Rocky Planet Discovery Challenges Planet Formation Theories | LHS 1903 System

Astronomers have uncovered a planetary system that challenges conventional understanding of how planets form. The system, orbiting the red dwarf star LHS 1903, features a rocky planet in an outer orbit – a configuration that contradicts established theories predicting rocky planets close to their star and gas giants further out.

For years, the prevailing model of planet formation has posited a clear pattern. Young stars emit intense radiation, stripping away gases from planets forming nearby, resulting in solid, rocky worlds. Further from the star, cooler temperatures allow planets to retain thick atmospheres, leading to the development of gas giants. Our own solar system – with its inner rocky planets (Mercury, Venus, Earth, and Mars) and outer gas giants (Jupiter, Saturn, Uranus, and Neptune) – has long served as the prime example of this arrangement.

The LHS 1903 system, located roughly 116 light-years away, initially appeared to conform to this pattern. Researchers, led by Professor Ryan Cloutier of McMaster University and Professor Thomas Wilson of the University of Warwick, first identified three planets. The innermost planet is rocky, followed by two gas-rich planets comparable in size to Neptune. However, subsequent observations from the European Space Agency’s CHEOPS satellite revealed a fourth planet, LHS 1903 e, orbiting farthest from the star. This outermost planet, surprisingly, also appears to be rocky.

“We’ve seen this pattern: rocky inside, gaseous outside, across hundreds of planetary systems,” explains Cloutier, an assistant professor in the Department of Physics and Astronomy. “But now, the discovery of a rocky planet in the outer part of a system forces us to rethink the timing and conditions under which rocky planets can form.”

The team investigated several potential explanations for this anomaly. They considered whether a significant impact could have stripped away the atmosphere of the outer planet, or if gravitational interactions might have caused the planets to migrate from their original positions. Detailed computer simulations and analysis of the planets’ orbits ruled out both of these scenarios.

Instead, the evidence suggests a different process: inside-out planet formation. This model proposes that planets don’t necessarily form simultaneously within a protoplanetary disc – the swirling cloud of gas and dust surrounding a young star. Instead, they develop sequentially, as conditions around the star evolve. The characteristics of each planet, whether rocky or gas-rich, are determined by the local environment at the time it finishes forming.

In the case of LHS 1903 e, the researchers hypothesize that by the time it began to coalesce, much of the gas in the surrounding disc had already dissipated. This left insufficient material to build a substantial atmosphere, resulting in a rocky planet forming in a region where a gas giant would typically be expected.

“It’s remarkable to see a rocky world forming in an environment that shouldn’t favour that outcome,” says Cloutier. “It challenges the assumptions built into our current models.” He adds that the discovery raises the question of whether LHS 1903 is an unusual case or an early indication of a pattern scientists have yet to fully recognize.

The discovery highlights the increasing sophistication of astronomical observation and the growing realization that planetary systems can exhibit a far greater diversity than previously imagined. As telescopes and detection methods become more precise, astronomers are uncovering systems that deviate from the established norms, forcing a reevaluation of long-held theories.

“As telescopes and detection methods become more precise, we are strengthening our ability to find planetary systems that don’t resemble our own and that don’t conform to longstanding theories,” Cloutier notes. “Each new system adds another data point to a growing picture of planetary diversity — one that forces scientists to rethink the processes that shape worlds across the galaxy.”

The findings, reported in the journal February 12, 2026 in Science, underscore the dynamic and complex nature of planet formation and the need for continued research to unravel the mysteries of exoplanetary systems.