A Four-planet System Orbiting The Old Thick Disk Star TOI-1203

TOI-1203 is a bright (V=8.6) G3 V star known to host a transiting warm sub-Neptune on a 25.5 d orbit.

Here we report on an intensive high-precision radial velocity and photometric follow-up campaign carried out with the HARPS spectrograph and the CHEOPS space telescope. We found that TOI-1203 has an enhancement of α elements relative to iron of [α/Fe]=0.21±0.04. With an age of ∼12.5 Gyr, TOI-1203 belongs to the old, α-element enhanced stellar population of the galactic thick disk.

We spectroscopically confirmed the planetary nature of the 25.5 d sub-Neptune TOI-1203 d, measured its mass (Md=7.39±0.62 M_⊕) and refined its radius (R_d=2.918+0.046−0.045 R_⊕). We discovered the presence of an additional transiting super-Earth on a 4.2 d orbit (TOI-1203 b) with a mass of M_b=3.51+0.33−0.32 M_⊕ and a radius of R_b=1.520+0.045−0.046 R_⊕. We also revealed the presence of two additional low-mass planets at 13.1 d and 204.6 d (TOI-1203 c and e), with minimum masses of 5.46+0.51−0.50 M_⊕ and 42.10+1.83−1.78 M_⊕.

We found that the outer planet TOI-1203 e lies on an eccentric orbit with ee=0.152±0.029. We performed a stability analysis of the system confirming that there are configurations consistent with the observed parameters that are dynamically stable over billion-year timescales. While analyzing the HARPS time series, we discovered that the FWHM of the HARPS cross-correlation function shows a significant long-period signal (∼615 d) that has no counterpart in the radial velocity data or in the remaining HARPS ancillary time series.

We significantly detected the same signal in the FWHM of the Th-Ar calibration lines used to compute the nightly wavelength solution, and attributed this systematic effect to a long-term variation of the HARPS instrumental profile.

D. Gandolfi, A. Alnajjarine, L. M. Serrano, J. A. Egger, K. W. F. Lam, J. Cabrera, A. P. Hatzes, M. Fridlund, M. Garbaccio Gili, T. G. Wilson, W. D. Cochran, A. Brandeker, E. Goffo, S. G. Sousa, G. Nowak, A. Heitzmann, C. Hellier, J. Venturini, J. Livingston, A. Bonfanti, O. Barragán, V. Adibekyan, E. Knudstrup, Y. Alibert, S. Grziwa, C. Ziegler, L. Fossati, F. Murgas, A. C. M. Correia, S. H. Albrecht, J. Laskar, E. W. Guenther, S. J.A.J. Salmon, S. Redfield, N. Billot, H. J. Deeg, L. Delrez, L. Palethorpe, V. Van Eylen, F. Rodler, J. Alarcon, J. M. Jenkins, J. D. Twicken, A. W. Mann, R. Alonso, J. Asquier, T. Bárczy, D. Barrado, S. C. C. Barros, W. Baumjohann, W. Benz, L. Borsato, C. Broeg, M. Buder, P. Chaturvedi, A. Collier Cameron, Sz. Csizmadia, P. E. Cubillos, M. B. Davies, M. Deleuil, A. Deline, O. D. S. Demangeon, B.-O. Demory, A. Derekas, B. Edwards, D. Ehrenreich, A. Erikson, A. Fortier, K. Gazeas, M. Gillon, M. Güdel, M. N. Günther, Ch. Helling, K. G. Isaak, L. L. Kiss, J. Korth, N. Law, A. Lecavelier des Etangs, A. Leleu, M. Lendl, P. Leonardi, D. Magrin, G. Mantovan, L. Marafatto, P. F. L. Maxted, M. Mecina, B. Merín, C. Mordasini, V. Nascimbeni, A. Nigioni, G. Olofsson, H. P. Osborn, R. Ottensamer, I. Pagano, E. Pallé, C. M. Persson, G. Peter, D. Piazza, G. Piotto, D. Pollacco et al. (23 additional authors not shown)

Comments: Accepted for publication in A&A. Abstract abridged as per arXiv directive
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:2509.10136 [astro-ph.EP] (or arXiv:2509.10136v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2509.10136
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From: Davide Gandolfi
[v1] Fri, 12 Sep 2025 11:03:01 UTC (25,394 KB)
https://arxiv.org/abs/2509.10136
Astrobiology,