Exoplanet Duo's 'Cold Start' Migration: First Atmosphere Data Revealed

Astronomers have uncovered a rare planetary system where a hot Jupiter and a smaller mini-Neptune have coexisted by migrating inward from a colder region of their star system. The research, which utilized data from the James Webb Space Telescope (JWST), offers the first detailed atmospheric analysis of a planet orbiting inside a hot Jupiter's path, shedding light on how such unlikely companions can form and survive.

The TOI-1130 system, located approximately 190 light-years from Earth, features a gas giant known as TOI-1130c that orbits its star extremely closely. Typically, hot Jupiters are solitary giants, as their immense gravity tends to eject any smaller planets that form or migrate too near. However, TOI-1130c appears to have maintained a companion, a mini-Neptune designated TOI-1130b, about three-and-a-half times Earth's diameter. This unusual pairing suggests a shared journey inward from the outer reaches of their protoplanetary disk.

Chelsea Huang of the University of Southern Queensland first identified the peculiar TOI-1130 system in 2020 while examining data from NASA's Transiting Exoplanet Survey Satellite (TESS). "This was a one-of-a-kind system," Huang stated. "Hot Jupiters are 'lonely', meaning that they don't have companion planets inside their orbits. They are so massive and their gravity so strong, that whatever is inside their orbit just gets scattered away. But somehow, with this hot Jupiter, an inner companion has survived, and that raises questions about how such a system could form."

An international team, led by Saugata Barat of MIT and including Huang, employed the advanced capabilities of the JWST to investigate TOI-1130b's atmosphere. By observing the starlight filtered through the mini-Neptune's atmosphere as it transited its star, researchers identified specific wavelengths of light being absorbed. This absorption pattern indicated the presence of a dense atmosphere rich in water vapor, carbon dioxide, sulfur dioxide, and methane. Crucially, the atmosphere is considered "heavy" not because it lacks lighter elements like hydrogen and helium, but because these heavier molecules are abundant, suggesting formation in a region where water could freeze.

Forming Beyond the Frost Line

The composition of TOI-1130b's atmosphere provides compelling evidence that both planets originated beyond the so-called "frost line" or "snow line." This is the orbital distance from a young star where temperatures are low enough for volatile compounds, such as water, to condense into ice. "This is the first time we've observed the atmosphere of a planet that is inside the orbit of a hot Jupiter," said Barat. "This measurement tells us this mini-Neptune indeed formed beyond the 'frost line'." The implication is that both the hot Jupiter and its mini-Neptune companion formed together in this frigid zone before migrating inward as a pair, rather than the hot Jupiter forming close to the star and later disrupting any inner planets.

The survival of TOI-1130b alongside the massive TOI-1130c is attributed to their gravitational dance. The planets are locked in a 2:1 orbital resonance, meaning the mini-Neptune completes two orbits for every single orbit of the hot Jupiter. This stable, synchronized movement likely prevented the larger planet from gravitationally ejecting its smaller companion. In their current configuration, TOI-1130b orbits its star every four days at an average distance of 4.2 million miles, experiencing temperatures around 1,025 degrees Fahrenheit (550 degrees Celsius). TOI-1130c orbits every eight days at 6.8 million miles, with temperatures reaching approximately 930 degrees Fahrenheit (500 degrees Celsius).

Observing this system presented significant challenges for the research team. The gravitational tug-of-war between the two planets causes transit timing variations (TTVs), meaning the precise moment of a planet's transit across its star can shift. With limited observation time on the JWST, accurately predicting these transits was critical. Judith Korth of Lund University in Sweden developed a sophisticated model based on prior system observations to pinpoint the exact timing for the JWST observations. "It was a challenging prediction and we had to be spot on," Barat noted.

The successful observation not only clarified the unique architecture of the TOI-1130 system but may also provide insights into the formation of other mini-Neptunes found in close proximity to their stars. "This system represents one of the rarest architectures that astronomers have ever found," Barat summarized. "The observations of TOI-1130b provide the first hint that such mini-Neptunes that form beyond the water/ice lines are indeed present in nature." The findings suggest that shared migration from cold regions can be a viable pathway for forming companion planets in systems dominated by hot Jupiters.