Astronomers studying a dim red dwarf star called LHS 1903 have found a four-planet system with an architecture that defies the standard rules of how rocky worlds take shape. The outermost planet in the system lacks any gaseous envelope, even though it orbits beyond two gas-rich neighbors, a layout that existing formation models struggle to explain. Reported in a new study and described in university and science-news coverage, the discovery challenges some leading ideas about how planets assemble in compact systems around small stars.
A Rocky Outsider Where Gas Should Be
In our own solar system and in most known exoplanet systems, a familiar pattern holds: small, dense rocky worlds sit close to their star, while larger gas-rich planets occupy the outer orbits. The LHS 1903 observations break that template. Its four planets follow a rocky, gaseous, gaseous, rocky sequence from inside out. The innermost world, LHS 1903 b, orbits every 2.2 days and is dense and rocky. The middle pair, planets c and d, carry extended hydrogen-helium atmospheres. Then comes the surprise: the outermost planet, LHS 1903 e, completing an orbit every 29.3 days, shows no sign of a gas envelope at all.
That bare outer world is the puzzle. Density measurements reported in the study listing and in the team’s write-ups indicate that LHS 1903 e is rocky, while c and d are consistent with retaining extended atmospheres. All four planets span what astronomers call the radius valley, the size boundary where worlds transition between rocky and gaseous compositions. Finding a stripped-down rocky planet beyond two gas-rich siblings upends the expectation that outer planets should have had more raw material to hold onto their atmospheres, not less. Instead of a smooth progression from compact rocky worlds to progressively puffier neighbors, LHS 1903 presents an alternating pattern that demands a more nuanced explanation.
Why Standard Explanations Fall Short
Researchers considered and rejected several conventional explanations. According to the university press materials, the researchers argue that planet swapping or large-scale migration is unlikely to explain the current ordering. They also argue that a major collision is an unlikely explanation for stripping LHS 1903 e of a primordial atmosphere. Neither mechanism fits the observed masses, orbits, and compositions, which together point to a system that has remained dynamically calm rather than one that has undergone violent rearrangements.
The failure of these standard models matters because it exposes a gap in how scientists think about planet formation around M-dwarf stars. LHS 1903 is a small, cool red dwarf that shines far less brightly than the Sun. Disks around such stars contain less total mass, and gas disperses on shorter timescales, making timing a critical factor in whether a young planet can capture and retain a thick envelope. If the outer reaches of the disk ran out of gas before LHS 1903 e finished assembling, the planet would have formed from gas-depleted material, arriving too late to collect a substantial atmosphere. That is the scenario the discovery team favors, and it points toward a formation pathway that may not be captured well by standard migration and photoevaporation-only explanations.
Inside-Out Formation Gets a Real Test Case
To explain the system, the researchers turned to a theoretical framework first proposed by Sourav Chatterjee and Jonathan Tan. Their inside-out formation concept, published in The Astrophysical Journal, describes a sequential process in which small rocky particles called pebbles drift inward through a protoplanetary disk and pile up at pressure maxima near the inner edge of a magnetically quiet zone. Planets form one after another, starting closest to the star, as these pebbles rapidly assemble into planetary cores that can then accrete gas if enough remains in the disk. Each new world assembles from whatever disk material is left after its inner neighbor has already swept through and consumed part of the supply.
The peer-reviewed treatment of this model lays out analytic scaling relations and predicted planet masses and separations for tightly packed systems. LHS 1903 fits the model’s logic almost too neatly: the inner rocky planet formed first from a gas-rich disk, the middle planets formed next and retained their atmospheres, and the outermost planet formed last from a disk that had already lost most of its gas. Until now, the inside-out framework lacked a clear observational case where sequential gas depletion produced a measurably different composition for the final planet in the chain. LHS 1903 e provides that test case, tying a specific orbital order directly to contrasting planetary densities and atmospheric contents.
Red Dwarfs as Formation Laboratories
The discovery also sharpens a broader question about what kinds of planets red dwarfs can produce. In mid-2025, a separate team reported finding a massive companion orbiting a tiny red dwarf, a configuration once considered highly unlikely. Later that year, NASA’s James Webb Space Telescope detected the strongest atmospheric signal yet from a broiling lava world. Together with LHS 1903, these results suggest that M-dwarf systems produce a far wider variety of planetary outcomes than theorists assumed even a few years ago, ranging from tightly packed rocky chains to unexpected gas giants and superheated worlds with surprisingly resilient atmospheres.
Most coverage of the LHS 1903 discovery has framed it as a single oddity. A sharper reading is that it exposes a systematic blind spot. Formation models calibrated on Sun-like stars may consistently underpredict the diversity of architectures around low-mass stars, where disk lifetimes are shorter and gas depletion gradients are steeper. If inside-out formation is more common around M-dwarfs than currently recognized, surveys of compact multi-planet systems should reveal more examples where the outermost planet is compositionally distinct, signaling that it formed from a disk in its final, gas-poor phase rather than from the rich conditions that prevailed closer in.
Open Data, Community Models, and What Comes Next
The LHS 1903 work also highlights the infrastructure that makes such reinterpretations possible. The study and the theoretical work it builds on are disseminated through open repositories that astronomers use to share preprints, refine models, and debate interpretations as results move through the publication process. Services like community preprint platforms provide guidance for authors and readers, helping standardize how new results are described and accessed. That rapid, open circulation of ideas allowed the inside-out formation framework, first proposed more than a decade ago, to be stress-tested against the new LHS 1903 data almost immediately.
Behind the scenes, those platforms are maintained by a network of institutional partners and individual contributors. The list of supporting institutions shows how universities, observatories, and research organizations collectively underwrite the infrastructure that exoplanet science now depends on. Voluntary financial contributions from the broader community help keep these repositories free to access, ensuring that complex cases like LHS 1903 can be examined from multiple theoretical angles by researchers around the world. As more red dwarf systems are discovered and characterized, that open ecosystem will be essential for iterating on models of inside-out formation, testing alternative scenarios, and ultimately building a more complete picture of how planetary systems emerge in some of the galaxy’s most common stellar environments.
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*This article was researched with the help of AI, with human editors creating the final content.
