A ‘Cosmic Sandwich’: LHS 1903 System Challenges Planet Formation Theories

A team of astronomers has discovered a unique planetary system around the red dwarf star LHS 1903 that defies conventional models of planet formation. Observations led by the European Space Agency’s (ESA) Cheops satellite have revealed a structure that contradicts the established theory, which, based on our Solar System, places rocky planets close to their star and gas giants on the periphery. In the LHS 1903 system, located about 116 light-years away, this expected order is upended, presenting a rare “inside-out” configuration.

An Unprecedented Planetary Arrangement

Initially, scientists identified three planets orbiting the cool, faint M-dwarf star: a rocky inner planet followed by two gas giants, a pattern that aligned perfectly with standard formation theories. This model suggests that intense stellar radiation near a star strips away light gases from nearby planets, leaving behind dense, rocky cores, while gas giants form in cooler, more distant regions. The groundbreaking discovery came when the Cheops satellite detected a fourth, outermost planet. To the researchers’ surprise, this distant world was also rocky, creating a “rocky-gaseous-gaseous-rocky” architecture never before observed.

Rewriting the Rules of Cosmic Birth

This unusual “sandwich” structure has prompted researchers to propose a new model for planetary development, published in the journal Science. The team, co-led by Thomas Wilson of the University of Warwick, suggests that the planets around LHS 1903 did not form simultaneously from a single protoplanetary disk. Instead, they likely formed sequentially. The fourth rocky world is theorized to be a “late bloomer,” having formed much later than the inner planets. By the time it began to coalesce, the system’s gas-rich disk had largely dissipated, leaving insufficient material for it to develop a thick gaseous envelope despite its distance from the star. This provides the first strong evidence that planets can form in a gas-depleted environment.

A Universe of Diverse Possibilities

The discovery of the LHS 1903 system is a significant reminder that our Solar System is not a universal template. The diversity of planetary formation scenarios is far broader than previously understood, challenging astronomers to refine their models. As Isabel Rebollido, a research fellow at ESA, noted, theories have historically been based on what is known about our own solar system, but observing more diverse exoplanet systems forces a re-evaluation.

Looking Ahead

Further investigation of LHS 1903 will be crucial for understanding how frequently such “anomalous” systems occur and what other factors influence the final architecture of planetary systems. This finding opens up new avenues for research, suggesting that the conditions for forming rocky planets might exist in a wider range of environments and at different times in a system’s life than previously thought. The work of Cheops and other advanced telescopes will continue to uncover the vast complexity of how worlds are born across the galaxy.