A newly studied planetary system located light-years away is prompting astronomers to reassess one of the most widely accepted ideas about how planets form. Scientists have identified an unexpected rocky planet in the outermost orbit of the red dwarf star LHS 1903 — a finding that defies conventional models of planetary architecture.
For decades, astronomers have observed a broadly consistent pattern across planetary systems in the Milky Way. Small, rocky planets typically orbit close to their host star, while larger gas-rich planets are found farther out. Our Solar System exemplifies this structure: Mercury, Venus, Earth and Mars are rocky worlds, whereas Jupiter, Saturn, Uranus and Neptune are dominated by thick gaseous envelopes.
This arrangement is supported by the leading theory of planet formation. Young stars emit intense radiation capable of stripping gases from nearby developing planets, leaving behind dense rocky cores. At greater distances, cooler conditions allow planets to retain lighter gases, leading to the emergence of gas giants.
However, new observations of the LHS 1903 system suggest that nature may not always follow this rulebook.
The discovery, published in the journal Science, was led by an international team of astronomers including Professor Ryan Cloutier of McMaster University and Professor Thomas Wilson of the University of Warwick. By combining data from both ground-based observatories and space telescopes, the researchers initially detected three planets orbiting the faint red dwarf star.
The inner configuration appeared textbook. The closest planet was identified as rocky, followed by two gas-rich worlds resembling smaller versions of Neptune — precisely the structure predicted by existing theories.
But extended monitoring over several years revealed a striking anomaly.
Fresh measurements from the European Space Agency’s CHEOPS (Characterising Exoplanet Satellite) mission uncovered a fourth planet, designated LHS 1903 e, orbiting farthest from the star. Contrary to expectations, this outermost planet appears to be rocky rather than gaseous, ScienceDaily reported.
“We’ve seen the pattern of rocky planets inside and gas-rich planets outside across hundreds of systems,” Cloutier explained. “Finding a rocky world in the outer region forces us to rethink the timing and environmental conditions that govern planet formation.”
To understand the unexpected result, the research team tested alternative explanations. They investigated whether a giant collision might have stripped away the planet’s atmosphere or whether orbital migration had rearranged the system over time. Detailed simulations and orbital analyses ruled out both scenarios.
Instead, the scientists propose that the planets around LHS 1903 may have formed sequentially rather than simultaneously.
Traditional models suggest planets originate within a protoplanetary disc — a rotating cloud of gas and dust surrounding a young star. In this framework, multiple planetary embryos form around the same time and gradually evolve into fully developed planets.
The LHS 1903 system, however, may represent a different evolutionary pathway known as “inside-out planet formation.” Under this scenario, planets emerge one after another as conditions within the disc change. The availability of gas at the time of formation determines whether a planet becomes gas-rich or remains rocky.
Researchers believe LHS 1903 may have assembled after much of the surrounding gas had already dissipated, preventing it from developing a thick atmosphere.
“It’s remarkable to observe a rocky planet forming in an environment that shouldn’t favour that outcome,” Cloutier said, noting that the discovery raises broader questions about planetary diversity.
Astronomers now aim to determine whether LHS 1903 is an unusual outlier or an early hint of a more complex formation process occurring elsewhere in the galaxy.
As detection technologies continue to improve, scientists expect more such surprises.
“Each newly characterised system expands our understanding of how worlds take shape,” Cloutier added. “And sometimes, those worlds challenge the very theories we rely on.”
