A decade of transit photometry for K2-19: Revised system architecture^★

¹ Observatoire de Genève, Département d’Astronomie, Université de Genève, Chemin Pegasi 51b, 1290 Versoix, Switzerland
² Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
³ School of Physics & Astronomy, Monash University, Victoria, 3800, Australia
⁴ Instituto Tecnológico de Buenos Aires (ITBA), Iguazú 341, Buenos Aires, CABA C1437, Argentina
⁵ Instituto de Ciencias Físicas (ICIFI; CONICET), ECyT-UNSAM, Campus Miguelete, 25 de Mayo y Francia, (1650) Buenos Aires, Argentina
⁶ Department of Physics and Institute of Astronomy, National Tsing-Hua University, Hsinchu 30013, Taiwan
⁷ Institute of Computational and Modeling Science, National Tsing-Hua University, Hsinchu 30013, Taiwan
⁸ College of Surveying and Geo-Informatics, Tongji University, Shanghai, 200092, PR China
⁹ Department of Physics and Astronomy, Vanderbilt University, Nashville, TN 37235, USA
¹⁰ National Astronomical Research Institute of Thailand, Chiang Mai 50180, Thailand
¹¹ SRON, Space Research Organisation Netherlands, Niels Bohrweg 4, 2333 CA, Leiden, The Netherlands
¹² NASA Ames Research Center, Moffett Field, CA 94035, USA
¹³ Institute of Space Sciences (ICE, CSIC), Carrer de Can Magrans S/N, Campus UAB, Cerdanyola del Valles, 08193 Spain
¹⁴ Institut d’Estudis Espacials de Catalunya (IEEC), 08860 Castellde-fels (Barcelona), Spain
¹⁵ Department of Astrophysical Sciences, Princeton University, NJ 08544, USA

^★★ Corresponding author: Jose.Almenara@unige.ch

Received: 16 July 2025
Accepted: 10 September 2025

Abstract

The star K2-19 hosts a pair of Neptunian planets deep inside the 3:2 resonance. They induce strong transit-timing variations with two incommensurate frequencies. Previous photodynamical modeling of 3.3 years of transit and radial velocity data produced mass estimates of 32.4 ± 1.7 M_⊕ and 10.8 ± 0.6 M_⊕ for planets b and c, respectively, and corresponding eccentricity estimates of 0.20 ± 0.03 and 0.21 ± 0.03. These high eccentricities raise questions about the formation origin of the system, and this motivated us to extend the observing baseline in an attempt to better constrain their values. We present a photodynamical analysis of 10 years of transit data that confirms the previous mass estimates (30.8 ± 1.3 M_⊕ and 11.1 ± 0.4 M_⊕), but reduces the median eccentricities to 0.04 ± 0.02 and 0.07 ± 0.02 for b and c, respectively. These values are more consistent with standard formation models, but still involve nonzero free eccentricity. The previously reported high eccentricities appear to be due to a single transit for which measurements taken at twilight mimicked ingress. This resulted in a 12-minute error in the midtransit time. The data that covered 1.3 and 5 so-called super and resonant periods were used to match a Fourier analysis of the transit-timing variation signal with simple analytic expressions for the frequencies and amplitudes to obtain planet mass estimates within 2% of the median photodynamical values, regardless of the eccentricities. Theoretical details of the analysis are presented in a companion paper. Additionally, we identified a possible planet candidate situated exterior to the b–c pair. Finally, in contrast to a previous study, our internal structure modeling of K2-19 b yields a metal mass fraction that is consistent with core accretion.