TOI-178 six transiting planets

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^+0.28^_-0.23_ to 0.177^+0.055^_-0.061_ times the Earth's density between planets c and d.