Rocky Outer Planet Defies Formation Models * Science and Technology Times

Astronomers have identified a distant planetary system that challenges long‑standing assumptions about how planets form.
A rocky world has been found in the outer orbit of the red dwarf LHS 1903, contradicting the typical pattern of rocky inner planets and gaseous outer giants.
The discovery suggests that planet formation may be more varied and sequential than current models predict.

A System That Breaks the Usual Pattern

Astronomers have discovered a planetary system around the red dwarf LHS 1903 that contradicts one of the most widely accepted rules of planet formation. Most systems observed across the Milky Way follow a familiar structure, with rocky planets orbiting close to their star and gas giants forming farther out. Our own Solar System fits this arrangement, with Mercury, Venus, Earth and Mars near the Sun and the gas giants positioned at greater distances. This pattern is typically explained by early stellar radiation stripping gases from inner planets while cooler outer regions allow atmospheres to accumulate.

LHS 1903 is a small, faint M‑dwarf star that emits less heat and light than the Sun. Researchers used both ground‑based and space‑based telescopes to identify four planets orbiting it. Initial observations revealed a familiar configuration: a rocky inner planet followed by two gas‑rich worlds resembling smaller versions of Neptune. These findings aligned with expectations based on hundreds of previously studied systems.

Years of additional monitoring, however, revealed an unexpected fourth planet. New measurements from the European Space Agency’s CHEOPS satellite showed that the outermost planet, LHS 1903 e, appears to be rocky rather than gaseous. This result contradicts the standard model of planetary formation. The discovery prompted researchers to re‑evaluate how and when rocky planets can emerge.

Prof. Ryan Cloutier of McMaster University noted that the finding challenges assumptions built into current theories. He emphasized that rocky planets have consistently been found closer to their stars, making this system a rare exception. The discovery raises questions about whether LHS 1903 is unique or part of a broader, previously overlooked pattern. Improved detection methods may reveal more such systems in the future.

Testing and Eliminating Alternative Explanations

The research team examined several possible explanations for the unusual composition of LHS 1903 e. One hypothesis considered whether a massive collision might have stripped away a thick atmosphere, leaving behind a rocky core. Another explored whether the planets could have migrated to new positions over time. Detailed simulations and orbital analyses ruled out both scenarios.

Instead, the evidence points toward a different formation timeline. The planets may not have formed simultaneously, as standard models suggest. Rather, they could have developed sequentially as conditions in the protoplanetary disc evolved. This interpretation aligns with a model known as inside‑out planet formation.

In this scenario, planets form one after another in changing environments. The amount of gas available at the time each planet assembles determines whether it becomes rocky or gas‑rich. By the time LHS 1903 e began forming, much of the disc’s gas may have already dissipated. This would have prevented the planet from developing a thick atmosphere.

Cloutier described the finding as remarkable because it shows a rocky world forming in conditions that should favor gas giants. He noted that the discovery challenges long‑held assumptions about the timing of planet formation. The system may represent an early example of a pattern that becomes clearer as detection techniques improve. Each new discovery adds to a growing picture of planetary diversity.

A New Framework for Understanding Planet Formation

Standard models propose that planets arise within a protoplanetary disc where multiple planetary embryos form at roughly the same time. These bodies grow over millions of years into fully developed planets with varying compositions. The structure of the LHS 1903 system suggests that this simultaneous formation model may not apply universally. Instead, sequential formation may play a larger role than previously recognized.

Inside‑out formation provides a compelling explanation for the rocky nature of LHS 1903 e. The model suggests that planets forming later in the disc’s evolution encounter different material conditions. Reduced gas levels would naturally lead to rocky planets even at greater distances from the star. This mechanism could account for the unexpected composition of the outermost world.

The discovery highlights the importance of studying systems that differ from our own. As telescopes and detection methods become more precise, astronomers are increasingly able to identify planets that do not fit established patterns. These findings help refine theories about how planetary systems evolve. They also broaden our understanding of the diversity of worlds across the galaxy.

Cloutier emphasized that each new system adds valuable data to the field. He noted that unusual configurations like LHS 1903 encourage scientists to revisit long‑standing assumptions. The discovery may prompt further investigation into how environmental changes influence planet formation. Future observations will help determine whether this system is an outlier or part of a larger trend.