More than 100 light-years from Earth, a planetary system is challenging astronomers and potentially forcing a reevaluation of established concepts regarding world formation. Observations made with telescopes from NASA and the European Space Agency (ESA) have revealed an unusual architecture around the star LHS 1903, a red dwarf – the most common type of star in the Universe.
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Four planets orbit the star, but their arrangement defies known patterns. The innermost planet is rocky; the next two are gas-rich; and the outermost, unexpectedly, is too rocky. This configuration contradicts the model observed in our own Solar System, where rocky planets occupy the inner orbits and gas giants concentrate in the outer regions.
Formation in Reverse Order
The traditional model posits that planets form from a disk of gas and dust around young stars. In the hotter regions, close to the star, only heat-resistant materials like iron and silicates can clump together, forming rocky bodies. Beyond the so-called “ice line,” where temperatures allow for the condensation of water and other compounds, planetary cores grow rapidly and can capture large amounts of hydrogen and helium, forming gas giants.
According to Thomas Wilson, assistant professor at the University of Warwick and lead author of the study published in the journal Science, this is the first time a rocky planet has been observed so far from its host star, situated beyond gas worlds. The outer object, classified as a “Super-Earth,” has a radius approximately 1.7 times that of Earth.
The team tested hypotheses such as planet collisions or the loss of atmosphere from a gas giant, but the simulations did not reproduce the observed system. Researchers believe the most plausible explanation is a formation mechanism with a progressive scarcity of gas. In this scenario, the planets would have emerged one after another, from the innermost to the outermost. When the last one began to form, the disk was already poor in gas and dust, favoring the emergence of a predominantly rocky body.
The system was initially identified by the TESS satellite and subsequently analyzed with CHEOPS, along with complementary data from ground-based observatories.
Sara Seager, of the Massachusetts Institute of Technology and a co-author of the study, suggests the discovery may represent one of the first pieces of evidence that planet formation around red dwarfs follows different paths than those observed in stars like our Sun. The findings underscore the ongoing effort to understand the diversity of planetary systems beyond our own. Experts emphasize that the debate remains open.
Heather Knutson, of the California Institute of Technology, pointed out that the outermost planet may be cold enough to allow water to condense and that future observations with the James Webb Space Telescope could reveal details about its atmosphere.
The case of LHS 1903 adds an unexpected data point to current models and is expected to stimulate novel simulations in the coming years. In a field still in its consolidation phase, this system serves as a reminder that the diversity of planetary architectures may be greater, and more complex, than previously imagined.
