Lead: Astronomers report a newly characterized exoplanet, L 98-59 d, 35 light-years away, that combines a global molten mantle with a sulfur-rich atmosphere. Published in Nature Astronomy, the study—led by researchers at the University of Oxford—finds the roughly 1.6-times-Earth-sized world hosts a persistent magma ocean that stores large sulfur inventories. Observations from the James Webb Space Telescope and advanced evolutionary models together indicate hydrogen sulfide and related sulfur gases persist in the atmosphere, forcing a rethink of how small planets evolve and retain volatiles.
Key Takeaways
- L 98-59 d lies 35 light-years from Earth and measures about 1.6 times the size of Earth, placing it outside standard small-planet categories.
- The discovery appears in Nature Astronomy and is led by the University of Oxford; simulations track the planet’s evolution over roughly 5 billion years.
- JWST spectroscopy indicates sulfur-bearing gases, including hydrogen sulfide, in the atmosphere—molecules associated with a rotten-egg odor on Earth.
- Models show a global magma ocean thousands of kilometers deep that acts as a reservoir, storing sulfur and moderating atmospheric loss from stellar X-ray irradiation.
- The planet’s low bulk density combined with the molten interior produces a novel class of sulfur-rich rocky worlds not represented in the solar system.
- Researchers infer L 98-59 d likely started as a larger, volatile-rich body and lost mass over time while retaining sulfur in its deep magma.
Background
Planet classification has traditionally separated small exoplanets into rocky terrestrial types, water-rich worlds, and low-density mini-Neptunes with significant volatile envelopes. That framework helps interpret transit and mass measurements but relies on assumptions about how atmospheres evolve and escape under stellar irradiation. L 98-59 d challenges that tidy picture: its combination of relatively low density, a retained volatile signature, and a molten subsurface does not fit neatly into existing bins. The host star’s X-ray and ultraviolet output was expected to drive strong atmospheric loss for close-in small planets, but a deep magma reservoir can sequester volatiles and release them more slowly, altering escape timelines.
Previous studies have modeled magma oceans on young rocky planets and their role in early volatile chemistry, but observational confirmation beyond our solar system has been scarce. The new work synthesizes JWST spectral detections with long-term thermal and compositional evolution models, providing a coherent narrative for how a magma ocean could both store and feed sulfur gases into the atmosphere. That interplay—between interior storage and atmospheric chemistry—creates planetary states that were underrepresented in formation and evolution theories to date.
Main Event
Using transit spectroscopy from JWST, the team identified spectral signatures consistent with sulfur-bearing molecules, notably hydrogen sulfide, in the transmission spectrum of L 98-59 d. Those spectral hints were combined with constraints on radius and mass that place the planet at about 1.6 times Earth’s size and with a lower-than-expected bulk density for a purely rocky world. To reconcile those data, researchers ran coupled thermal and chemical evolution models spanning roughly five billion years of the planet’s history.
The simulations indicate L 98-59 d sustained a global silicate magma ocean beneath its atmosphere for an extended period. That molten interior can dissolve and retain sulfur species that would otherwise be lost, then slowly outgas them to maintain a sulfur-rich atmosphere. In this scenario, stellar X-ray-driven escape removes lighter volatiles preferentially, while sulfur remains buffered by the deep melt.
Lead author Dr. Harrison Nicholls (University of Oxford) summarized the implication concisely: the team argues current small-planet categories may be too simple. Co-author Professor Raymond Pierrehumbert added that combining high-precision JWST spectra with forward models allows researchers to infer interior states—something once thought purely speculative for distant worlds.
Analysis & Implications
The finding expands the known parameter space of exoplanet types by introducing sulfur-rich, magma-ocean planets as a distinct outcome of planetary evolution. If magma oceans can sequester volatiles and later feed them into atmospheres, the survival times and composition of atmospheres for many close-in small planets may be substantially different from prior estimates. That affects how scientists interpret transmission spectra and mass–radius relationships when inferring composition.
For atmospheric escape theory, the result highlights a mechanism—interior buffering—by which heavier or chemically reactive species persist despite high-energy stellar flux. Models of atmospheric evolution often assume a monotonic loss of volatiles; L 98-59 d suggests interior–atmosphere exchange can produce long-lived, chemically unusual envelopes. This has downstream consequences for interpreting potential biosignatures on other worlds: the mere presence of certain gases no longer implies surface habitability without understanding interior processes.
Observationally, the result motivates targeted follow-up: deeper JWST observations across additional bands, phase-curve measurements, and complementary high-resolution spectroscopy could refine abundances and search for temporal variability tied to outgassing. On the theoretical side, planetary formation and evolution models must better incorporate prolonged magma oceans and multi-component volatile chemistry to predict how common such sulfur-rich planets might be.
Comparison & Data
| Property | L 98-59 d | Earth | Typical Small Exoplanet |
|---|---|---|---|
| Distance from Earth | 35 light-years | 0 ly | Varies |
| Size | ≈1.6× Earth | 1× Earth | 1–3× Earth |
| Surface/interior state | Global magma ocean beneath atmosphere | Solid crust, molten core | Rocky or volatile-rich |
| Atmospheric signature | Sulfur gases (incl. H2S) | N2-O2; traces | H2, H2O, CO2, or thin |
The table places L 98-59 d in context: its radius and distance are measured quantities, while the magma ocean and sulfur-rich atmosphere come from the combined interpretation of JWST spectroscopy and evolution models. These comparisons show why the object resists simple classification and why interior chemistry matters when deducing atmospheric histories.
Reactions & Quotes
“This discovery suggests the categories astronomers currently use to describe small planets may be too simple.”
Dr. Harrison Nicholls, University of Oxford (lead author)
Dr. Nicholls framed the result as a prompt to broaden classification schemes. The team emphasizes that a planet’s current radius and mass do not tell the whole story without models that include prolonged interior–atmosphere exchange.
“We can use computer models to probe the hidden interior of planets we will never visit, revealing types with no counterpart in our solar system.”
Prof. Raymond Pierrehumbert, University of Oxford (co-author)
Professor Pierrehumbert highlighted how modeling and observations together provide indirect but robust constraints on internal states, a growing capability in exoplanet science.
“Our simulations show how deep magma reservoirs can retain sulfur inventories that would otherwise escape under strong stellar X-rays.”
Dr. Richard Chatterjee, University of Leeds (co-author)
Independent experts caution that the interpretation depends on model assumptions and that further observations are needed to confirm abundance estimates.
Unconfirmed
- The exact atmospheric abundances of sulfur species on L 98-59 d remain model-dependent and require additional spectral coverage to confirm.
- Whether the planet’s atmosphere produces an odor analogous to rotten eggs is hypothetical—smell detection is not possible remotely and depends on local chemistry and concentration.
- The long-term stability of the magma ocean and its sulfur inventory over geological timescales is inferred from models but not directly observed.
Bottom Line
L 98-59 d represents a demonstrable expansion of known planetary outcomes: a relatively small world that retains a sulfur-rich atmosphere through deep interior storage and prolonged outgassing. The combined JWST observations and multi-billion-year evolution models argue for a new category of sulfur-buffered, magma-ocean planets that evade neat classification schemes derived from solar-system examples.
The finding reshapes how scientists should read transmission spectra and interpret mass–radius data for close-in small planets. Confirming the prevalence of such worlds will require targeted JWST follow-ups, different observational techniques, and more sophisticated models that integrate interior chemistry with atmospheric escape dynamics.