, showed only two transits separated by $\sim$ 734 days, leading to 36 possible values of its period. We performed follow-up observations with the CHaracterising ExOPlanet Satellite (CHEOPS) to confirm the true period of HD 15906 c and improve the radius precision of the two planets. From TESS, CHEOPS and additional ground-based photometry, we find that HD 15906 b has a radius of 2.24 $\pm$ 0.08 R$_\oplus$ and a period of 10.924709 $\pm$ 0.000032 days, whilst HD 15906 c has a radius of 2.93$^{+0.07}_{-0.06}$ R$_\oplus$ and a period of 21.583298$^{+0.000052}_{-0.000055}$ days. Assuming zero bond albedo and full day-night heat redistribution, the inner and outer planet have equilibrium temperatures of 668 $\pm$ 13 K and 532 $\pm$ 10 K, respectively. The HD 15906 system has become one of only six multiplanet systems with two warm ($\lesssim$ 700 K) sub-Neptune sized planets transiting a bright star (G $\leq$ 10 mag). It is an excellent target for detailed characterisation studies to constrain the composition of sub-Neptune planets and test theories of planet formation and evolution.
description: 30 pages, 20 figures, 11 tables (including appendix). Published in MNRAS