Rocky planet discovered in outer orbit challenges planet formation theory

Astronomers have identified a planetary system around the red dwarf star LHS 1903 that includes a rocky planet in the outermost orbit. The discovery was reported in the journal Science. Researchers led by Prof. Ryan Cloutier of McMaster University and Prof. Thomas Wilson of the University of Warwick combined observations from ground-based telescopes and the European Space Agency's CHEOPS satellite. The system contains an inner rocky planet and two gas-rich planets similar to smaller versions of Neptune. A fourth planet, LHS 1903 e, was later confirmed in the farthest orbit and appears to be rocky. This arrangement challenges the standard model of planet formation. That model holds that intense radiation from young stars strips gases from nearby planets, leaving rocky worlds close in, while cooler outer regions allow gas giants to form. The team used computer simulations to rule out explanations based on collisions or later orbital shifts. The findings instead suggest the planets formed sequentially as conditions in the surrounding disk changed over time.