Radii, masses, and transit-timing variations of the three-planet system orbiting the naked-eye star TOI-396

A. Bonfanti; I. Amateis; D. Gandolfi; L. Borsato; J. A. Egger; P. E. Cubillos; D. Armstrong; I. C. Leão; M. Fridlund; B. L. Canto Martins; S. G. Sousa; J. R. De Medeiros; L. Fossati; V. Adibekyan; A. Collier Cameron; S. Grziwa; K. W. F. Lam; E. Goffo; L. D. Nielsen; F. Rodler; J. Alarcon; J. Lillo-Box; W. D. Cochran; R. Luque; S. Redfield; N. C. Santos; S. C. C. Barros; D. Bayliss; X. Dumusque; M. A. F. Keniger; J. Livingston; F. Murgas; G. Nowak; A. Osborn; H. P. Osborn; E. Pallé; C. M. Persson; L. M. Serrano; P. A. Strøm; S. Udry; P. J. Wheatley

doi:10.1051/0004-6361/202451300

Radii, masses, and transit-timing variations of the three-planet system orbiting the naked-eye star TOI-396

A. Bonfanti, I. Amateis, D. Gandolfi, L. Borsato, J. A. Egger, P. E. Cubillos, D. Armstrong, I. C. Leão, M. Fridlund, B. L. Canto Martins, S. G. Sousa, J. R. De Medeiros, L. Fossati, V. Adibekyan, A. Collier Cameron, S. Grziwa, K. W. F. Lam, E. Goffo, L. D. Nielsen, F. RodlerJ. Alarcon, J. Lillo-Box, W. D. Cochran, R. Luque, S. Redfield, N. C. Santos, S. C. C. Barros, D. Bayliss, X. Dumusque, M. A. F. Keniger, J. Livingston, F. Murgas, G. Nowak, A. Osborn, H. P. Osborn, E. Pallé, C. M. Persson, L. M. Serrano, P. A. Strøm, S. Udry, P. J. Wheatley

Research output: Contribution to journal › Article › peer-review

Abstract

Context. TOI-396 is an F6 V bright naked-eye star (V ≈ 6.4) orbited by three small (R_p ≈ 2 R⊕) transiting planets discovered thanks to space-based photometry from two TESS sectors. The orbital periods of the two innermost planets, namely TOI-396 b and c, are close to the 5:3 commensurability (P_b ~ 3.6 d and P_c ~ 6.0 d), suggesting that the planets might be trapped in a mean motion resonance (MMR).

Aims. To measure the masses of the three planets, refine their radii, and investigate whether planets b and c are in MMR, we carried out HARPS radial velocity (RV) observations of TOI-396 and retrieved archival high-precision transit photometry from four TESS sectors.

Methods. We extracted the RVs via a skew-normal fit onto the HARPS cross-correlation functions and performed a Markov chain Monte Carlo joint analysis of the Doppler measurements and transit photometry, while employing the breakpoint method to remove stellar activity from the RV time series. We also performed a transit timing variation (TTV) dynamical analysis of the system and simulated the temporal evolution of the TTV amplitudes of the three planets following an N-body numerical integration.

Results. Our analysis confirms that the three planets have similar sizes (R_b = 2.004_−0.047^+0.045 R⊕ ; R_c = 1.979_−0.051^+0.054 R⊕; R_d = 2.001_−0.064^+0.063 R⊕) and is thus in agreement with previous findings. However, our measurements are ~ 1.4 times more precise thanks to the use of two additional TESS sectors. For the first time, we have determined the RV masses for TOI-396 b and d, finding them to be M_b = 3.55_−0.96^+0.94 M⊕ and M_d = 7.1 ± 1.6 M⊕, which implies bulk densities of ρ_b = 2.44_−0.68^+0.69 g cm⁻³ and ρ_d = 4.9_−1.1^+1.2 g cm⁻³, respectively. Our results suggest a quite unusual system architecture, with the outermost planet being the densest. Based on a frequency analysis of the HARPS activity indicators and TESS light curves, we find the rotation period of the star to be P_rot,⋆ = 6.7 ± 1.3 d, in agreement with the value predicted from log R′_HK-based empirical relations. The Doppler reflex motion induced by TOI-396 c remains undetected in our RV time series, likely due to the proximity of the planet’s orbital period to the star’s rotation period. We also discovered that TOI-396 b and c display significant TTVs. While the TTV dynamical analysis returns a formally precise mass for TOI-396 c of M_c,dyn= 2.24_−0.67^+0.13M⊕, the result might not be accurate, owing to the poor sampling of the TTV phase. We also conclude that TOI-396 b and c are close to but out of the 5:3 MMR.

