Astronomers have used NASA’s James Webb Space Telescope (JWST) to analyze the atmosphere of a rare mini-Neptune orbiting inside the orbit of a hot Jupiter star system 190 light years away. The findings offer the first direct evidence that these two unusual planets likely formed far from their star beyond the frost line, before migrating inward while retaining their atmospheres.
The planetary system, called TOI-1130, was originally discovered in 2020 using NASA’s Transiting Exoplanet Survey Satellite (TESS). It consists of a mini-Neptune orbiting every four days and a hot Jupiter orbiting every eight days—a rare pairing since hot Jupiters typically lack close companion planets.
By capturing detailed spectroscopic data during the mini-Neptune’s transit using JWST, the researchers identified an atmosphere rich in water vapor, carbon dioxide, sulfur dioxide, and traces of methane. These heavier molecules indicate a “heavy” atmosphere, which contradicts expectations for a planet that formed in its current close-in, hot orbit, where lighter hydrogen- and helium-dominated atmospheres would be expected.
The atmospheric composition suggests the mini-Neptune formed in the colder, outer region of the protoplanetary disk where ices could accumulate—beyond the frost line, the distance from the star where temperatures are low enough for water to freeze. Over time, both the mini-Neptune and its hot Jupiter companion likely migrated inward through a slow process that preserved their atmospheres, arriving in their current orbits still closely bound.
This finding is the first observational confirmation that mini-Neptunes can form beyond a star’s frost line and later move close to their host star. It also challenges assumptions about hot Jupiter systems being “lonely,” as this system features a stable close-orbiting companion.
The team, led by researchers from MIT’s Kavli Institute for Astrophysics and Space Research, Lund University, and other international institutions, faced challenges in timing the JWST observations due to the gravitational interactions causing orbital period variations between the two planets. Their success offers a rare glimpse into the formation and migration history of exoplanets in complex systems.
Why it matters
The study provides key insights into planet formation and migration, demonstrating that mini-Neptunes with substantial atmospheres can originate in icy outer regions and later migrate closer to their stars. This challenges prior models of planetary system evolution and influences how astronomers interpret exoplanet population data across the Milky Way. It also expands understanding of the diversity of planetary system architectures, especially those containing hot Jupiters and close-in companion planets.
Background
Mini-Neptunes are gas dwarf exoplanets smaller than Neptune but common throughout the galaxy, though absent in our solar system. Hot Jupiters are giant gas planets orbiting very close to their stars and are known for their strong gravitational influence, often clearing out other planets in their orbital vicinity. The discovery of a mini-Neptune orbiting inside a hot Jupiter’s orbit is highly unusual, prompting detailed study of their origins.
NASA’s TESS first identified the TOI-1130 system in 2020, revealing its unique planetary configuration. Follow-up studies with JWST enabled the detailed atmospheric characterization that supported the migration hypothesis, marking a milestone in exoplanetary science.
Sources
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