Astronomers have uncovered a planetary arrangement that defies conventional formation models, featuring a rocky world that formed beyond the orbits of its gaseous neighbors—perhaps after much of the primordial material had already been consumed.
This discovery, captured with the European Space Agency’s Cheops telescope, reveals a four-planet system around a faint red-dwarf star situated roughly 117 light-years away toward the Lynx constellation. The system hosts two rocky planets and two gaseous ones, all circling a star that is about half the mass of the Sun and shines at roughly 5% of its brightness.
A striking aspect is the arrangement of the planets. The closest member is rocky, followed by two gaseous planets, and the outermost world is rocky as well, a pattern that challenges standard expectations. According to the prevailing planet-formation model, inner planets should be small and rocky because the inner disk is too hot for substantial gas or ice, and any forming atmospheres would be stripped away by stellar irradiation. By contrast, planets further out are expected to accumulate thick atmospheres from gas-rich, icy environments. Instead, this system presents a rocky outer planet where gas-rich worlds would be anticipated, effectively an “inside-out” configuration.
Explaining this anomaly, lead author Thomas Wilson of the University of Warwick notes that the arrangement disrupts the typical inside-out sequence we expect: a rocky inner pair and a gaseous outer pair, not the other way around. The discovery invites us to rethink how planetary systems assemble, especially around smaller, dimmer stars.
In our own solar system, the inner four planets are rocky, while the outer ones are gas giants, and dwarf rocky bodies beyond Neptune are far smaller than the planets closer to the Sun. Since the 1990s, astronomers have detected roughly 6,100 exoplanets beyond our solar system.
All four planets in this system orbit nearer to their star than Mercury does to the Sun, with the outermost planet circling at only about 40% of Mercury’s orbital distance. This compact arrangement is typical of planets around red dwarfs, which emit far less energy than the Sun.
Both rocky planets in the system are classified as super-Earths—rocky worlds with masses two to ten times that of Earth. The two gaseous planets are labeled mini-Neptunes, smaller than Neptune yet larger than Earth.
The team suggests that the planets did not form all at once from a single, large disk of gas and dust. Instead, they may have formed sequentially, with gas that would have fed the fourth planet’s atmosphere being consumed by the inner planets before it could coalesce fully.
Wilson describes the fourth planet as probably a late bloomer: it formed later in a gas-poor environment and had less material to build a substantial atmosphere. Another scenario is that it formed with a thick atmosphere but later lost it due to a catastrophic event, leaving a rocky core behind. Co-author Andrew Cameron from the University of St Andrews even likens this to Earth’s moon-forming collision, suggesting a similar dramatic past could explain the present state.
The fourth planet also raises questions about habitability. If its mass is about 5.8 Earth masses and its surface temperature hovers around 60°C, it sits near conditions that are intriguingly similar to the hottest temperatures recorded on Earth, making a habitable zone plausible in theory. Future observations with the James Webb Space Telescope could probe its atmosphere and surface conditions to assess whether such a planet might truly harbor Earth-like climate niches.
