A newly studied planetary system appears to break the expected order of worlds, revealing an unusual arrangement that current theories struggle to explain.
“Many Vile Earthlings Munch Jam Sandwiches Under Newspapers” and “My Very Educated Mother Just Served Us Nachos” may sound like nonsense, but they are classic memory tricks used to teach the order of the planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
These eight planets fall into two broad categories: rocky and gaseous. The inner worlds closest to the Sun, from Mercury through Mars, are solid and rocky, while the outer planets, from Jupiter to Neptune, are mostly made of gas.
Astronomers have long observed this same arrangement across the universe. Planet formation theories predict that rocky planets form near their stars, while gas giants develop farther out, and most observations have supported this idea.
That was until scientists closely studied a system around a star called LHS 1903 using ESA’s CHaracterizing ExOPlanet Satellite (Cheops). What they found could reshape how researchers think planets form.
The four planets of LHS 1903
LHS 1903 is a small, cool red M-dwarf star that emits far less light than our Sun. Thomas Wilson from the University of Warwick and his international team used both space-based and ground-based telescopes to identify three planets orbiting the star. Their analysis suggested a familiar pattern: a rocky planet closest in, followed by two gaseous planets.
But further observations with Cheops revealed something unexpected. Data pointed to a fourth planet orbiting even farther out, and surprisingly, this distant world also appears to be rocky.
“That makes this an inside-out system, with a planet order of rocky-gaseous-gaseous-and then rocky again. Rocky planets don’t usually form so far away from their home star,” says Thomas.
Current models explain that intense radiation near a star strips away gas, leaving behind rocky planets. Farther out, cooler conditions allow thick atmospheres to build, creating gas giants.
ESA Cheops project scientist Maximilian Günther highlights the importance of the discovery: “Much about how planets form and evolve is still a mystery. Finding clues like this one for solving this puzzle is precisely what Cheops set out to do.”
Born to be weird?
Researchers were cautious about drawing conclusions from a single unusual system. They explored other possibilities, including whether the outer planet lost its atmosphere after a major impact or whether the planets shifted positions over time. Simulations and orbital analyses ruled out these scenarios.
Was the planet, for example, at some point in its past hit by a giant asteroid, comet, or another big object, that blew away its atmosphere? Or had the planets around LHS 1903 swapped places at some point during their evolution? After testing these scenarios through simulations and calculations of the planets’ orbital times, the team of scientists ruled them out.
Instead, their investigation led them to a more intriguing explanation: the planets may have formed one after the other, instead of at the same time. According to our current understanding, planets form from discs of gas and dust (protoplanetary discs) by clumping into planetary embryos at roughly the same time. These clumps then evolve into planets of different sizes and compositions over millions of years.
In contrast, here Thomas and his team discovered a planetary system where the star might have given birth to its four planets one after the other, instead of bearing quadruplets at once. This idea – known as inside-out planet formation – was proposed by scientists as a theory about a decade ago, but until now, never has the evidence been so strong.
A late bloomer defying expectations
This conclusion comes with an additional catch: Much like how our younger siblings are growing up in a world that is different from the one of our childhoods, this small rocky planet seems to have evolved and formed in a very different environment than its older sibling-planets.
“By the time this outer planet formed, the system may have already run out of gas, which is considered vital for planet formation. Yet here is a small, rocky world, defying expectations. It seems that we have found first evidence for a planet which formed in what we call a gas-depleted environment”, says Thomas.
The small rocky world is either an odd outlier, or the first evidence for a trend we hadn’t known about yet. Either way, its discovery begs for an explanation that lies beyond our usual planet formation theories.
Our Solar System as a one-size-fits-all
“Historically, our planet formation theories are based on what we see and know about our Solar System,” Isabel Rebollido who is currently a Research Fellow at ESA points out. “As we are seeing more and more different exoplanet systems, we are starting to revisit these theories.”
As our instruments improve, we continue to discover more and more ‘weird’ planetary systems in the vastness of space. They force us to question our understanding and make us reconsider established theories of planet formation. Ultimately, these discoveries are helping us learn about how our Solar System fits into the big family of diverse planetary systems. They make us wonder how special the order of the planets is that we teach our children, and if maybe it is our home Solar System that is the weird one after all.
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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