Astronomers have discovered a planetary system with an unusual arrangement, challenging conventional understanding of planetary formation. The findings, revealed on February 12, detail a system where the order of planets deviates from the typical pattern observed in our solar system and others.
In our solar system, the four planets closest to the Sun are small and rocky, whereas the four furthest are gas giants. This arrangement has long been considered a standard model for planetary systems.
Scientists previously believed this rocky-then-gas planet sequence was consistent throughout the universe. However, the newly discovered system, designated LHS 1903, located in the Thick Disk of the Milky Way galaxy, suggests otherwise.
An international team of astronomers analyzing data from multiple telescopes identified four planets orbiting a red dwarf star – a star smaller and cooler than our Sun. The planets’ arrangement is what sets this system apart.
The planet closest to its star is rocky, followed by two gas giants. Surprisingly, the fourth and outermost planet is also rocky. “That makes this system an inside-out system, with a planet order of rock-gas-gas-and then rock again,” explained Thomas Wilson, an astrophysicist at the University of Warwick in the United Kingdom and a member of the discovery team. “Typically, rocky planets don’t form that far from their host star.”
Inner planets are generally small and rocky because intense radiation from the nearby star strips away most of the gas from their cores. However, in the cooler regions of a system, thick atmospheres can form around cores, resulting in gas giants. This discovery challenges those established models and could reshape our understanding of planetary formation.
Intrigued by the unusual arrangement of LHS 1903, the team sought to understand how it came to be. After considering several possibilities, they explored a scenario where planets form one at a time.
The most widely accepted theory posits that planets form simultaneously within a large ring of gas and dust called a protoplanetary disk. This process involves the accretion of small dust particles, which eventually coalesce into larger cores that develop into full-sized planets.
However, by the time the fourth planet orbiting LHS 1903 formed, “the gas may have already dissipated from the system.”
“But in this system, there’s a small rocky planet that defies expectations,” Wilson stated. “It seems we’ve found the first evidence of a planet forming in what we call a gas-poor environment.”
Since the 1990s, astronomers have discovered over 6,000 exoplanets, providing more data than ever before. This wealth of information is driving a reevaluation of existing planetary formation theories.
Isabelle Rebollido, a researcher in planetary disks at the European Space Agency (ESA), noted, “In the past, theories of planet formation were based on what we saw and knew about our solar system. But as we find more and more different exoplanetary systems, we have to start revising these theories.”
This discovery highlights the diversity of planetary systems and the demand for more flexible models to explain their formation. The findings, published in research available through Science Alert, could have significant implications for our understanding of planet formation and the potential for life beyond Earth.
