Astronomers have identified a planetary system that does not fit standard models of planet formation, with a rocky planet positioned farther from its star than two gas giants, possibly forming after most surrounding material had already been consumed.

The system, studied using the European Space Agency Cheops space telescope, contains four planets, two rocky super-Earths and two gaseous mini-Neptunes, orbiting a dim red dwarf star named LHS 1903, roughly 117 light-years from Earth in the Lynx constellation. The star has about half the mass and 5% of the luminosity of the Sun.

The configuration drew attention because the outermost planet, which would typically be expected to be gaseous in standard planetary formation theory, is instead rocky. Thomas Wilson of the University of Warwick, lead author of a study published in Science, described the system as “built inside-out.”

“Planets close to their host star are expected to be small and rocky due to high temperatures and atmospheric erosion, while planets farther out form in colder regions rich in gas and ice,” Wilson explained. “Here, a rocky planet exists beyond two gas-rich planets, which challenges that paradigm.”

All four planets orbit much closer to LHS 1903 than Mercury does to the Sun, with the outermost planet at only 40% of Mercury’s orbital distance. The two rocky planets are super-Earths, with masses two to ten times that of Earth, while the two gaseous planets are mini-Neptunes, larger than Earth but smaller than Neptune.

Researchers suggest the planets may have formed sequentially rather than simultaneously, with the outermost rocky planet emerging after gas needed to form its atmosphere had already been used by its siblings. Wilson referred to it as a “late bloomer.” Another possibility is that it initially had a gaseous envelope that was later stripped away, potentially by a collision.

Andrew Cameron of the University of St Andrews, a co-author of the study, noted, “Did it form just as the gas ran out, or was its atmosphere removed by a collision? The latter is plausible—Earth’s Moon formed similarly.”

The fourth planet is also notable for its potential habitability. With a mass 5.8 times that of Earth and an estimated temperature of 140°F (60°C), it exists in a climate comparable to the hottest conditions recorded on Earth. Wilson said future observations with the James Webb Space Telescope could provide further insights into its conditions and habitability potential.