Artist’s impression of the TOI-1130 system, showing a mini-Neptune (TOI-1130b) orbiting inside the orbit of a hot Jupiter (TOI-1130c).
For the first time, astronomers have measured the composition of the atmosphere of a planet orbiting within the orbit of a so-called hot Jupiter—and found that both worlds most likely formed far from their star and then gradually migrated closer to it. The results of the study were published in 2026 in the Astrophysical Journal Letters.
An unusual pair
The TOI-1130 star system is located 190 light-years from Earth. Back in 2025, researcher Chelsea Huang discovered a rare pair of planets there: a mini-Neptune and a hot Jupiter, which orbit their host star every four and eight days, respectively.
Hot Jupiters are considered solitary planets because their gravity typically pushes any other bodies out of their inner orbits. The very existence of such a duo immediately raised the question of how such a system could have come into being.
What the James Webb Space Telescope revealed
A team of scientists from the Massachusetts Institute of Technology (MIT) used NASA’s James Webb Space Telescope (JWST) to analyze the atmosphere of the mini-Neptune TOI-1130b. The telescope detects which wavelengths the planet absorbs, which makes it possible to determine the chemical composition of its atmosphere.
It turned out that it was rich in water vapor, carbon dioxide, sulfur dioxide, and traces of methane—molecules heavier than hydrogen and helium. Such a composition could not have formed in the vicinity of a star.
Icy cradle
According to scientists, the only explanation is that both planets formed far from the star, beyond the so-called frost line. This is the minimum distance from a star at which the temperature is low enough for water vapor to condense directly into ice.
In that cold region, ice particles settle onto dust grains, and the young planet gradually builds up a dense atmosphere. As the two bodies gradually began to move closer to the star, the ice evaporated—but the composition of the gaseous envelope remained unchanged.
“This is the first indication that such mini-Neptunes, formed beyond the ice line, actually exist in nature,” notes lead author Saugata Barat of the Kavli Institute for Astrophysics and Space Research at the Massachusetts Institute of Technology.
According to doi.org