Scientists say a nearby red dwarf star hosts a planetary lineup that looks like a mirror image of our own solar system, with rocky worlds bookending a pair of gas-rich planets. Around the star LHS 1903, the sequence runs rocky, gas, gas, rocky, and a new Science paper reports that the outer planet, LHS 1903 e, appears to be strikingly depleted in gas. With orbital periods ranging from about 2.2 to 29.3 days and what one European Space Agency scientist calls a “late bloomer” outer world, the finding is forcing astronomers to rethink how solar systems like ours first came together.
The Surprising Architecture of LHS 1903
The system around LHS 1903 features four known planets arranged in a pattern that immediately caught astronomers’ attention: two inner rocky worlds, labeled LHS 1903 b and LHS 1903 c, followed by a pair of larger gaseous planets, LHS 1903 d and LHS 1903 e, with the outermost object unexpectedly turning out to be rocky again. In the Science paper associated with DOI 10.1126/science.adl2348, the team reports that the planets’ measured radii and densities fall on both sides of the so-called “radius valley,” the statistical divide between compact, likely rocky super-Earths and puffier, gas-rich mini-Neptunes. Lead author Thomas Wilson attributes the precise density measurements to a combination of transit data and stellar characterization that allowed the team to pin down the planets’ sizes and masses with unusual confidence.
Those measurements relied heavily on the CHEOPS space telescope, which was able to watch LHS 1903 for repeated, shallow dips in brightness as each planet crossed the face of the star. According to the Preprint version of the study, CHEOPS recorded distinct transit depths for all four worlds, with the inner planets producing relatively small signals consistent with compact rocky bodies and the middle planets generating deeper transits that match larger, gas-enveloped planets. The authors note that LHS 1903 e, despite occupying the system’s outermost known orbit, presents a transit signature and inferred density that indicate a rocky composition with little or no gaseous envelope, a configuration they argue is best explained by “gas-depleted” formation at a late stage in the system’s history.
How Astronomers Spotted This Anomaly
The discovery grew out of a multi-year observing campaign by the CHEOPS mission, which has been monitoring bright nearby stars for small variations in light that betray the presence of transiting planets. In its Authoritative write-up, the European Space Agency describes how CHEOPS first identified the innermost planet, LHS 1903 b, before additional observations revealed three more transiting companions. The Science Preprint contains the key orbital periods that define the system’s compact structure: LHS 1903 b circles the star in about 2.2 days, LHS 1903 c in roughly 7.5 days, LHS 1903 d in about 14.4 days, and LHS 1903 e in approximately 29.3 days. Those tightly packed orbits all lie far closer to LHS 1903 than Mercury is to the Sun, yet the planets still manage to display a wide range of sizes and compositions.
Maximilian Günther, identified in the ESA release as the CHEOPS project scientist, describes a careful sequence of interpretive steps that turned raw light curves into a physical story about planetary formation. After confirming that each transit signal came from a distinct planet and not from stellar activity or background objects, the team combined CHEOPS data with follow-up observations to refine the planets’ orbital parameters and densities. Günther emphasizes that the unusual rocky–gas–gas–rocky pattern only became clear once all four orbits and sizes were in hand, at which point the system stood out as an anomaly compared with the more common “hot Jupiter” or “compact Neptune” configurations seen in many other exoplanet systems.
Why This Mirrors Our Early Solar System
The authors of the Science paper argue that the LHS 1903 system offers a tantalizing parallel to what our own solar system might have looked like in its earliest stages, albeit compressed into a much smaller volume. In their “inside-out” formation narrative, rocky LHS 1903 b and LHS 1903 c formed first, close to the star, followed by the accretion of gas-rich LHS 1903 d and LHS 1903 e while the protoplanetary disk still contained abundant hydrogen and helium. As the disk evolved and its gas content dwindled, the outermost planet would have been left with a solid core but little envelope, producing the gas-depleted rocky world inferred for LHS 1903 e. ESA’s account highlights Wilson’s description of LHS 1903 e as a planetary “late bloomer,” a phrase meant to capture the idea that it finished forming after most of the disk’s gas had already vanished.
