paper

The multi-planet system TOI-5624: Four transiting sub-Neptunes with an outer companion revealed by transit-timing variations

arXiv:2604.15035 · doi:10.1051/0004-6361/202558740

Abstract

Following the 2022 alert of a TESS object of interest transiting TOI-5624 (a G7 V star $\sim$ 100 pc away), a CHEOPS campaign in 2023 detected four planetary signals at $P_{b} \approx 3.4$ , $P_{c} \approx 7.9$ , $P_{d} \approx 13.7$ , and $P_{e} \approx 21.5$ days, later confirmed by additional TESS and CHEOPS photometry in 2024-2025. After analysing the TESS & CHEOPS photometric data, we extracted and modelled the HARPS-N & SOPHIE RV time series using two independent methodologies both within an MCMC framework. We further integrated the N-body equations of motion, while simultaneously fitting the transit times and the detrended RVs, to dynamically characterise the system. We present the discovery of four transiting sub-Neptunes with radii of $R_{b} = 2.314 \pm 0.035 R_{\oplus}$ , $R_{c} = 2.474 \pm 0.042 R_{\oplus}$ , $R_{d} = 3.58 4_{- 0.050}^{+ 0.051} R_{\oplus}$ , and $R_{e} = 3.24 7_{- 0.043}^{+ 0.042} R_{\oplus}$ and masses of $M_{b} = 9.4 \pm 1.4 M_{\oplus}$ , $M_{c} = 4.8 \pm 1.9 M_{\oplus}$ , $M_{d} = 4.9 \pm 2.2 M_{\oplus}$ , and $M_{e} = 8. 9_{- 3.0}^{+ 2.9} M_{\oplus}$ . Our photometric analysis reveals that the outermost transiting planet TOI-5624 e shows significant TTVs. We find a robust Keplerian signal in the RV time series close to the 2:1 period commensurability with TOI-5624 e, which explains the TTV pattern exhibited by TOI-5624 e according to our dynamical analysis. We label this non-transiting planet as TOI-5624 f and find its minimum mass to be $M_{f} sin i_{f} = 13.0 \pm 3.7 M_{\oplus}$ . Among the known systems hosting more than four planets, the remarkable precision with which the radii have been measured (<1.7%) and the firm assessment (>3 $I ¨ $ ) of the mass for at least three planets has been previously reached only for TRAPPIST-1. Additional photometric observations will enable a better sample of the TTV modulation and a more robust dynamical determination of the masses.

Accepted for publication in A&A. 22 pages (13 in the main text), 4 figures (+9 in the Appendix), 2 tables (+7 in the Appendix). Abstract abridged to satisfy the arXiv submission requirements