Conclusions. A TTV dynamical analysis of additional transit photometry evenly covering the TTV phase and super-period is likely the most effective approach for precisely and accurately determining the mass of TOI-396 c. Our numerical simulation suggests TTV semi-amplitudes of up to five hours over a temporal baseline of ~ 5.2 years, which should be duly taken into account when scheduling future observations of TOI-396.

Original language	English
Article number	A90
Number of pages	18
Journal	Astronomy & Astrophysics
Volume	693
Early online date	7 Jan 2025
DOIs	https://doi.org/10.1051/0004-6361/202451300
Publication status	Published - Jan 2025

Keywords

Techniques: photometric
Techniques: radial velocities
Planets and satellites: fundamental parameters
Stars: fundamental parameters

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Copyright © The Authors 2025. Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. This article is published in open access under the Subscribe to Open model. Subscribe to A&A to support open access publication.
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https://arxiv.org/abs/2411.14911Licence: CC BY

Radii, masses, and transit-timing variations of the three-planet system orbiting the naked-eye star TOI-396 (dataset)
Cameron, A. C. (Creator), VizieR On-line Data Catalog, 2025
Dataset

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Bonfanti, A., Amateis, I., Gandolfi, D., Borsato, L., Egger, J. A., Cubillos, P. E., Armstrong, D., Leão, I. C., Fridlund, M., Martins, B. L. C., Sousa, S. G., De Medeiros, J. R., Fossati, L., Adibekyan, V., Cameron, A. C., Grziwa, S., Lam, K. W. F., Goffo, E., Nielsen, L. D., ... Wheatley, P. J. (2025). Radii, masses, and transit-timing variations of the three-planet system orbiting the naked-eye star TOI-396. Astronomy & Astrophysics, 693, Article A90. https://doi.org/10.1051/0004-6361/202451300

@article{9ec4591bf72a4aa1b246351fb889ab80,

title = "Radii, masses, and transit-timing variations of the three-planet system orbiting the naked-eye star TOI-396",

abstract = "Context. TOI-396 is an F6 V bright naked-eye star (V ≈ 6.4) orbited by three small (Rp ≈ 2 R⊕) transiting planets discovered thanks to space-based photometry from two TESS sectors. The orbital periods of the two innermost planets, namely TOI-396 b and c, are close to the 5:3 commensurability (Pb ~ 3.6 d and Pc ~ 6.0 d), suggesting that the planets might be trapped in a mean motion resonance (MMR).Aims. To measure the masses of the three planets, refine their radii, and investigate whether planets b and c are in MMR, we carried out HARPS radial velocity (RV) observations of TOI-396 and retrieved archival high-precision transit photometry from four TESS sectors.Methods. We extracted the RVs via a skew-normal fit onto the HARPS cross-correlation functions and performed a Markov chain Monte Carlo joint analysis of the Doppler measurements and transit photometry, while employing the breakpoint method to remove stellar activity from the RV time series. We also performed a transit timing variation (TTV) dynamical analysis of the system and simulated the temporal evolution of the TTV amplitudes of the three planets following an N-body numerical integration.Results. Our analysis confirms that the three planets have similar sizes (Rb = 2.004−0.047+0.045 R⊕ ; Rc = 1.979−0.051+0.054 R⊕; Rd = 2.001−0.064+0.063 R⊕) and is thus in agreement with previous findings. However, our measurements are ~ 1.4 times more precise thanks to the use of two additional TESS sectors. For the first time, we have determined the RV masses for TOI-396 b and d, finding them to be Mb = 3.55−0.96+0.94 M⊕ and Md = 7.1 ± 1.6 M⊕, which implies bulk densities of ρb = 2.44−0.68+0.69 g cm−3 and ρd = 4.9−1.1+1.2 g cm−3, respectively. Our results suggest a quite unusual system architecture, with the outermost planet being the densest. Based on a frequency analysis of the HARPS activity indicators and TESS light curves, we find the rotation period of the star to be Prot,⋆ = 6.7 ± 1.3 d, in agreement with the value predicted from log R′HK-based empirical relations. The Doppler reflex motion induced by TOI-396 c remains undetected in our RV time series, likely due to the proximity of the planet{\textquoteright}s orbital period to the star{\textquoteright}s rotation period. We also discovered that TOI-396 b and c display significant TTVs. While the TTV dynamical analysis returns a formally precise mass for TOI-396 c of Mc,dyn = 2.24−0.67+0.13 M⊕, the result might not be accurate, owing to the poor sampling of the TTV phase. We also conclude that TOI-396 b and c are close to but out of the 5:3 MMR.Conclusions. A TTV dynamical analysis of additional transit photometry evenly covering the TTV phase and super-period is likely the most effective approach for precisely and accurately determining the mass of TOI-396 c. Our numerical simulation suggests TTV semi-amplitudes of up to five hours over a temporal baseline of ~ 5.2 years, which should be duly taken into account when scheduling future observations of TOI-396.",

