Lead: Astronomers announced on 12 February 2026 the discovery of an unusual four-planet system around the red dwarf LHS 1903, located about 116 light-years from Earth. Using data from NASA’s TESS and ESA’s Cheops plus ground-based follow-up, the team found an unexpected sequence: a small rocky world close in, two gas-rich planets at intermediate distances, and a distant outer planet that is rocky. The outer planet, LHS 1903 e, is a roughly 1.7 Earth-radius super-Earth — a configuration that conflicts with the standard model in which rocky planets form inside and gas giants form outside a star’s snow line. Researchers report their interpretation in a paper published this week in the journal Science and say the system may require rethinking how planets form around the galaxy’s most common stars.
Key takeaways
- Distance and discovery: LHS 1903 lies about 116 light-years away; the system was identified with TESS and characterized with ESA’s Cheops (NASA/ESA missions, launched 2018 and 2019 respectively).
- Architecture: Four planets orbit the M-dwarf in an order of rocky–gaseous–gaseous–rocky, with the outermost, LHS 1903 e, being a 1.7 R⊕ super-Earth.
- Formation tension: This “inside-out” ordering contradicts the typical pattern tied to the protoplanetary disk’s snow line and the rapid core growth that leads to gas giants beyond it.
- Ruling out collisions and stripping: Dynamical simulations by the discovery team indicate giant impacts or envelope loss are unlikely to produce the observed architecture.
- Proposed mechanism: The authors favor a gas-depleted, sequential formation scenario in which planets form from the inside out as the disk’s gas supply dwindles.
- Implications for M-dwarfs: If confirmed, the system provides a counterexample that will force planet-formation models for low-mass stars to account for very different formation timelines.
- Follow-up prospects: LHS 1903 e may be within reach of detailed atmospheric study with observatories like the James Webb Space Telescope, offering a test of its composition and formation history.
Background
Planet-formation theory has long linked a star’s temperature gradient and the disk’s snow line to the distribution of planet types. Close to a young star, high temperatures vaporize volatile compounds, leaving refractory materials such as silicates and iron to coagulate into rocky cores; farther out, ices can condense and speed core growth, enabling rapid accumulation of hydrogen and helium when cores exceed roughly 10 Earth masses. That dichotomy explains why the Solar System’s four small rocky planets sit inside and the gas giants lie outside.
Red dwarf stars (M dwarfs) like LHS 1903 are the most abundant stellar type in the Galaxy and have long been known to host compact planetary systems. But relative to Sun-like stars, their lower luminosity pushes the snow line closer in, altering where ices and gas are available during planet formation. Observational surveys over the past decade have shown many compact M-dwarf systems with mixed planet sizes, but the clear reversal — a distant rocky world sitting beyond gas-rich neighbors — is novel enough to challenge standard expectations.
Main event
The system was first flagged by TESS, which detects transiting planets by measuring periodic dips in starlight. The candidate configuration was then followed up with ESA’s Cheops to refine planet radii and transit timing, and by an international network of ground-based facilities to validate the signals. The combined dataset resolved four transiting planets around LHS 1903 and allowed precise determination that the outer planet is denser and smaller than its two inner gaseous neighbors.
Lead author Thomas Wilson (University of Warwick) and colleagues performed dynamical simulations to test whether collisions, tidal stripping, or late-stage envelope loss could explain the outer planet’s rockier nature. According to the team, those scenarios could not consistently reproduce the observed masses, separations and long-term system stability. The simulations included numerous configurations and impact histories but failed to recreate the system without invoking an unlikely fine tuning of events.
After excluding those pathways, the authors propose a gas-depleted, sequential formation model: the inner planets formed first when disk gas was abundant; the middle two accreted more gas and became mini-Neptunes, while the outermost formed later, when the disk had already lost much of its gas. In that late stage there was insufficient gas to form a hydrogen-helium envelope, leaving a rocky super-Earth at a larger orbital distance — effectively an “inside-out” formation order.
