Astronomers have identified a planetary system that challenges conventional understanding of planet formation. Orbiting the red dwarf star LHS 1903, located roughly 116 light-years away, the system exhibits an unusual order: rocky planet, gas giant, gas giant and then another rocky planet. This configuration, dubbed “inside-out” by researchers, upends the widely accepted model where rocky planets form closer to their star and gas giants reside further out.
The discovery, detailed in a study published in the journal Science on , began with observations from NASA’s Transiting Exoplanet Survey Satellite (TESS) in 2019. Subsequent analysis utilizing data from ground- and space-based telescopes, including Europe’s Cheops space telescope, revealed the unexpected arrangement. The team, led by Thomas Wilson of the University of Warwick in the UK, initially identified three planets: a rocky planet closest to LHS 1903, followed by two gas giants. However, further investigation with Cheops uncovered a fourth, rocky planet orbiting furthest from the star.
“That makes this an inside-out system, with a planet order of rocky-gaseous-gaseous-and then rocky again,” explained Wilson. “Rocky planets don’t usually form so far away from their home star.” The standard model posits that intense radiation from a star strips away the atmospheres of planets in close proximity, leaving behind rocky cores. Further out, where temperatures are colder, gas can accumulate, forming gas giants like Jupiter.
LHS 1903 is a red dwarf star, smaller and cooler than our Sun. Its four planets all orbit very closely, completing orbits in less than 30 days. The planets range in size from approximately 1.4 to 2.5 times the radius of Earth, placing them in the range between super-Earths and mini-Neptunes. Precise measurements of their masses and densities, made possible by the combined observations, have helped scientists determine their compositions.
The unusual arrangement suggests that the system’s history may have involved significant gravitational disturbances. Andrew Cameron, an astronomer at the University of St. Andrews in Scotland, described the system as having “the look of something that’s been turned inside out,” implying a chaotic past where planetary orbits were significantly altered. This upheaval could have migrated planets from their original formation locations, leading to the current configuration.
The discovery raises questions about the universality of planet formation models. While the rocky-then-gaseous pattern observed in our solar system was long considered a common blueprint, LHS 1903 demonstrates that other pathways are possible. The system’s architecture suggests that planetary formation isn’t always a smooth, predictable process, and that gravitational interactions can play a crucial role in shaping planetary systems.
The fact that a rocky planet can form, or survive, at such a distance from its star is particularly puzzling. The outer rocky planet in the LHS 1903 system challenges the understanding of how rocky planets accumulate material in the colder regions of a protoplanetary disk. It’s possible that the planet formed closer to the star and migrated outwards, or that the conditions in the disk were somehow conducive to rocky planet formation at a greater distance.
Scientists are now focusing on further observations of LHS 1903 to refine their understanding of the system’s dynamics and history. Detailed atmospheric studies of the planets, if feasible, could provide clues about their origins and compositions. The discovery highlights the diversity of planetary systems beyond our own and underscores the need for continued exploration to refine our models of planet formation. As noted in a report by Science News, “Bad stuff does happen in young planetary systems.” LHS 1903 appears to be a prime example of that “bad stuff” – a system that defies expectations and forces astronomers to reconsider the processes that shape planetary worlds.
The discovery of this “inside-out” system, as described by ScienceAlert, is a reminder that our solar system may not be typical. The universe is full of surprises, and LHS 1903 is a compelling example of a planetary system that challenges our assumptions and expands our knowledge of the cosmos.
