CHEOPS Discovers Rocky Exoplanet that Defies Conventional Formation Theories
ESA’s ExOPlanet Characterization Satellite (CHEOPS) has revealed a four-planet system in which the farthest world is a small rocky planet and not a gas giant. The unexpected arrangement around the nearby red dwarf star LHS 1903 suggests that the outermost member of the system formed long after the others, providing the strongest observational clue yet that planets can emerge in poor gas conditions once thought impossible.
Artist’s impression of the four-planet system around LHS 1903. Image credit: ESA / ATG Europe.
LHS 1903 is a small M dwarf located 116.3 light years away in the constellation Lynx.
Also known as TOI-1730 or G 107-55, the star is cooler and dimmer than our Sun.
The planets around LHS 1903 start with the rocky planet LHS 1903b orbiting nearby, then two gaseous worlds, LHS 1903c and LHS 1903d, the expected planetary model.
However, using the CHEOPS spacecraft, University of Warwick astronomers Thomas Wilson and colleagues discovered that a surprising fourth planet at the outer edge of the system was rocky, not gaseous.
“This makes it an inverted system, with a rocky-gaseous-gaseous planetary order, then rocky again,” Dr Thomas said.
“Rocky planets generally don’t form that far from their home star.”
Current models suggest that the planets closest to stars are rocky because stellar radiation sweeps away their gaseous atmospheres, leaving behind dense, solid cores.
Gas giants form farther away in colder regions where gas can accumulate and planets can trap it.
However, LHS 1903e appeared to have lost its gaseous atmosphere or never to have formed one.
“How planets form and evolve is still a mystery,” said Dr Maximilian Günther, CHEOPS project scientist and ESA astronomer.
“Finding clues like this to solve this puzzle is precisely what CHEOPS set out to do.”
Astronomers then explored various explanations for why this strange rocky planet breaks the familiar pattern.
Was the planet, for example, hit at some point in its past by a giant asteroid, comet, or other large object, which destroyed its atmosphere?
Or did the planets around LHS 1903 change places at some point during their evolution?
After testing these scenarios through simulations and calculations of the orbital times of the planets, the researchers ruled them out.
Instead, their investigation led them to a more intriguing explanation: the planets might have formed one after the other, rather than at the same time.
“By the time this outer planet formed, the system may have already run out of gas, which is considered vital for planet formation. Yet here is a small, rocky world that defies expectations,” Dr Wilson said.
“It appears we have found the first evidence of a planet that formed in what we call a gas-depleted environment.”
A paper describing the discovery was published today in the journal Science.
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Thomas G. Wilson and others. Gas-depleted planet formation occurred in the four-planet system around the red dwarf LHS 1903. Sciencepublished online February 12, 2026; doi: 10.1126/science.adl2348