keywords = "Techniques: photometric, Techniques: radial velocities, Planets and satellites: fundamental parameters, Stars: fundamental parameters",

author = "A. Bonfanti and I. Amateis and D. Gandolfi and L. Borsato and Egger, {J. A.} and Cubillos, {P. E.} and D. Armstrong and Le{\~a}o, {I. C.} and M. Fridlund and Martins, {B. L. Canto} and Sousa, {S. G.} and {De Medeiros}, {J. R.} and L. Fossati and V. Adibekyan and Cameron, {A. Collier} and S. Grziwa and Lam, {K. W. F.} and E. Goffo and Nielsen, {L. D.} and F. Rodler and J. Alarcon and J. Lillo-Box and Cochran, {W. D.} and R. Luque and S. Redfield and Santos, {N. C.} and Barros, {S. C. C.} and D. Bayliss and X. Dumusque and Keniger, {M. A. F.} and J. Livingston and F. Murgas and G. Nowak and A. Osborn and Osborn, {H. P.} and E. Pall{\'e} and Persson, {C. M.} and Serrano, {L. M.} and Str{\o}m, {P. A.} and S. Udry and Wheatley, {P. J.}",

note = "Funding: This research was funded in part by the UKRI, (Grants ST/X001121/1, EP/X027562/1). P.J.W. acknowledges support from the UK Science and Technology Facilities Council (STFC) through consolidated grants ST/P000495/1, ST/T000406/1 and ST/X001121/1. N.C.S. is funded by the European Union (ERC, FIERCE, 101052347). X.D. acknowledges the support from the European Research Council (ERC) under the European Union{\textquoteright}s Horizon 2020 research and innovation programme (grant agreement SCORE No 851555).",

year = "2025",

month = jan,

doi = "10.1051/0004-6361/202451300",

language = "English",

volume = "693",

journal = "Astronomy & Astrophysics",

issn = "0004-6361",

publisher = "EDP SCIENCES S A",

}

Bonfanti, A, Amateis, I, Gandolfi, D, Borsato, L, Egger, JA, Cubillos, PE, Armstrong, D, Leão, IC, Fridlund, M, Martins, BLC, Sousa, SG, De Medeiros, JR, Fossati, L, Adibekyan, V, Cameron, AC, Grziwa, S, Lam, KWF, Goffo, E, Nielsen, LD, Rodler, F, Alarcon, J, Lillo-Box, J, Cochran, WD, Luque, R, Redfield, S, Santos, NC, Barros, SCC, Bayliss, D, Dumusque, X, Keniger, MAF, Livingston, J, Murgas, F, Nowak, G, Osborn, A, Osborn, HP, Pallé, E, Persson, CM, Serrano, LM, Strøm, PA, Udry, S & Wheatley, PJ 2025, 'Radii, masses, and transit-timing variations of the three-planet system orbiting the naked-eye star TOI-396', Astronomy & Astrophysics, vol. 693, A90. https://doi.org/10.1051/0004-6361/202451300

TY - JOUR

T1 - Radii, masses, and transit-timing variations of the three-planet system orbiting the naked-eye star TOI-396

AU - Bonfanti, A.

