The GAPS programme at TNG: LXXVI. TOI-1533: a compact system hosting a super-Neptune-mass pair with disparate radii
G. Mantovan, V. Nascimbeni, S. Desidera, L. Malavolta, J. J. Lissauer, P. Leonardi, T. Azevedo Silva, C. Guerra, D. Polychroni, L. Borsato, M. Baratella, K. Biazzo, D. Nardiello, K. A. Collins, M. Damasso, J. De Leon, M. E. Everett, D. Gandolfi, S. Giacalone, L. Naponiello, G. Piotto, G. Scandariato, K. Stassun, S. W. Yee, L. Affer, F. Amadori, A. Bignamini, S. Colombo, M. D'Arpa, C. D. Dressing, A. Fukui, A. Ghedina, J. Higuera, K. Ikuta, Kawauchi, J. Korth, V. Lorenzi, L. Mancini, M. Mori, F. Murgas, N. Narita, E. Palle, H. Parviainen, A. Ruggieri, A. Savel, R. P. Schwarz, A. Shporer, G. Srdoc, Stockdale, D. Watanabe, R. Zambelli, T. Zingales
astro-ph.EP
Submitted: 2026-06-29
Comments: 19 pages, 22 figures, 5 tables. Accepted for publication in Astronomy & Astrophysics on 29 June 2026, first submission to A&A on 15 May
Code: https://github.com/hpparvi/MuSCAT2_transit_pipelinehttps:
License: http://creativecommons.org/licenses/by/4.0/
The gist: The present-day architecture of planetary systems contains information about their formation and migration histories.
Terminology
Abstract
The present-day architecture of planetary systems contains information about their formation and migration histories. The origin of hot Jupiters (HJs, P 10 d, R p > 8 R) has long been a matter of debate. While most of them are found to be ``lonely'', there is a rare population of HJs hosting small companions on inner orbits (eight known as of May 2026). Their peculiar architecture suggests a gentle disc-migration mechanism. In this study, we present the discovery and characterisation of the multi-planet system TOI-1533, comprising an inner sub-Neptune (TOI-1533 b, P orb = 3.63 d, R p = 3.15 R) and an outer hot giant planet (TOI-1533 c, P orb = 8.06 d, R p > 7.5 R) with substantial H/He by mass (rho p < 0.48 g cm-3), both transiting an active K-dwarf star (T eff about 5130 K; V (mag) about 11). Our joint modelling of stellar activity and planetary signals from radial velocities (HARPS-N) and transits (TESS) allows us to detect their Keplerian signals (approximately 10 sigma) and to isolate the stellar modulation. The inclusion of simultaneous photometry in the multi-dimensional Gaussian processes formalism was a fundamental addition to the spectroscopic activity indicators, enabling the disentanglement of stellar activity from planetary signals. The mass ratio of the two confirmed planets (M b / M c about 0.8), together with the super-Neptune mass of the large outer companion (M c about 40 M), makes this system unusual compared to the other few HJs with low-mass inner companions.
Sources
- The HARPS search for southern extra-solar planets. XXXI. Magnetic activity cycles in solar-type stars: statistics and impact on precise radial velocities
- Transit Timing Variations in TESS: A Catalog from the First Five Years
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