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 the possible presence of a near 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 1.02−0.23+0.28 to 0.177−0.061+0.055 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 (H = 8.76 mag, J = 9.37 mag, V = 11.95 mag) 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.
Highlights
Since the discovery of the first exoplanet orbiting a Sun-like star by Mayor & Queloz (1995), the diversity of observed planetary systems has continued to challenge our understanding of their formation and evolution
Our current understanding of planetary system formation theory implies that such configurations are a common outcome of protoplanetary discs: Slow convergent migration of a pair of planets in quasi-circular orbits leads to a high probability of capture in first-order mean-motion resonances (MMRs) – the period ratio of the two planets is equal to (k + 1)/k, with k an integer (Lee & Peale 2002; Correia et al 2018)
For all of these potential solutions, we modelled the transits of the five candidates – 1.91 d, 3.23 d, 6.55 d, and ci, j – using the batman package (Kreidberg 2015) and ran an Markov chain Monte Carlo (MCMC) on the pre-detrended light curve to estimate the relative likelihood of the different ci, j
Summary
Since the discovery of the first exoplanet orbiting a Sun-like star by Mayor & Queloz (1995), the diversity of observed planetary systems has continued to challenge our understanding of their formation and evolution. TOI-178 was identified as a potential co-orbital system (Leleu et al 2019) with two planets oscillating around the same period This prompted ESPRESSO RV measurements and two sequences of simultaneous ground-based photometric observations with NGTS and SPECULOOS. In order to identify new possible solutions for the available data (TESS Sector 2, NGTS visits in September and October 2019, and CHEOPS visits 1 and 2), we individually predetrended each light curve and subtracted the signal of the 6.55 d, 3.23 d, and 1.91 d candidates. We explored a large range of models for the CHEOPS light curves, consisting of first- to fourth-order polynomials in the recorded external parameters (most importantly: time, background level, position of the point-spread-function (PSF) centroid, spacecraft roll angle, and contamination), as well as GPs (celerite ForemanMackey et al 2017 and george Ambikasaran et al 2015) against. The uncertainty on the inclination of the inner planets allows for the near-coplanarity of the entire system, a feature that was found in the TRAPPIST-1 system (Agol et al 2021)
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