This exoplanet system formed backwards, and nobody can explain why

Rocky planets usually come first, but there is nothing ordinary about LHS 1903

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The takeaway: An international team of astronomers has published a study on LHS 1903, a red dwarf star hosting a truly unusual planetary system. Unlike our solar system and many previously discovered systems across the galaxy, this configuration appears to be "inside-out," defying the typical arrangement of alternating rocky and gaseous planets.

In a conventional system like our own, rocky planets such as Mercury, Venus, Earth, and Mars orbit closest to the host star. Farther out, gas giants like Jupiter and Saturn occupy the outer regions. Uranus and Neptune are classified as ice giants because they are composed largely of heavier volatile elements.

Until now, this broad pattern – smaller, rocky worlds near the star and gas or ice giants farther away – has been commonly observed in planetary systems across the Milky Way. The prevailing theory holds that intense radiation from the host star strips lighter gases from planets in close orbits, leaving behind rocky bodies. By contrast, planets forming farther out can retain thick gaseous envelopes and grow into gas giants.

The traditional model of planetary formation does not hold for LHS 1903. Located about 116 light-years from Earth in the Lynx constellation, the red dwarf is orbited by four unusual exoplanets. Data from the European Space Agency's CHaracterising ExOPlanet Satellite (CHEOPS) shows that the system includes a rocky planet, LHS 1903 b, closest to the star; two gaseous planets, LHS 1903 c and d; and a fourth rocky planet, LHS 1903 e, at the outer edge.

"Rocky planets don't usually form far from their home star, on the outside of the gaseous worlds," said Dr. Thomas Wilson, assistant professor at the University of Warwick.

Wilson led the international team that studied LHS 1903, describing the system as an "inside-out" planetary formation never seen before. The team is now investigating this reversed configuration, and several hypotheses – including planet swapping or catastrophic collisions – have already been ruled out.

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CHEOPS data also suggest that the four exoplanets did not form simultaneously but emerged sequentially in a process called inside-out planet formation. According to Wilson, LHS 1903 e may have formed after most of the system's gas had already been depleted – an unprecedented, expectation-defying occurrence.

ESA researcher Isabel Rebollido noted that more and more exoplanet systems are being discovered that do not follow the patterns observed in our solar system. Theories of planetary formation are evolving rapidly, with ESA scientist Maximilian Günther suggesting that traditional models barely scratch the surface of this cosmic puzzle.