Astronomers have announced the discovery of an unusual planetary system, dubbed an “inside-out” system, challenging conventional understanding of planet formation. The system, centered around the red dwarf star LHS 1903, exhibits a planetary arrangement that defies expectations based on our own solar system and previously observed exoplanetary systems.
Typically, planetary systems are believed to form with smaller, rocky planets closer to their star and larger, gaseous planets further out. This pattern is thought to arise from the intense radiation near a star, which strips away the gaseous atmospheres of inner planets, leaving behind rocky cores. Further out, where temperatures are cooler, gas giants can accumulate and retain their atmospheres. However, LHS 1903 presents a different picture.
Initial observations revealed three planets: a rocky planet closest to the star (LHS 1903 b), followed by two gas giants (LHS 1903 c and d). This arrangement initially aligned with established theories. However, further analysis using the European Space Agency’s Cheops space telescope uncovered a fourth planet, located further from the star, that is also rocky. This unexpected finding creates a planetary order of rocky-gaseous-gaseous-rocky, hence the “inside-out” designation.
“That makes this an inside-out system, with a planet order of rocky-gaseous-gaseous-and then rocky again,” explained Thomas Wilson, a planetary astrophysicist from the University of Warwick in the UK, and lead author of the study published in the journal Science. “Rocky planets don’t usually form so far away from their home star.”
The discovery raises questions about the processes governing planet formation, particularly in systems with red dwarf stars. LHS 1903 is a cooler and less bright star than our Sun, and approximately seven billion years old, with about half the mass of our Sun. The unusual arrangement suggests that the standard model of planetary formation may not be universally applicable.
One hypothesis being explored is that the planets didn’t necessarily form in their current positions. The team considered the possibility that gas had already dissipated from the system when the outermost rocky planet formed. “But at this system, we find a small rocky planet that challenges expectations. It seems we’re finding the first evidence of a planet forming in what we call a gas-poor environment,” Wilson stated.
The conventional understanding of planet formation involves the gradual accumulation of dust and gas within a protoplanetary disk surrounding a young star. This process typically results in planets forming concurrently within the disk. However, the LHS 1903 system suggests an alternative scenario where planets can form sequentially, even after the gas disk has largely dissipated.
Isabelle Rebollido, a researcher specializing in protoplanetary disks at the ESA, highlighted the need to re-evaluate existing theories. “In the past, theories of planet formation were based on what we saw and knew about our own solar system. But as we find more and more different exoplanetary systems, we have to start revising these theories.”
The discovery of LHS 1903 is significant because it adds to the growing body of evidence demonstrating the diversity of planetary systems in the galaxy. Since the mid-1990s, astronomers have identified over 6,000 exoplanets, revealing a wide range of planetary configurations that differ significantly from our own solar system. These findings are forcing scientists to broaden their understanding of how planets form, and evolve.
The prevalence of “super-Earths” and “mini-Neptunes” – planets with masses between Earth and Neptune – in other systems, but absent in our own, has already prompted a reassessment of planetary formation models. The LHS 1903 system adds another layer of complexity, suggesting that the conditions necessary for rocky planet formation may be more varied than previously thought.
Further research will be crucial to unraveling the mysteries of the LHS 1903 system and determining the precise mechanisms that led to its unusual configuration. Continued observations with advanced telescopes will help astronomers refine their understanding of planet formation and assess the prevalence of “inside-out” systems throughout the galaxy. The discovery underscores the fact that our solar system may not be representative of planetary systems as a whole, and that the universe is full of surprises.