AU - Amateis, I.

AU - Gandolfi, D.

AU - Borsato, L.

AU - Egger, J. A.

AU - Cubillos, P. E.

AU - Armstrong, D.

AU - Leão, I. C.

AU - Fridlund, M.

AU - Martins, B. L. Canto

AU - Sousa, S. G.

AU - De Medeiros, J. R.

AU - Fossati, L.

AU - Adibekyan, V.

AU - Cameron, A. Collier

AU - Grziwa, S.

AU - Lam, K. W. F.

AU - Goffo, E.

AU - Nielsen, L. D.

AU - Rodler, F.

AU - Alarcon, J.

AU - Lillo-Box, J.

AU - Cochran, W. D.

AU - Luque, R.

AU - Redfield, S.

AU - Santos, N. C.

AU - Barros, S. C. C.

AU - Bayliss, D.

AU - Dumusque, X.

AU - Keniger, M. A. F.

AU - Livingston, J.

AU - Murgas, F.

AU - Nowak, G.

AU - Osborn, A.

AU - Osborn, H. P.

AU - Pallé, E.

AU - Persson, C. M.

AU - Serrano, L. M.

AU - Strøm, P. A.

AU - Udry, S.

AU - Wheatley, P. J.

N1 - Funding: This research was funded in part by the UKRI, (Grants ST/X001121/1, EP/X027562/1). P.J.W. acknowledges support from the UK Science and Technology Facilities Council (STFC) through consolidated grants ST/P000495/1, ST/T000406/1 and ST/X001121/1. N.C.S. is funded by the European Union (ERC, FIERCE, 101052347). X.D. acknowledges the support from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement SCORE No 851555).

PY - 2025/1

Y1 - 2025/1

N2 - Context. TOI-396 is an F6 V bright naked-eye star (V ≈ 6.4) orbited by three small (Rp ≈ 2 R⊕) transiting planets discovered thanks to space-based photometry from two TESS sectors. The orbital periods of the two innermost planets, namely TOI-396 b and c, are close to the 5:3 commensurability (Pb ~ 3.6 d and Pc ~ 6.0 d), suggesting that the planets might be trapped in a mean motion resonance (MMR).Aims. To measure the masses of the three planets, refine their radii, and investigate whether planets b and c are in MMR, we carried out HARPS radial velocity (RV) observations of TOI-396 and retrieved archival high-precision transit photometry from four TESS sectors.Methods. We extracted the RVs via a skew-normal fit onto the HARPS cross-correlation functions and performed a Markov chain Monte Carlo joint analysis of the Doppler measurements and transit photometry, while employing the breakpoint method to remove stellar activity from the RV time series. We also performed a transit timing variation (TTV) dynamical analysis of the system and simulated the temporal evolution of the TTV amplitudes of the three planets following an N-body numerical integration.Results. Our analysis confirms that the three planets have similar sizes (Rb = 2.004−0.047+0.045 R⊕ ; Rc = 1.979−0.051+0.054 R⊕; Rd = 2.001−0.064+0.063 R⊕) and is thus in agreement with previous findings. However, our measurements are ~ 1.4 times more precise thanks to the use of two additional TESS sectors. For the first time, we have determined the RV masses for TOI-396 b and d, finding them to be Mb = 3.55−0.96+0.94 M⊕ and Md = 7.1 ± 1.6 M⊕, which implies bulk densities of ρb = 2.44−0.68+0.69 g cm−3 and ρd = 4.9−1.1+1.2 g cm−3, respectively. Our results suggest a quite unusual system architecture, with the outermost planet being the densest. Based on a frequency analysis of the HARPS activity indicators and TESS light curves, we find the rotation period of the star to be Prot,⋆ = 6.7 ± 1.3 d, in agreement with the value predicted from log R′HK-based empirical relations. The Doppler reflex motion induced by TOI-396 c remains undetected in our RV time series, likely due to the proximity of the planet’s orbital period to the star’s rotation period. We also discovered that TOI-396 b and c display significant TTVs. While the TTV dynamical analysis returns a formally precise mass for TOI-396 c of Mc,dyn = 2.24−0.67+0.13 M⊕, the result might not be accurate, owing to the poor sampling of the TTV phase. We also conclude that TOI-396 b and c are close to but out of the 5:3 MMR.Conclusions. A TTV dynamical analysis of additional transit photometry evenly covering the TTV phase and super-period is likely the most effective approach for precisely and accurately determining the mass of TOI-396 c. Our numerical simulation suggests TTV semi-amplitudes of up to five hours over a temporal baseline of ~ 5.2 years, which should be duly taken into account when scheduling future observations of TOI-396.

