Six transiting planets and a chain of Laplace resonances in TOI-178

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.

A Tidal Origin for a Three-body Resonance in Kepler-221

“…The values of |B|/〈n〉 closest to zero are shown in Table 3 along with basic information about the system and whether the planet triplet is adjacent. TRAPPIST-1, TOI-178, Kepler-80, Kepler-60, K2-138, K2-72, and Kepler-223 are known resonant chain systems (Migaszewski et al 2012;Goździewski et al 2016;MacDonald et al 2016;Mills et al 2016;Gillon et al 2017;Leleu et al 2021;Christiansen et al 2018). The fact that |B|/〈n〉 for Kepler-221 is characteristic of resonant chain values is indicative that its closeness to zero is not coincidental.…”

An integrable model for first-order three-planet mean motion resonances

“…On the other hand, the study of three-body resonances has long been restricted to the asteroids dynamics in the solar system (Nesvorný and Morbidelli 1998;Cachucho et al 2010). The question has gained a renewed interest thanks to the discovery of close-in exoplanet systems trapped into zeroth-order resonances such as Trappist-1 (Gillon et al 2017;Agol et al 2021) or more recently TOI-178 (Leleu et al 2021). Three-planet resonances are also thought to be the main driver of the early instability of tightly packed systems (Quillen 2011;Petit et al 2020).…”

TOI-1749: an M dwarf with a Trio of Planets including a Near-resonant Pair

“…This figure shows that additional planets to planetary pairs near 2:1 commensurability around M (FGK) dwarfs tend to have an inside (outside) orbit. Note that the TOI-178 system shows an opposite property; the host star however has an effective temperature that is close to the boundary (T eff = 4316 K, Leleu et al 2021).…”

“…However, near 2:1 period-ratio planetary pairs in FGK systems tend to have an additional planet at an outer orbit rather than inner one, as shown in the bottom panels of Figure 19. Note that the TOI-178 system is an exception to this trend; i.e., the near 2:1 period ratio pair of TOI-178 c and d has an additional inner planet (TOI-178b) despite the fact that the host star is a K dwarf; however, the host star has an effective temperature of T eff = 4316 K (Leleu et al 2021), which is close to the temperature range of M dwarfs, <4000 K.…”