Astronomers have discovered a distant planetary system that contradicts established theories of planet formation. The findings, reported on February 14, 2026, focus on the star LHS 1903, where a surprising configuration of planets has emerged. Researchers from McMaster University and the University of Warwick were initially led to believe they would find a familiar arrangement of rocky planets close to the star and gas giants further out. Instead, they have identified a rocky planet orbiting on the outer edge of the system.
In a typical planetary system, like our own Solar System, small, rocky planets are found nearer to their star, while larger gas giants are located farther away. This pattern has been observed in many systems throughout the Milky Way, where younger stars emit radiation that strips gas from nearby developing planets. As a result, rocky worlds form closer to the star, while gas giants develop in cooler, outer regions where they can retain thick atmospheres.
A Surprising Discovery Around LHS 1903
The new research centers on the star LHS 1903, a small red dwarf that is cooler and less massive than the Sun. The team, led by Professor Ryan Cloutier of McMaster University and Professor Thomas Wilson from the University of Warwick, utilized data from both ground-based and space telescopes to investigate the system. Initially, they identified three planets: a rocky inner planet and two gas-rich planets resembling smaller versions of Neptune.
However, subsequent observations from the European Space Agency’s CHEOPS satellite revealed an additional planet, designated LHS 1903 e, which orbits the star at a greater distance. This planet, unexpectedly, appears to be rocky, challenging the long-standing model of planetary formation.
“We’ve seen this pattern of rocky planets inside and gaseous planets outside across hundreds of planetary systems. The discovery of a rocky planet in the outer part of a system compels us to reevaluate the timing and conditions under which rocky planets can form,” stated Cloutier, who is an assistant professor in the Department of Physics and Astronomy.
Rethinking Planetary Formation Models
The research team explored various hypotheses to explain the unusual presence of a rocky planet in the outer orbit. They investigated whether a significant impact could have stripped away its atmosphere and examined the possibility of planets shifting positions over time. However, detailed computer simulations and orbital studies ruled out these scenarios.
Their findings suggest an alternative theory: the planets in this system may not have formed simultaneously. Instead, they could have developed sequentially as environmental conditions around LHS 1903 evolved. This theory, known as inside-out planet formation, posits that the local conditions when each planet formed dictate whether it becomes gas-rich or remains rocky.
This insight may explain the characteristics of LHS 1903 e. By the time it began to form, much of the gas in the surrounding disc may have dissipated, leaving insufficient material to create a thick atmosphere.
“It’s remarkable to see a rocky world forming in an environment that shouldn’t favour that outcome. It challenges the assumptions built into our current models,” Cloutier added. He emphasized that this discovery raises broader questions about whether LHS 1903 represents an anomaly or an early indication of a new pattern not yet recognized by scientists.
As astronomical instruments improve, researchers are increasingly able to identify planetary systems that diverge from the conventional models. Each new discovery adds to the growing understanding of planetary diversity and compels scientists to reconsider the processes that shape worlds across the galaxy.