ESA/CHEOPS discovers unusual four-planet system around LHS 1903 with rocky outer planet challenging formation models

Scientists analyzing observations from the European Space Agency's Cheops satellite have discovered a fourth planet orbiting the star LHS 1903. The planet is rocky and lies farthest from the star, creating an unusual order in the system. The star is a small red M-dwarf. Three previously identified planets consist of a rocky inner world followed by two gaseous planets. The newly detected outer planet is also rocky, forming a sequence of rocky, gaseous, gaseous and then rocky planets again. Thomas Wilson from the University of Warwick led an international team that combined Cheops data with other telescope observations. The team concluded that the outer planet formed later than the others in a gas depleted environment after much of the system's gas had dissipated. Standard formation models predict rocky planets near the star and gaseous planets farther out because of radiation and temperature conditions. The new findings suggest sequential formation of planets rather than simultaneous creation from a protoplanetary disc. Simulations ruled out explanations involving orbital exchanges or giant impacts. The discovery provides evidence supporting the theory of planet formation proceeding from the inside outward. It indicates that rocky planets can form far from their star under certain conditions.