LHS 1903 System Reveals Rare Inside-Out Planetary Pattern

LHS 1903, also known as TOI-1730 or G 107-55, is a small M-dwarf star located 116.3 light-years away in the constellation Lynx. Cooler and dimmer than the Sun, it hosts a compact planetary system that has challenged prevailing models of planet formation. Observations by the European Space Agency’s CHEOPS spacecraft have revealed an unusual configuration that defies established expectations.

Unusual Planetary Arrangement

The system initially appeared conventional. Closest to the star is LHS 1903b, a rocky planet orbiting at short range. Beyond it lie two gaseous planets, LHS 1903c and LHS 1903d, consistent with the standard pattern in which rocky worlds form near stars and gas giants form farther out in cooler regions.

However, astronomers from the University of Warwick, led by Thomas Wilson, identified a fourth planet, LHS 1903e, at the system’s outer edge. Contrary to predictions, this distant planet is rocky rather than gaseous. This creates an “inside-out” sequence: rocky, gaseous, gaseous, and then rocky again. Such a configuration is rare and unexpected.

Challenge to Planet Formation Models

Current models suggest intense stellar radiation strips atmospheres from close-in planets, leaving dense rocky cores. Farther from the star, cooler conditions allow planets to retain thick gaseous envelopes and grow into gas giants.

The discovery of LHS 1903e contradicts this framework. A rocky planet at such a distance implies either it lost a previously existing atmosphere or never accumulated one. Both possibilities challenge assumptions about how protoplanetary discs evolve and distribute gas.

Testing Possible Explanations

Researchers explored several scenarios. One possibility was that a massive collision stripped the planet of its atmosphere. Another considered orbital migration, where planets may have swapped positions during early system evolution.

Simulations and orbital calculations ruled out these explanations. The evidence instead suggests that the planets formed sequentially rather than simultaneously. By the time LHS 1903e formed, the protoplanetary disc may have been depleted of gas, preventing atmospheric accumulation.

Important Facts for Exams

• M-dwarf stars are smaller, cooler and less luminous than the Sun.
• CHEOPS stands for Characterising Exoplanet Satellite, launched by ESA in 2019.
• Protoplanetary discs are composed of gas and dust surrounding young stars.
• Gas giants typically form beyond the “snow line” where volatile compounds condense.

Evidence of Gas-Depleted Formation

The findings indicate that LHS 1903e may be the first observed planet formed in a gas-depleted environment. This suggests planet formation can continue even after most disc gas has dissipated. The discovery provides critical insight into the diversity of planetary systems and highlights that much about planetary evolution remains unresolved.