Astronomers are expressing surprise after discovering a star system with a planetary arrangement that challenges previous scientific assumptions about planet formation. In our Solar System, the order of planets is relatively straightforward: the four closest to the Sun are rocky, while the outer four are gas giants. Scientists had believed that this pattern—rocky planets nearer to their star and gaseous giants further away—was a universal principle. However, the recently studied star LHS 1903, located in the Milky Way, offers a different scenario.
An international team of researchers, utilizing data from multiple telescopes, found three planets orbiting the red dwarf star, which is less bright and cooler than our Sun. As expected, the closest planet to LHS 1903 is rocky, followed by two gas giants. However, an analysis of observations from the Cheops space telescope revealed an unexpected fourth planet further out in the system, which is also rocky. This arrangement creates what the lead author of the study, Thomas Wilson from the University of Warwick, termed an “inside-out system.”
“Rocky planets typically do not form so far from their home star,” Wilson explained. Under normal conditions, rocky inner planets form close to their star due to the intense radiation that disperses gaseous materials. In contrast, gas giants evolve in the cooler, outer regions where they can develop thick atmospheres.
Confronted with this puzzling planetary order, the research team hypothesized a new formation scenario: the planets may have formed sequentially rather than simultaneously. Traditionally, planet formation theories are based on the idea that planets develop concurrently within a massive disc of gas and dust. However, by the time the fourth planet in the LHS 1903 system formed, resources like gas might have been limited, creating a rocky world in an atypical environment.
This discovery opens new avenues for understanding planetary formation, as noted by Isabel Rebollido from the European Space Agency. The more varied exoplanet systems astronomers examine, the more they realize that existing theories may need to be reevaluated. Since the 1990s, over 6,000 exoplanets have been discovered, underscoring the vast diversity of planetary systems existing beyond our own.
The findings regarding LHS 1903 not only highlight the complexities of planetary formation but also inspire hope for further exploration that may reveal even more exotic worlds and broaden our understanding of the universe.