Astronomers have discovered a planetary system orbiting a small, ancient star that appears to defy conventional theories of planet formation. Instead of the typical arrangement – rocky worlds close to the star and gas giants farther out – this system, designated LHS 1903, features larger planets near the star and a smaller, rocky world at the outer edge. This unexpected structure raises fundamental questions about how planets assemble around low-mass stars.
The Anomaly: Inside-Out Planetary Architecture
The LHS 1903 system contains at least four planets: three sub-Neptunes (LHS 1903 b, c, and d) and a dense, rocky planet (LHS 1903 e) at the outermost orbit. Initial observations suggested a somewhat familiar arrangement, but high-precision data from the European Space Agency’s CHEOPS satellite revealed the anomaly. The outer planet, LHS 1903 e, is a bare, rocky core, lacking the thick atmosphere expected in colder, more distant regions where gas and ice are abundant.
This is significant because current models predict that planets forming farther from a star should accumulate substantial gas atmospheres. The presence of a rocky world at that distance suggests that the formation process was drastically different than previously assumed.
Gas-Depleted Formation: A New Hypothesis
To explain this unusual structure, the team proposed a “gas-depleted formation” mechanism. This theory posits that the planets formed sequentially, starting with those closest to the star. As the star aged, the surrounding gas and dust dissipated, leaving fewer resources for outer planets to grow. The outermost planet, LHS 1903 e, would have coalesced slowly from the remaining rocky debris in a gas-poor environment, resulting in its small size and lack of atmosphere.
Simulations support this hypothesis, though other scenarios – such as a past atmospheric loss due to a collision – cannot be entirely ruled out. The stability of the planetary orbits also lends credibility to the sequential formation model.
Implications for Exoplanet Research
The discovery of LHS 1903 has broader implications for understanding planet formation, particularly around M-dwarf stars, which are the most common type in the Milky Way. This system could provide a natural laboratory for studying the “radius valley”—the gap in size distribution between rocky and gaseous exoplanets.
By observing planets orbiting the same star, astronomers can control variables like stellar age and composition, allowing for more precise constraints on planet formation histories. Further observations with the James Webb Space Telescope will be crucial to analyze planetary atmospheres and refine these models.
“Finding more of these systems will really help us refine and constrain planet formation models in the near future.”
The LHS 1903 system represents a key step towards a more complete understanding of how planetary systems evolve, challenging long-held assumptions and opening new avenues for research.
