Six transiting planets and a chain of Laplace resonances in TOI-178 (2101.09260v1)

Abstract: Determining the architecture of multi-planetary systems is one of the cornerstones of understanding planet formation and evolution. Resonant systems are especially important as the fragility of their orbital configuration ensures that no significant scattering or collisional event has taken place since the earliest formation phase when the parent protoplanetary disc was still present. In this context, TOI-178 has been the subject of particular attention since the first TESS observations hinted at a 2:3:3 resonant chain. Here we report the results of observations from CHEOPS, ESPRESSO, NGTS, and SPECULOOS with the aim of deciphering the peculiar orbital architecture of the system. We show that TOI-178 harbours at least six planets in the super-Earth to mini-Neptune regimes, with radii ranging from 1.152(-0.070/+0.073) to 2.87(-0.13/+0.14) Earth radii and periods of 1.91, 3.24, 6.56, 9.96, 15.23, and 20.71 days. All planets but the innermost one form a 2:4:6:9:12 chain of Laplace resonances, and the planetary densities show important variations from planet to planet, jumping from 1.02(+0.28/-0.23) to 0.177(+0.055/-0.061) times the Earth's density between planets c and d. Using Bayesian interior structure retrieval models, we show that the amount of gas in the planets does not vary in a monotonous way, contrary to what one would expect from simple formation and evolution models and unlike other known systems in a chain of Laplace resonances. The brightness of TOI-178 allows for a precise characterisation of its orbital architecture as well as of the physical nature of the six presently known transiting planets it harbours. The peculiar orbital configuration and the diversity in average density among the planets in the system will enable the study of interior planetary structures and atmospheric evolution, providing important clues on the formation of super-Earths and mini-Neptunes.

• The paper reveals six exoplanets in TOI-178 with a unique 2:4:6:9:12 Laplace resonance chain.
• It combines data from CHEOPS, ESPRESSO, NGTS, SPECULOOS, and TESS to accurately measure planetary radii, densities, and compositions.
• The findings support resonance-driven migration models and encourage follow-up studies with advanced telescopes like JWST.

Analysis of the TOI-178 Planetary System: Orbital Architecture and Resonance Dynamics

This paper presents a comprehensive paper centered on the TOI-178 system, revealing its complex orbital arrangement and resonant interactions. The investigation draws upon observational data from multiple advanced instruments: CHEOPS, ESPRESSO, NGTS, and SPECULOOS, as well as pre-existing observations from TESS. This collaborative and multi-faceted approach has proved crucial in decrypting the perplexing architecture of TOI-178, culminating in the identification of six distinct exoplanets.

Planetary Configuration and Discovery

The planets within the TOI-178 system reside in a compact zone, with orbits ranging from 1.91 to 20.71 days. Despite previous uncertainties and mis-assignations in early TESS data, the subsequent detailed analyses have successfully corroborated the presence of all six planets. Among these, planets exhibit a range from super-Earth to mini-Neptune class bodies, characterized by radii spanning 1.152 to 2.87 times that of Earth. The detectability and precise characterization of these planets, even when their radial velocity signals overlap with stellar activity, underscore the precision achieved in the observations.

Resonant Chains and Orbital Dynamics

A standout feature of the TOI-178 system is the chain of Laplace resonances displayed by five of its planets. These planets are locked into a 2:4:6:9:12 resonant chain, each pair displaying close-to-integral period ratios. Such elaborate configurations are extremely fragile, indicative of a quiescent dynamical history dominated by smooth, resonance-driven migration within the proto-planetary disk. This chain presents an unusual stability, with Laplace angles indicating resonant equilibrium configurations, reminiscent of the Galilean moons around Jupiter.

Mass, Density, and Composition

Mass determination through radial velocity data complements the transit-based radii measurements, providing critical insights into the density and internal structure of these planets. The planets show a diversity in density and composition, largely at odds with standard models of planet formation and evolution, revealing unexpectedly high densities among some planets compared to their neighbors. Such anomalies challenge existing theories, suggesting that additional mechanisms may be at play, such as varying capture into resonance or differential atmospheric escape dynamics. The presence of substantial gas envelopes in all but the inner planets further enriches their characterization, contrasting sharply with more uniform systems like Kepler-80.

Implications and Future Directions

The findings presented underline TOI-178 as a pivotal system for understanding resonance dynamics and planet formation theories. The near-coplanarity of the architecture, coupled with its diversity in internal structure, presents a unique laboratory for testing models of planetary assembly and migration. The robust prospects for follow-up investigations, especially using the upcoming James Webb Space Telescope and the E-ELT, could unveil atmospheric compositions and further refine planetary masses and densities.

The heterogeneity in densities and atmospheric retentions challenges models that predict smooth transitions in planetary characteristics—a divergence likely rooted in complex interplays during the system's formation. Further observations are suggested to pursue possible additional planets continuing the resonance chain, potentially extending into the habitable zone.

In summary, the TOI-178 system offers an unprecedented opportunity to investigate the intricacies of resonant planetary systems, encouraging revisions to current paradigms of orbital dynamics and evolution within multi-planetary systems. This paper sets a significant precedence for the synergistic use of varied observational platforms in exoplanetary science.