Researchers using the James Webb Space Telescope (JWST) have discovered that “mini-Neptune” exoplanets may hide vast quantities of water beneath hydrogen-rich atmospheres. Recent studies of planets TOI-270 d and TOI-1130 b suggest these worlds often form far from their stars before migrating inward, challenging previous assumptions about planetary composition.
Sub-Neptunes are the most common type of planet in the Milky Way, with nearly 3,300 of the more than 6,300 confirmed exoplanets fitting this description. Despite their prevalence, they lack a direct analog in our own solar system, and their thick, hazy atmospheres have historically shielded their true compositions from observers.
Hidden Oceans on TOI-270 d
A study published in The Astrophysical Journal, led by the University of Chicago, utilized computer models to analyze TOI-270 d, a sub-Neptune located approximately 73 light-years away. The planet, which orbits a red dwarf star every 11.4 days, possesses a mass 4.2 times that of Earth and a radius roughly twice as large. While JWST previously detected methane, carbon dioxide, and hydrogen in its atmosphere, the physical state of the water remained a mystery.
The research team found that a specific combination of temperature and the water-hydrogen ratio determines whether these elements mix or separate. Because TOI-270 d reaches temperatures of 537 degrees Celsius (1,000 degrees Fahrenheit) and contains more water than hydrogen, the water may actually sink beneath the hydrogen layer. This layering effect creates a shield that hides deep-level water from telescope observations.
This finding contradicts long-standing hypotheses that sub-Neptunes possess well-mixed interiors. According to the research led by UChicago, this suggests that current data from powerful telescopes must be interpreted with more nuance to accurately determine what is happening beneath the haze.
The Unusual Pairing of TOI-1130 b and c
While TOI-270 d reveals the mystery of hidden water, the TOI-1130 system—located 190 light-years away—challenges the rules of planetary architecture. In this system, a mini-Neptune (TOI-1130 b) orbits inside the path of a “hot Jupiter” (TOI-1130 c). This arrangement is rare because the massive gravity of hot Jupiters typically scatters any smaller inner companion planets away.
The inner planet, TOI-1130 b, completes an orbit in just over four days, while the outer giant orbits every 8.35 days. The two worlds exist in a 2:1 mean motion resonance, meaning the inner planet orbits twice for every one orbit of the outer giant. This lockstep pattern suggests the planets migrated slowly together through the protoplanetary disk rather than experiencing a violent history.
“Hot Jupiters are ‘lonely,’ meaning they don’t have companion planets inside their orbits. They are so massive, and their gravity is so strong, that whatever is inside their orbit just gets scattered away. But somehow, with this hot Jupiter, an inner companion has survived.”
Chelsea X. Huang, University of South Queensland
Atmospheric Weights and the Frost Line
Using NIRSpec and NIRISS instruments in August 2024, an MIT-led team analyzed the atmosphere of TOI-1130 b. They found an atmosphere rich in heavier molecules, including sulfur dioxide, carbon dioxide, and water vapor, with a tentative hint of methane. This “heavy” atmosphere has a mean molecular weight of about 5.5 atomic mass units, placing it between a light hydrogen-helium envelope and a heavy steam-dominated one.
This chemistry serves as a forensic clue to the planet’s origin. Because the atmosphere is too volatile-rich to have formed in its current position—where solid material is mostly rock—the team argues that both planets likely formed beyond the system’s frost line, where water could condense into ice, before moving inward.
“This measurement tells us this mini-Neptune indeed formed beyond the frost line, giving confirmation that this formation channel does exist.”
Saugata Barat, postdoc in MIT’s Kavli Institute for Astrophysics and Space Research
Capturing this data required precise timing. Because the planets gravitationally tug on one another, transit timings can vary by up to five hours. To succeed, researchers combined data from TESS, CHEOPS, and ground-based telescopes to refine their predictions.
Future Observation Capabilities
The ability to detect these compositions relies on the JWST’s capacity to observe across different wavelengths. As Saugata Barat noted, the specific wavelengths a planet absorbs reveal the composition of its atmosphere
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The search for these watery, distant worlds will expand with upcoming hardware.
- Nancy Grace Roman Space Telescope: Slated for launch on August 30, 2026, this NASA mission will use a coronagraph to directly image exoplanets.
- Extremely Large Telescope (ELT): An ESO ground-based facility currently under construction, expected to begin scientific operations around December 2030.
These discoveries collectively suggest that our current understanding of sub-Neptunes is incomplete. By identifying that water can be hidden deep within a planet or that “lonely” hot Jupiters can have survivors in their orbit, astronomers are finding that the most common planets in the galaxy are also the most deceptive.
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