Recent JWST observations of exoplanet GJ 3090b reveal intriguing qualities of one of the galaxy’s most abundant planet types.
Many Sub-Neptunes, Many Mysteries
Over the past 30 years, astronomers have discovered thousands of distant worlds. Within the growing exoplanet population, the sub-Neptune — a planet with a mass and radius between Earth’s and Neptune’s — has emerged as the most common planet type in the Milky Way. However, the composition and structure of these planets are not yet well understood. While sub-Neptunes orbiting Sun-like stars seem to separate into a small, rocky population and a larger, gaseous population, sub-Neptunes around cooler, lower-mass stars do not show such a clear division.
For sub-Neptunes around Sun-like stars, the population separation between rocky and gaseous planets is inferred from a dearth of planets with radii between 1.8 and 2 times the Earth’s radius (a gap dubbed the “radius valley”). This gap provides hints to the formation and evolutionary processes that shaped the planet, but with a murkier radius valley for sub-Neptunes around lower-mass stars, their origins are more unclear. Recent observations of sub-Neptunes have revealed a diverse range of atmospheric compositions, further bolstering the need to continue exploring the galaxy’s most prevalent planet type.
Planet equilibrium temperature versus planet radius plot showing where GJ 3090b falls with respect to the general sub-Neptune population. Click to enlarge. [Ahrer et al 2025]
JWST Observations of GJ 3090b
Aiming to add to the sample of well-characterized sub-Neptune atmospheres, Eva-Maria Ahrer (Max Planck Institute for Astronomy) and collaborators collected JWST spectroscopy of the sub-Neptune GJ 3090b. Previously discovered with the Transiting Exoplanet Survey Satellite (TESS), GJ 3090b orbits a late-type, low-mass star and has a radius that places it at the outer edge of the radius valley.
With the JWST spectroscopy, the authors search for a helium signature that traces how hydrogen and helium may be escaping the planet’s atmosphere. The authors detect a low-amplitude helium signature that suggests GJ 3090b has a metal-enriched atmosphere that slows down mass loss and weakens the helium feature. Additionally, the spectrum reveals the presence of heavy molecules like water, carbon dioxide, and sulfur dioxide, which points again to the planet’s atmosphere being metal enriched. What does this composition tell us about the planet?
JWST spectrum of GJ 3090b’s atmosphere. The spectrum is best fit with a high-metallicity model. Click to enlarge. [Modified from Ahrer et al 2025]
Exploring Metal Enrichment and Planet Structure
Given the makeup of GJ 3090b’s atmosphere, the authors explore various theoretical models to understand why this planet may have a high-metallicity atmosphere. If the planet formed past its system’s snow line, where it is cool enough for water to freeze, the planet could have accreted water ice laced with enriched material. As the planet migrated inwards, the heat from the star melted the ice, allowing the enriched material to escape into the atmosphere.
Alternatively, sub-Neptunes close to their host stars are expected to lose significant amounts of hydrogen and helium due to hydrodynamic effects. When these light elements escape the atmosphere, the remaining gas contains a higher fraction of metals as it is now less diluted with hydrogen and helium. Additionally, using structure models, the authors predict that GJ 3090b has a well-mixed core of rock and ice, further supporting the idea that it formed farther out where water could freeze and then migrated inward.
However, further high-resolution observations are needed to better constrain the true abundances of the metals in GJ 3090b’s atmosphere, which leaves the planet’s history and structure still a little hazy. With JWST opening the window for detailed atmospheric studies of exoplanets, more observations will allow astronomers to understand and characterize planets across the Milky Way.
Citation
“Escaping Helium and a Highly Muted Spectrum Suggest a Metal-enriched Atmosphere on Sub-Neptune GJ3090b from JWST Transit Spectroscopy,” Eva-Maria Ahrer et al 2025 ApJL 985 L10. doi:10.3847/2041-8213/add010