Scientists have identified a rocky outer planet in a system where a gas giant was expected. The discovery challenges traditional models and supports the idea that planets may form one by one in changing environments.
When scientists survey planetary systems across the Milky Way, they tend to see a familiar arrangement. Small, rocky worlds circle close to their parent star, while large gas giants orbit much farther out. Our Solar System fits this pattern perfectly. The inner planets: Mercury, Venus, Earth, and Mars, are made mainly of rock and metal. Beyond them lie Jupiter, Saturn, Uranus, and Neptune, which are dominated by thick gaseous envelopes.
This layout is explained by a widely accepted model of planet formation. Young stars emit powerful radiation that can strip lightweight gases from nearby developing planets, leaving dense, rocky cores behind. At greater distances, cooler temperatures allow planets to retain thick atmospheres, enabling them to grow into gas giants.
A Rule-Breaking System Around LHS 1903
A newly studied system orbiting the star LHS 1903 does not follow this expected structure. The results were published in Science.
LHS 1903 is a red dwarf, smaller and dimmer than our Sun. Researchers led by Prof. Ryan Cloutier of McMaster University and Prof. Thomas Wilson of the University of Warwick combined observations from telescopes on Earth and in space to analyze the system. They initially identified three planets. Closest to the star is a rocky world, followed by two gas-rich planets similar to scaled-down versions of Neptune. That arrangement aligns with standard theories.
However, after years of additional measurements, new observations from the European Space Agency’s CHEOPS satellite uncovered an unexpected fourth planet. Known as LHS 1903 e, it orbits farthest from the star and appears to be rocky rather than gaseous.
“We’ve seen this pattern: rocky inside, gaseous outside, across hundreds of planetary systems. But now, the discovery of a rocky planet in the outer part of a system forces us to rethink the timing and conditions under which rocky planets can form,” says Cloutier, who is an assistant professor in the Department of Physics and Astronomy.
Testing Collisions and Orbital Shifts
The researchers explored several alternative explanations. They asked whether a massive impact might have stripped away a thick atmosphere. They also examined whether the planets might have migrated and swapped positions over time. Detailed computer simulations and studies of their orbital dynamics ruled out both ideas.
Instead, the team arrived at a more unexpected conclusion. The planets around LHS 1903 may not have formed simultaneously. Rather, they could have developed one after another as the environment around the star gradually changed.
Inside Out Planet Formation Theory
Most models suggest that planets emerge within a protoplanetary disc, a rotating cloud of gas and dust surrounding a young star. In this disc, clumps of material form multiple planetary embryos at roughly the same time. Over millions of years, those embryos grow into fully formed planets with different sizes and compositions.
The unusual structure of the LHS 1903 system points to another possibility known as inside out planet formation. In this scenario, planets form sequentially in shifting conditions. The makeup of each planet depends on the local environment at the time it completes its formation, determining whether it becomes gas-rich or remains rocky.
This framework helps explain why LHS 1903 e stands out from its neighbors. By the time it began assembling, much of the surrounding gas in the disc may have already dissipated, leaving too little material to build a thick atmosphere.
“It’s remarkable to see a rocky world forming in an environment that shouldn’t favour that outcome. It challenges the assumptions built into our current models,” says Cloutier, who adds that the discovery raises broader questions about whether LHS 1903 is an anomaly or an early example of a pattern scientists have yet to recognize.
“As telescopes and detection methods become more precise, we are strengthening our ability to find planetary systems that don’t resemble our own and that don’t conform to longstanding theories,” he says.
“Each new system adds another data point to a growing picture of planetary diversity – one that forces scientists to rethink the processes that shape worlds across the galaxy.”
