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The multi-planet system TOI-5624: four transiting sub-Neptunes with an outer companion revealed by transit-timing variations
Authors: Bonfanti, A.; Gandolfi, D.; Leonardi, P.; ...; Correia, A.C.M.;
Ref.: Astron. Astrophys. 709, A265 (2026)
Abstract: Context. Following the 2022 alert of a TESS object of interest transiting TOI-5624 (a G7 V star similar to 100 pc away), a CHEOPS campaign in 2023 detected four planetary signals at P-b approximate to 3.4, P-c approximate to 7.9, P-d approximate to 13.7, and P-e approximate to 21.5 days. These signals were later confirmed by additional TESS and CHEOPS photometry in 2024-2025. Aims. By using TESS and CHEOPS photometry, along with HARPS-N and SOPHIE high-resolution spectra, we determined the planet properties and performed a dynamical analysis of the system. Methods. After analysing the photometric data, we extracted and modelled the radial velocity (RV) time series using two independent methodologies, both within a Markov chain Monte Carlo 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. Results. We present the discovery of four transiting sub-Neptunes with radii of R-b = 2.314 +/- 0.035 R-circle plus, R-c = 2.474 +/- 0.042 R-circle plus, R-d = 3.584(-0.050)(+0.051)R(circle plus), and R-e = 3.247(-0.043)(+0.042)R(circle plus), along with masses of M-b = 9.4 +/- 1.4M(circle plus), M-c = 4.8 +/- 1.9M(circle plus), Md = 4.9 +/- 2.2M(circle plus), and M-e = 8.9(-3.0)(+2.9)M(circle plus). Our photometric analysis revealed that the outermost transiting planet TOI-5624e shows significant transit-timing variations (TTVs). Indeed, we found 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 labelled this non-transiting planet as TOI-5624 f and determined its minimum mass to be M-f sin i(f) = 13.0 +/- 3.7 M-circle plus. Conclusions. 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 sigma) of the mass for at least three planets has previously been 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.
