A steam-filled atmosphere detected on a distant 'hot Neptune' has provided strong new evidence that some planets form far from their host star before migrating inward – a long-debated process in planetary science.
University of Southern Queensland (UniSQ) researchers played a key role in the discovery as part of an international team that included collaborators from the Massachusetts Institute of Technology (MIT) and Harvard University.
UniSQ Associate Professor Chelsea Huang led observations of the atmosphere of TOI-1130 b, 190 light-years from Earth, as part of NASA's James Webb Space Telescope program, offering rare insight into how planetary systems form and evolve.
The planet's atmosphere – rich in water vapour – reveals it formed well beyond its star's "water ice line", where water can freeze, before moving into a much closer orbit.
"The observation of TOI-1130 b provides some of the clearest direct atmospheric evidence yet that planets have migrated," Associate Professor Huang said.
"The large amount of water in the atmosphere is a direct indicator that the planet didn't form at its current location.
"The only way for the planet to accumulate a quarter of its atmosphere in water is to obtain it in ice form at a much colder place far away from its star."
TOI-1130 b is similar in size to Neptune but orbits its host star every four days, with surface temperatures exceeding 600 degrees. Despite these extreme conditions, researchers found around a quarter of its upper atmosphere is composed of water vapour.
The Webb observations detected strong absorption features from water, along with carbon dioxide and sulphur dioxide, and trace amounts of methane – providing a detailed picture of the planet's atmospheric composition.
The findings help resolve a key question in exoplanet science: whether planets found close to their stars form in place or migrate inward after forming further out.
The TOI-1130 system also presents an unusual configuration, with two planets of very different sizes orbiting in close proximity, including a Jupiter-sized planet just beyond TOI-1130 b.
"The planets gravitationally tug on each other, causing their orbits to slowly oscillate over time," Associate Professor Huang said.
"We recognised they would be great targets for Webb, and it was exciting to be awarded observing time."
The study builds on years of international collaboration. Data analysis and publication were led by MIT postdoctoral researcher Dr Saugata Barat, who is also an adjunct research fellow at UniSQ, working under the supervision of Associate Professor Huang and Harvard Assistant Professor Andrew Vanderburg.
The TOI-1130 system was first identified in 2020 using NASA's TESS telescope by Associate Professor Huang and Professor Vanderburg.
Ongoing observations from the European Space Agency's CHEOPS satellite and a global network of ground-based telescopes – including UniSQ's Minerva-Australis array – helped refine the planet's orbit and secure the narrow Webb observation window.
"The observation window can drift many hours from year to year, but we had just one shot from Webb," Associate Professor Huang said.
"We caught the transit right in the middle of our scheduled Webb window. It was such a relief when we saw the detection."
The findings have been published in The Astrophysical Journal Letters, with co-authors from UniSQ – Associate Professor Chelsea Huang, Dr George Zhou, Dr Duncan Wright and Emma Nabbie – alongside collaborators from MIT, Harvard University, the University of Chicago, University of Texas, Smithsonian Institution, University of Geneva, Lund University and a global network of astronomers.
The team will now turn their attention to analysing the atmosphere of the second planet in the system, with further insights expected.
The study of the TOI-1130 system was funded by the Australian Research Council as part of Associate Professor Huang's Future Fellowship, and is the only medium-sized Webb observation program awarded to the Australian exoplanet research community to date.
