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Astronomers studying a distant solar system have uncovered an unexpected cosmic arrangement that could challenge long-standing theories about how planets form and evolve.
Using the CHaracterising ExOPlanet Satellite (Cheops) operated by the European Space Agency (ESA), researchers discovered that the planetary system around the red dwarf star LHS 1903 does not follow the familiar pattern seen in our Solar System and many others across the Universe.
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For decades, scientists have believed that planetary systems typically form with rocky planets close to their star and gas-rich planets farther away.
This arrangement is explained by planet-formation models: intense radiation near a star strips away gas, leaving behind small rocky worlds, while cooler outer regions allow thick atmospheres to accumulate, forming gas giants.
The LHS 1903 system initially appeared to follow this rule. Observations by an international team led by researchers at the University of Warwick identified three planets orbiting the small, cool M-dwarf star, an inner rocky planet followed by two gaseous ones.
But new Cheops observations revealed something unexpected: a fourth planet orbiting farthest from the star that appears to be rocky.
“That makes this an inside-out system, with a planet order of rocky-gaseous-gaseous-rocky again,” said lead researcher Thomas Wilson. “Rocky planets don’t usually form so far away from their home star.”
The discovery prompted scientists to test several explanations. They considered whether a collision might have stripped the planet’s atmosphere or whether planets in the system could have migrated and swapped positions over time. Computer simulations and orbital calculations, however, ruled out these possibilities.
Instead, researchers arrived at a more intriguing explanation: the planets may have formed sequentially rather than simultaneously.
In this scenario, known as inside-out planet formation, the star’s protoplanetary disc may have produced planets one after another over time. By the time the outermost rocky planet formed, much of the gas in the system may already have disappeared, leaving only solid material to build a small rocky world.
This would mean the outer planet formed in a gas-depleted environment, something rarely observed before.
Scientists caution that a single system is not enough to overturn established theories. Still, the finding highlights how the growing number of discovered exoplanets is reshaping planetary science.
As more unusual planetary systems are observed with advanced telescopes, astronomers are increasingly questioning whether our own Solar System’s orderly structure is typical, or simply one example in a much more diverse cosmic family.
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