LHS 1903, also known as TOI-1730 or G 107-55, is a small M-dwarf star about 116.3 light-years away in the constellation Lynx. Astronomers studying the system with the CHEOPS spacecraft have identified an unusual planetary arrangement – a rocky inner planet, two gas-rich planets, and then an outer rocky planet. This breaks the expected pattern of planetary formation and has raised fresh questions about how planets grow and evolve.
Unusual Planetary Sequence
The system contains four known planets. LHS 1903b is a rocky planet close to the star. It is followed by LHS 1903c and LHS 1903d, which are gas worlds. The surprise is LHS 1903e, the outermost planet, which appears to be rocky. This creates an inside-out pattern of rocky-gaseous-gaseous-rocky.
Why the Discovery Matters
Standard models suggest that planets close to a star lose their atmospheres because of intense radiation, leaving behind dense rocky cores. Farther away, cooler conditions allow gas to accumulate and form gas-rich planets. LHS 1903e does not fit this model. It either never formed a thick atmosphere or lost it under unusual conditions.
Possible Explanations Tested
Researchers examined several possibilities, including:
- A giant impact stripping away the atmosphere.
- Planetary migration causing the planets to swap positions.
- Formation in a gas-poor environment.
Simulations and orbital calculations ruled out the first two ideas. The most likely explanation is that the planets formed one after another, and by the time the outer planet formed, the system had already run out of gas needed for atmospheric growth.
Significance for Exoplanet Studies
The finding provides rare evidence of a planet forming in a gas-depleted environment. It may help scientists better understand planet formation in low-mass star systems. The CHEOPS mission continues to search for such unusual systems to refine current models of planetary evolution.
Last Modified: April 28, 2026