2026/06/29 by G. Mantovan, V. Nascimbeni, S. Desidera +49 · 1 voice
#astro-ph.EP
The present-day architecture of planetary systems contains information about their formation and migration histories. The origin of hot Jupiters (HJs, P \lesssim 10 d, R\rm 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\rm orb = 3.63 d, R\rm p = 3.15 R_⊕) and an outer hot giant planet (TOI-1533 c, P\rm orb = 8.06 d, R\rm p > 7.5 R_⊕) with substantial H/He by mass (ρ\rm p < 0.48 g cm-3), both transiting an active K-dwarf star (T\rm eff ≈ 5130 K; V (mag) ≈ 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~σ) 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\rm b / M\rm c about 0.8), together with the super-Neptune mass of the large outer companion (M\rm c ≈ 40 M_⊕), makes this system unusual compared to the other few HJs with low-mass inner companions.