The Characterising ExOPlanet Satellite (CHEOPS), operated by the European Space Agency (ESA), has provided insights into a planetary configuration comprising four celestial bodies, with the most distant one identified as a modest, terrestrial planet rather than a gas giant. This unanticipated arrangement around the proximate red dwarf star, designated LHS 1903, suggests that this outermost member of the system originated significantly later than its counterparts. This observation offers the most compelling empirical evidence to date indicating that planetary development can indeed occur under conditions of scarce gaseous material, a scenario previously deemed improbable.
An artist’s impression of the four-planet system around LHS 1903. Image credit: ESA / ATG Europe.
LHS 1903, also recognized as TOI-1730 or G 107-55, is a diminutive M-dwarf star situated 116.3 light-years distant within the constellation of Lynx. This star is characterized by a lower temperature and diminished luminosity compared to our Sun.
The planetary constituents orbiting LHS 1903 commence with LHS 1903b, a rocky planet in close proximity, followed by two gaseous worlds, LHS 1903c and LHS 1903d, aligning with the conventional understanding of planetary sequencing.
However, through the utilization of the CHEOPS spacecraft, a team of astronomers from the University of Warwick, led by Thomas Wilson and his associates, detected an unexpected fourth planet at the solar system’s periphery. This outermost body was determined to be rocky, contrary to expectations of it being gaseous.
Dr. Thomas commented, “This configuration renders it an inside-out system, exhibiting a planetary sequence of rocky, then gaseous, gaseous, and subsequently rocky once more.”
He further elaborated, “Terrestrial planets do not typically form at such considerable distances from their parent star.”
Prevailing theoretical frameworks posit that planets closest to stars are rocky due to stellar radiation effectively ablating their gaseous envelopes, leaving behind dense, solid nuclei. Gaseous giants are understood to coalesce in more remote, cooler zones where atmospheric material can accumulate, enabling planets to retain it.
Nevertheless, LHS 1903e appeared to have either lost its gaseous atmosphere or never possessed one to begin with.
Dr. Maximilian Günther, a CHEOPS project scientist and an astronomer at ESA, remarked, “Numerous aspects of planetary formation and evolution remain enigmatic. The discovery of evidence such as this, which aids in resolving these puzzles, is precisely the objective for which CHEOPS was designed.”
Following this revelation, the astronomers embarked on an investigation to elucidate the reasons behind this atypical rocky planet’s deviation from established patterns.
Consideration was given to whether the planet, at some juncture in its history, might have sustained a collision with a colossal asteroid, comet, or another substantial celestial object, resulting in the expulsion of its atmosphere.
Alternatively, the question was posed as to whether the planets orbiting LHS 1903 had undergone orbital migration, effectively swapping positions during their evolutionary trajectory.
Upon subjecting these hypotheses to rigorous computational simulations and analyses of the planets’ orbital periods, the research team ultimately discounted these possibilities.
Instead, their inquiry converged upon a more compelling hypothesis: that the planets may have formed sequentially, rather than concurrently.
Dr. Wilson stated, “By the time this outermost planet coalesced, the system likely had already been depleted of the gas considered essential for planetary formation. Yet, this small, rocky world emerged, defying all prior predictions.”
He concluded, “It appears we have unearthed the initial evidence of a planet that formed within what is termed a gas-depleted environment.”
A detailed account of this discovery has been published in the scientific journal Science.
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Thomas G. Wilson et al. Gas-depleted planet formation occurred in the four-planet system around the red dwarf LHS 1903. Science, published online February 12, 2026; doi: 10.1126/science.adl2348