Analysis & implications
If the authors’ interpretation holds, LHS 1903 demonstrates that planet formation is more temporally variable than models that assume concurrent core formation predict. Disk lifetimes, local solid surface density, pebble drift and migration rates can all differ from system to system; a late-forming outer core in a gas-depleted environment offers one clear route to produce the observed architecture. Modelers will need to incorporate time-dependent disk depletion and stochastic growth histories to see how common such outcomes might be.
The result is particularly important for M-dwarfs because their lower luminosity and closer-in snow lines change the radial structure where volatile-rich solids and gas are available. If sequential, inside-out growth is efficient around many low-mass stars, population-level predictions for planet radii and atmospheric retention around M-dwarfs will shift — with consequences for habitability estimates and target prioritization for atmospheric characterization.
Observationally, LHS 1903 e presents a high-value target. A confirmed rocky composition and a measurable atmosphere (or lack thereof) would discriminate between formation scenarios: a secondary, thin atmosphere would support late, gas-poor formation; a retained thick hydrogen envelope would contradict the gas-depleted hypothesis. JWST and next-generation ground-based spectrographs could constrain atmospheric scale height and composition if transit spectroscopy is feasible given the system’s brightness and transit geometry.
Comparison & data
| System | Typical inner/outer type | Notable numbers |
|---|---|---|
| Solar System | Inner rocky, outer gas giants | Inner planets small; gas giants >10 M⊕ formed early |
| LHS 1903 | Rocky, gas-rich, gas-rich, rocky (outer) | Distance ≈116 ly; LHS 1903 e ≈1.7 R⊕ |
The table highlights the contrast between the canonical Solar System ordering and LHS 1903’s unusual sequence. While the Solar System’s giants reached runaway gas accretion early, LHS 1903’s architecture suggests a staggered timeline with varying local gas availability. The core mass threshold of ~10 Earth masses for runaway gas accretion remains a key benchmark in these comparisons.
Reactions & quotes
Wilson, the study’s first author, framed the system as a direct challenge to prevailing formation intuition. He emphasized that the outer rocky planet is hard to reconcile with standard models without invoking a late, gas-poor formation epoch.
“This is the first time we see a rocky planet so far out after gas-rich neighbors — it shouldn’t happen under the standard picture,”
Thomas Wilson, University of Warwick (study lead)
Coauthor Sara Seager described the result as an intriguing but still-debatable interpretation that will drive further work. Independent experts welcomed the data point while urging caution until atmospheric measurements or mass determinations tighten the constraints.
“New discoveries remind us how much we still must learn about building planetary systems,”
Sara Seager, MIT (coauthor)
External commentators stressed the system’s value as a laboratory. Heather Knutson (Caltech), who was not part of the team, highlighted LHS 1903 e as an excellent JWST target to probe atmospheres and test formation scenarios observationally.
“Planet e could host several kinds of atmospheres and may be cool enough for condensates — a compelling JWST target,”
Heather Knutson, Caltech (external scientist)
Unconfirmed
- Atmospheric composition of LHS 1903 e is not yet measured; the presence of water or secondary volatiles remains speculative.
- While the gas-depleted, inside-out formation scenario fits current data, alternative pathways (e.g., atypical migration histories) have not been exhaustively excluded.
Bottom line
LHS 1903 offers a rare and robust data point that challenges conventional expectations about where rocky and gaseous planets should form. The discovery team’s gas-depleted, sequential-formation hypothesis is plausible and grounded in dynamical tests, but it rests on interpretations that future mass and atmospheric measurements must confirm.
For planet-formation theory and exoplanet surveys, this system underscores the need for models that include time-dependent disk evolution and stochastic growth. Observational follow-up — particularly precise mass determinations and transit spectroscopy with facilities like JWST — will be decisive in determining whether LHS 1903 is an outlier or the first-known example of a common, previously overlooked formation pathway.
Sources
- CNN — news report on the discovery (news).
- Science (AAAS) — peer-reviewed journal hosting the study (peer-reviewed).
- TESS (MIT/NASA) — mission overview and instrument documentation (official mission page).
- CHEOPS (ESA) — mission overview and instrument documentation (official mission page).