This scenario contrasts with standard models that expect outer planets in compact systems to retain thick gaseous envelopes or to have migrated inward from colder regions while still wrapped in gas. A High-quality context piece quotes an anonymous astronomer who weighs competing explanations: one possibility is that LHS 1903 e once carried a substantial atmosphere that was later stripped away by a giant impact, while another is that it truly formed in a gas-depleted environment, as Wilson’s team suggests. The critic points out that both scenarios are still on the table and that distinguishing between them will require more detailed measurements of the planet’s composition and any residual atmosphere. For now, the gas-depleted explanation is favored in the Preprint because the data do not show the clear dynamical scars that a catastrophic impact might be expected to leave.
Expert Reactions and Stunned Responses
Reactions from astronomers quoted in the ESA release and follow-up reporting convey a sense of genuine surprise at how neatly LHS 1903 seems to invert expectations. Thomas Wilson is quoted describing the outer planet as a “late bloomer from another era,” language that reflects how its composition appears out of step with its position in the system. ESA’s account also notes that Maximilian Günther calls the configuration rare, framing LHS 1903 as a valuable outlier that can stress-test existing formation theories. For researchers used to seeing gas giants clinging tightly to their stars, the idea of a compact system where the outermost known world is rocky and gas-poor feels like a direct challenge to standard migration-based explanations.
Secondary coverage amplifies that sense of puzzlement. A detailed LiveScience explainer highlights how the rocky outer planet defies the expectation that bodies further from the star should be better at holding onto thick atmospheres, since they experience less intense radiation and stellar wind. Reporting described as Secondary context notes that the discovery team explicitly tested and largely ruled out a simple giant-impact explanation for LHS 1903 e’s lack of gas, as well as straightforward migration scenarios in which the planets would have swapped places after formation. Those checks leave the gas-depleted, late-formation picture as the leading hypothesis, although even its proponents acknowledge that it remains a working model rather than a proven history.
Uncertainties and Future Probes
Despite the excitement, the authors and outside commentators are clear about how much remains unknown about LHS 1903’s past. Disk dissipation models that would explain precisely when and how the gas vanished around LHS 1903 e have not yet been directly tested for this specific system, and the Scientific American analysis stresses that the evidence for a particular formation timeline is still thin. The giant-impact scenario, while disfavored by the discovery team, cannot be fully excluded without tighter constraints on the planet’s interior structure and any remaining atmosphere. In other words, the data clearly show a rocky outer planet where theory expects a gas-rich one, but they do not yet pin down the exact sequence of events that produced it.
Several experts quoted across the ESA and LiveScience coverage point to future observations as the key to resolving those uncertainties. One priority is to use instruments such as the James Webb Space Telescope to perform spectroscopy of LHS 1903 e during transits, searching for signatures of any tenuous atmosphere or escaping gas that could hint at its history. Wilson is quoted in the ESA release projecting that CHEOPS will continue to play a role by targeting similar systems and refining the parameters of LHS 1903’s planets with additional follow-up. If more “inside-out” architectures emerge from that work, astronomers will gain a statistical sample that can either support or overturn the idea that gas-depleted late bloomers are a common outcome of disk evolution.
Broader Implications for Exoplanet Hunting
The LHS 1903 system matters not only because it is unusual, but because it broadens the menu of possible planetary architectures that exoplanet hunters must consider. A Space.com report describes it as one of fewer than 10 known systems with such an inverted, rocky–gas–gas–rocky layout, a statistic that highlights both its rarity and its potential importance. If gas-depleted outer worlds like LHS 1903 e turn out to be more common than current surveys suggest, that could reshape theories of how super-Earths and mini-Neptunes form and evolve, especially near the radius valley identified in the Science analysis. Finding more examples would also help clarify whether our own solar system’s mix of rocky inner planets and gas giants further out is typical or an outlier in its own right.
There are also implications for the search for habitable environments. The LiveScience coverage notes that the presence of a rocky outer planet in a compact system raises fresh questions about where temperate, potentially life-friendly worlds might reside around small stars like LHS 1903. If rocky planets can form and survive in positions that standard models would have reserved for gas-rich mini-Neptunes, then the range of orbits worth scrutinizing for Earth-sized worlds may be broader than assumed. For readers wondering how our own planetary neighborhood came to be, LHS 1903 offers a provocative hint that solar systems can build themselves from the inside out, leaving behind late-blooming rocky worlds in places that once seemed off-limits.
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*This article was researched with the help of AI, with human editors creating the final content.