Reference: “Gas-depleted planet formation occurred in the four-planet system around the red dwarf LHS 1903” by Thomas G. Wilson, Anna M. Simpson, Andrew Collier Cameron, Ryan Cloutier, Vardan Adibekyan, Ancy Anna John, Yann Alibert, Manu Stalport, Jo Ann Egger, Andrea Bonfanti, Nicolas Billot, Pascal Guterman, Pierre F. L. Maxted, Attila E. Simon, Sérgio G. Sousa, Malcolm Fridlund, Mathias Beck, Anja Bekkelien, Sébastien Salmon, Valérie Van Grootel, Luca Fossati, Alexander James Mustill, Hugh P. Osborn, Tiziano Zingales, Matthew J. Hooton, Laura Affer, Suzanne Aigrain, Roi Alonso, Guillem Anglada, Alexandros Antoniadis-Karnavas, Tamas Bárczy, David Barrado Navascues, Susana C. C. Barros, Wolfgang Baumjohann, Thomas Beck, Willy Benz, Federico Biondi, Xavier Bonfils, Luca Borsato, Alexis Brandeker, Christopher Broeg, Lars A. Buchhave, Maximilian Buder, Juan Cabrera, Sebastian Carrazco Gaxiola, David Charbonneau, Sébastien Charnoz, David R. Ciardi, Karen A. Collins, Kevin I. Collins, Rosario Cosentino, Szilard Csizmadia, Patricio E. Cubillos, Shweta Dalal, Mario Damasso, James R. A. Davenport, Melvyn B. Davies, Magali Deleuil, Laetitia Delrez, Olivier D. S. Demangeon, Brice-Olivier Demory, Victoria DiTomasso, Diana Dragomir, Courtney D. Dressing, Xavier Dumusque, David Ehrenreich, Anders Erikson, Emma Esparza-Borges, Andrea Fortier, Izuru Fukuda, Akihiko Fukui, Davide Gandolfi, Adriano Ghedina, Steven Giacalone, Holden Gill, Michaël Gillon, Yilen Gómez Maqueo Chew, Manuel Güdel, Pere Guerra, Maximilian N. Günther, Nathan Hara, Avet Harutyunyan, Yuya Hayashi, Raphaëlle D. Haywood, Rae Holcomb, Keith Horne, Sergio Hoyer, Chelsea X. Huang, Masahiro Ikoma, Kate G. Isaak, James A. G. Jackman, Jon M. Jenkins, Eric L. N. Jensen, Daniel Jontof-Hutter, Yugo Kawai, Laszlo L. Kiss, Ben S. Lakeland, Jacques Laskar, David W. Latham, Alain Lecavelier des Etangs, Adrien Leleu, Monika Lendl, Jerome de Leon, Florian Lienhard, Mercedes López-Morales, Christophe Lovis, Michael B. Lund, Rafael Luque, Demetrio Magrin, Luca Malavolta, Aldo F. Martínez Fiorenzano, Andrew W. Mayo, Michel Mayor, Christoph Mordasini, Annelies Mortier, Felipe Murgas, Norio Narita, Valerio Nascimbeni, Belinda A. Nicholson, Göran Olofsson, Roland Ottensamer, Isabella Pagano, Larissa Palethorpe, Enric Pallé, Hannu Parviainen, Marco Pedani, Francesco A. Pepe, Gisbert Peter, Matteo Pinamonti, Giampaolo Piotto, Don Pollacco, Ennio Poretti, Didier Queloz, Samuel N. Quinn, Roberto Ragazzoni, Nicola Rando, David Rapetti, Francesco Ratti, Heike Rauer, Federica Rescigno, Ignasi Ribas, Ken Rice, George R. Ricker, Paul Robertson, Thierry de Roche, Laurence Sabin, Nuno C. Santos, Dimitar D. Sasselov, Arjun B. Savel, Gaetano Scandariato, Nicole Schanche, Urs Schroffenegger, Richard P. Schwarz, Sara Seager, Ramotholo Sefako, Damien Ségransan, Avi Shporer, André M. Silva, Alexis M. S. Smith, Alessandro Sozzetti, Manfred Steller, Gyula M. Szabó, Motohide Tamura, Nicolas Thomas, Amy Tuson, Stéphane Udry, Andrew Vanderburg, Roland K. Vanderspek, Julia Venturini, Francesco Verrecchia, Nicholas A. Walton, Christopher A. Watson, Robert D. Wells, Joshua N. Winn, Roberto Zambelli and Carl Ziegler, 12 February 2026,Science.
DOI: 10.1126/science.adl2348
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