For decades, the architectural blueprint of our own Solar System has served as a foundational model for understanding how planetary systems take shape across the cosmos. The familiar pattern of smaller, rocky worlds orbiting closer to their star, with sprawling gas giants occupying the outer reaches, was long considered a universal standard. This conventional wisdom, derived from detailed observations of our cosmic neighborhood, posited that such an arrangement was the natural outcome of planetary formation within a protoplanetary disc. However, new findings surrounding a distant star known as LHS 1903 are now prompting astronomers to revisit these long-held assumptions, suggesting that the universe may be far more diverse in its planetary configurations than previously imagined.
The LHS 1903 system, located within the Milky Way’s thick disc, presents a striking departure from this established order. Initial analyses of data from various telescopes had already identified three planets circling this red dwarf star, which is considerably cooler and dimmer than our Sun. The innermost planet conformed to expectations, appearing rocky. It was followed by two gas giants, a sequence that aligned perfectly with the prevailing theories of planetary development. Yet, further scrutiny, particularly of observations gathered by the European Space Agency’s (ESA) exoplanet-probing Cheops space telescope, unveiled a crucial fourth planet situated even farther from the central star. This outermost world, to the surprise of researchers, also appears to be rocky.
This revelation creates what Thomas Wilson, the lead author of the study and a planetary astrophysicist from the University of Warwick, described in an ESA statement as an “inside-out system.” The sequence of rocky-gaseous-gaseous-rocky defies the expected gradient where intense stellar radiation near the star typically strips away lighter elements, leaving behind dense, rocky cores, while colder, more distant regions allow for the accretion of vast gaseous envelopes. “Rocky planets don’t usually form so far away from their home star,” Wilson emphasized, highlighting the anomaly.
The traditional model for planet formation posits that planets coalesce simultaneously within a massive disc of gas and dust. This process involves microscopic dust grains gradually clumping together, forming ever-larger cores that eventually evolve into fully-fledged planets. The presence of a small, rocky world in the outer reaches of the LHS 1903 system challenges this synchronous formation scenario. Instead, researchers are now exploring alternative explanations, including the intriguing possibility that the planets in this system may have formed sequentially, rather than all at once.
One leading hypothesis suggests that the outermost rocky planet in the LHS 1903 system might have formed in an environment where most of the gas had already dissipated. This “gas-depleted environment” would have prevented the accretion of a thick atmosphere, even at a distance where gas giants would typically emerge. Wilson noted that this could represent “the first evidence for a planet which formed in what we call a gas-depleted environment,” a concept that could significantly broaden our understanding of planetary genesis.
Since the 1990s, astronomers have cataloged over 6,000 exoplanets, primarily through detecting subtle dips in stellar brightness as planets transit across their host stars. This growing catalog of diverse planetary systems is increasingly forcing a reevaluation of theories that were largely shaped by our singular experience within the Solar System. Isabel Rebollido, a planetary disc researcher at ESA, articulated this evolving perspective, stating, “Historically, our planet formation theories are based on what we see and know about our Solar System. As we are seeing more and more different exoplanet systems, we are starting to revisit these theories.” The LHS 1903 system stands as a powerful testament to the universe’s capacity for unexpected configurations, pushing the boundaries of our cosmic understanding and reminding us that our own Solar System may be less “normal” than once presumed.
