Astronomers have uncovered a celestial arrangement that challenges decades of planetary science. Roughly 116 light-years from Earth, the LHS 1903 system hosts four worlds in a configuration that defies established formation theories, suggesting our cosmic neighborhood may be stranger than we imagined.
The star at the center, LHS 1903, belongs to the most common stellar category—red dwarfs. Its four planets follow a puzzling sequence: a rocky inner world, two gas planets in the middle, and unexpectedly, another rocky planet in the outermost orbit. This pattern contradicts the galactic norm, where rocky planets typically orbit close to their stars while gas giants occupy distant positions, as seen in our solar system.
The Conventional Formation Framework
The dominant theory of planet formation describes worlds emerging from a swirling disk of gas and dust encircling a newborn star. Temperature variations across this protoplanetary disk dictate planetary composition.
In the blistering inner regions, volatile substances like water and carbon dioxide remain gaseous. Only heat-resistant materials—iron and rocky minerals—can exist as solids and gradually accumulate into terrestrial planets. This explains the composition of Mercury, Venus, Earth, and Mars.
Beyond the snow line, where temperatures plummet, water vapor freezes into ice, providing abundant material for rapid core growth. Once a developing world reaches about ten Earth masses, its gravity captures hydrogen and helium, ballooning into a gas giant like Jupiter or Saturn.
This elegant model has explained countless systems—until now.
An Unexpected Discovery
The outermost planet, LHS 1903 e, is a super Earth measuring 1.7 times Earth's radius. Its rocky nature, positioned beyond two gas-rich worlds, directly contradicts formation predictions.
"The paradigm of planet formation is that we have rocky inner planets very close to the stars, like in our solar system," noted Thomas Wilson, assistant professor at the University of Warwick and lead author of the study published in Science. "This is the first time we have a rocky planet so far from its host star, after these gas-rich planets."
The research team believes LHS 1903 e formed significantly later than its siblings. While the inner three planets coalesced early from the primordial disk, the outer rocky world likely emerged from residual material that persisted after the disk had cleared elsewhere. This suggests planetary systems can evolve over extended formation timelines.
Observational Breakthrough
NASA's Transiting Exoplanet Survey Satellite (TESS) first detected the system in 2018 by spotting periodic starlight dimming as planets transited LHS 1903. ESA's CHaracterising ExOPlanet Satellite (Cheops) then provided precise measurements confirming the planets' properties.
This partnership showcases how complementary missions advance discovery—TESS surveys vast sky regions while Cheops conducts detailed follow-up investigations.
Rethinking Planetary Science
The LHS 1903 system forces astronomers to reconsider formation timelines and disk dynamics. If rocky planets can appear in outer orbits after gas giants form, potentially habitable worlds might exist in previously overlooked locations, expanding our concept of the habitable zone.
The discovery also implies that gravitational interactions and material distribution in young systems are more complex than models suggest. As observational technology advances, more rule-breaking systems will likely emerge, each refining our understanding of planetary diversity.
The universe, it appears, refuses to be confined by a single formation narrative—and that makes exploration infinitely more compelling.