Discovery of Rocky Outer Planet Challenges Planet Formation Theories

Astronomers have discovered a planetary system around the red dwarf star LHS 1903, located approximately 116 light-years from Earth in the constellation Lynx. This system includes four planets, with the outermost, LHS 1903 e, exhibiting characteristics that challenge traditional models of planetary formation.

Conventional theories suggest that rocky planets form close to their stars, where intense heat prevents the accumulation of thick atmospheres, while gas giants develop farther out in cooler regions. However, LHS 1903 e, despite its distant orbit, appears to be a dense, rocky world without a substantial atmosphere. This unexpected configuration has led scientists to reconsider the processes governing planet formation.

The discovery was made using data from NASA's Transiting Exoplanet Survey Satellite (TESS) and the European Space Agency's Characterizing Exoplanet Satellite (CHEOPS). Initial observations identified three planets: a rocky inner planet followed by two gas-rich planets. Subsequent measurements revealed the fourth planet, LHS 1903 e, in the system's outer region.

Researchers explored various explanations for LHS 1903 e's rocky nature, including the possibility of a massive impact stripping away its atmosphere or orbital migrations altering its position. However, simulations and analyses ruled out these scenarios. The prevailing hypothesis now suggests an 'inside-out' formation process, where planets form sequentially over time. In this model, the inner planets form first in a gas-rich environment, while outer planets like LHS 1903 e form later, after much of the gas has dissipated, resulting in a rocky composition.

This finding challenges existing models and suggests a more complex picture of planetary system development. The study detailing these observations was published in the journal Science on February 12, 2026.