TOI-178: a window into the formation and evolution of planetary systems

Laplacian resonant chains — where astronomical bodies are in mean motion resonance with two or more other bodies — are rare phenomena observed in systems such as the TRAPPIST-1 exoplanets and the Galilean moons of Jupiter. Laplacian chains are an important tool to study the history of planetary systems occupying this configuration, as the fragility of the chain significantly constrains the possible pathways through which the planets can form and evolve. Whilst initial TESS observations suggested that TOI-178 — a nearby system of exoplanets orbiting a relatively cool K-dwarf — hosted the first known planets occupying a horseshoe-coorbital configuration, follow-up observations by CHEOPS, NGTS and SPECULOOS revealed a compact system of six transiting exoplanets all smaller than Neptune: five of which form a chain of Laplacian resonance. Precise measurements of the host's radial velocity using the ESPRESSO spectrograph revealed uncommon planet-to-planet density variations: a stark departure from the monotonic decrease in density with orbital separation common to most systems. JWST time awarded in Cycle 1 to acquire transmission spectroscopy of planets b, d and g promises to make the evolution of the TOI-178 planets amongst the best-understood of any planetary system for the foreseeable future.