Astronomers have discovered a planetary system around the red dwarf LHS 1903 with an unusual “inside-out” architecture, challenging conventional planet formation models.

The system has four planets: three innermost planets follow the expected pattern of rocky then gas giants, but the outermost planet, far from the star, is small and rocky—similar to Venus.

Researchers ruled out planetary migration or collisions and propose the planets formed sequentially, with the outer planet emerging in a gas-depleted environment, preventing it from becoming a gas giant.

This arrangement—rocky–gaseous–gaseous–rocky—is extremely rare and suggests planetary systems can evolve in more diverse ways than previously thought, Space.com has reported.

The study, led by Thomas Wilson of the University of Warwick, was published in Science.