The European Space Agency’s (ESA) pioneering ExOPlanet satellite, CHEOPS, has uncovered a remarkable four-planet system. Notably, the outermost planet is a small rocky world instead of a gas giant, challenging previous assumptions about planet formation. This intriguing configuration surrounding the nearby red dwarf star LHS 1903 indicates that the outermost planet may have formed significantly later than its counterparts, providing compelling evidence that planets can develop in gas-poor environments—conditions once deemed impossible.
LHS 1903 is an M dwarf star located approximately 116.3 light-years away in the constellation Lynx.
This star, also referred to as TOI-1730 or G 107-55, is cooler and less luminous than our Sun.
The planet sequence around LHS 1903 begins with the rocky planet LHS 1903b, followed by two gaseous planets, LHS 1903c and LHS 1903d—a pattern that aligns with expectations.
However, astronomer Thomas Wilson from the University of Warwick, along with his team, discovered a surprising fourth planet on the system’s outer edge, which is rocky rather than gaseous.
“This creates an inside-out planetary arrangement: rock → gas → gas → rock,” said Dr. Wilson.
“Typically, rocky planets are expected to form close to their host star.”
Current scientific models suggest that planets closest to a star are often rocky, as stellar radiation can strip away gaseous atmospheres, leaving behind dense, solid cores.
In contrast, gas giants form in cooler regions where they can retain gas.
However, LHS 1903e seems to have lost its gaseous atmosphere or may never have had one.
“The mystery surrounding planet formation and evolution remains significant,” stated Dr. Maximilian Günther, CHEOPS project scientist and ESA astronomer.
“Discovering clues like this is precisely what CHEOPS aims to achieve.”
The research team explored various hypotheses about the unusual formation of this rocky planet.
For instance, could it have been impacted by a giant asteroid or other large object that stripped away its atmosphere?
Or was there a gravitational exchange in the configuration of the planets over time?
Through simulations and orbital calculations, researchers ruled these scenarios out.
Instead, they presented a more fascinating explanation: the planets may have formed sequentially rather than simultaneously.
“By the time this exoplanet formed, the system may have been void of the gases typically necessary for planet formation, yet here we find a small, rocky world defying all expectations,” Dr. Wilson added.
“This could be the first evidence of planets forming in so-called gas-deficient conditions.”
A study detailing this groundbreaking discovery has been published in the latest issue of Science.
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Thomas G. Wilson et al. Formation of gas-deficient planets in a four-planet system around the red dwarf star LHS 1903. Science published online on February 12, 2026. doi: 10.1126/science.adl2348
Source: www.sci.news
