HARPS-N and the two Mini-Neptunes in resonant orbit around the star TOI-1803

TOI-1803 is an orange star located 388 light years away from the Sun with a mass of 76% of the mass of the Sun. The star hosts two Neptune-sized planets named TOI-1803 b and TOI-1803 c, which reside in an orbital configuration close to a 2:1 mean motion resonance. In particular TOI-1803 b orbits the star every ~6.3 days, while TOI-1803 c has a period of ~12.9 days. The discovery of such a resonance is of great interest as it provides insights into planetary migration and the dynamical evolution of exoplanetary systems.

Thanks to the high resolution spectrophaph HARPSN installed at the Telescopio Nazionale Galileo is was possible to determine the mass of the two planets: 10.3 ± 2.5 Earth masses and 6.0 ± 3.0 Earth masses respectively for TOI-1803 b and TOI-1803 c. Their respective radii are estimated to be 2.99 ± 0.08 and 4.29 ± 0.08 Earth radii. The remarkably low density of TOI-1803 c suggests the presence of a thick, extended atmosphere, making it a prime target for atmospheric characterization studies with the James Webb Space Telescope (JWST).

Ground-based follow-up observations have also detected significant transit timing variations (TTVs) for TOI-1803 c, while space-based observations allowed precise mid-transit measurement for both planets. Thanks to dynamical simulations (TRADES simulations) it was possible to estimate a TTV amplitude of ~10 minutes for TOI-1803 b and ~40 minutes for TOI-1803 c. Such TTV measurements will enable a more precise determination of the planets’ masses and orbital properties in future studies.

TOI-1803 c’s inflated atmosphere raises questions about its origin and evolution. Did it retain a primordial hydrogen-helium envelope, or has it undergone secondary atmospheric accretion? Theoretical models suggest that its atmosphere may have been influenced by stellar irradiation, leading to significant atmospheric loss over time. Future observations with JWST’s NIRSpec instrument could provide critical data to distinguish between these atmospheric scenarios and refine our understanding of mini-Neptune formation.

Fig 1: O-C diagram for the planets b (left) and c (right) comparing the observations (different marker and color) with TRADES simulations (black circles). The black line is the oversampled best-fit model, and the gray shaded areas are the one, two, and three σ computed from 100 samples drawn from the posterior distribution. More detailsn in the paper.

Fig 2: Top panel: Radial velocity plot. Observations are shown as purple circles, and TRADES simulations are indicated as black circles. The black line is the oversampled best-fit model. Bottom panel: Residuals plot. The black error bars are the observed RV errors with the jitter term added in quadrature. More details in the paper.

More information in the paper:
Astronomy & Astrophysics, Zingales et al. (2025) doi: 10.1051/0004-6361/202451180