AB - Context. TOI-396 is an F6 V bright naked-eye star (V ≈ 6.4) orbited by three small (Rp ≈ 2 R⊕) transiting planets discovered thanks to space-based photometry from two TESS sectors. The orbital periods of the two innermost planets, namely TOI-396 b and c, are close to the 5:3 commensurability (Pb ~ 3.6 d and Pc ~ 6.0 d), suggesting that the planets might be trapped in a mean motion resonance (MMR).Aims. To measure the masses of the three planets, refine their radii, and investigate whether planets b and c are in MMR, we carried out HARPS radial velocity (RV) observations of TOI-396 and retrieved archival high-precision transit photometry from four TESS sectors.Methods. We extracted the RVs via a skew-normal fit onto the HARPS cross-correlation functions and performed a Markov chain Monte Carlo joint analysis of the Doppler measurements and transit photometry, while employing the breakpoint method to remove stellar activity from the RV time series. We also performed a transit timing variation (TTV) dynamical analysis of the system and simulated the temporal evolution of the TTV amplitudes of the three planets following an N-body numerical integration.Results. Our analysis confirms that the three planets have similar sizes (Rb = 2.004−0.047+0.045 R⊕ ; Rc = 1.979−0.051+0.054 R⊕; Rd = 2.001−0.064+0.063 R⊕) and is thus in agreement with previous findings. However, our measurements are ~ 1.4 times more precise thanks to the use of two additional TESS sectors. For the first time, we have determined the RV masses for TOI-396 b and d, finding them to be Mb = 3.55−0.96+0.94 M⊕ and Md = 7.1 ± 1.6 M⊕, which implies bulk densities of ρb = 2.44−0.68+0.69 g cm−3 and ρd = 4.9−1.1+1.2 g cm−3, respectively. Our results suggest a quite unusual system architecture, with the outermost planet being the densest. Based on a frequency analysis of the HARPS activity indicators and TESS light curves, we find the rotation period of the star to be Prot,⋆ = 6.7 ± 1.3 d, in agreement with the value predicted from log R′HK-based empirical relations. The Doppler reflex motion induced by TOI-396 c remains undetected in our RV time series, likely due to the proximity of the planet’s orbital period to the star’s rotation period. We also discovered that TOI-396 b and c display significant TTVs. While the TTV dynamical analysis returns a formally precise mass for TOI-396 c of Mc,dyn = 2.24−0.67+0.13 M⊕, the result might not be accurate, owing to the poor sampling of the TTV phase. We also conclude that TOI-396 b and c are close to but out of the 5:3 MMR.Conclusions. A TTV dynamical analysis of additional transit photometry evenly covering the TTV phase and super-period is likely the most effective approach for precisely and accurately determining the mass of TOI-396 c. Our numerical simulation suggests TTV semi-amplitudes of up to five hours over a temporal baseline of ~ 5.2 years, which should be duly taken into account when scheduling future observations of TOI-396.

KW - Techniques: photometric

KW - Techniques: radial velocities

KW - Planets and satellites: fundamental parameters

KW - Stars: fundamental parameters

U2 - 10.1051/0004-6361/202451300

DO - 10.1051/0004-6361/202451300

M3 - Article

SN - 0004-6361

VL - 693

JO - Astronomy & Astrophysics

JF - Astronomy & Astrophysics

M1 - A90

ER -

Radii, masses, and transit-timing variations of the three-planet system orbiting the naked-eye star TOI-396

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Radii, masses, and transit-timing variations of the three-planet system orbiting the naked-eye star TOI-396 (dataset)

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