Hot Jupiters, those gas giants that orbit very close to their star, were thought to sweep away any other planet during their migration. Yet the TOI-1130 system, located 190 light-years away, contradicts this rule: one of them, TOI-1130c, shares its intimate space with a smaller planet, a mini-Neptune named TOI-1130b.
This unexpected cohabitation surprised astronomers when it was discovered in 2020 by Chelsea Huang, then at MIT, using data from NASA's TESS satellite. "This system is unique in its kind," Huang explained. "Hot Jupiters are considered solitary: their gravity is so strong that any planet located inside their orbit is ejected. Yet here, an inner companion has survived."
Artist's impression of the mini-Neptune TOI-1130b and its hot Jupiter companion TOI-1130c, both formed far from their star before migrating inward.
Credit: Jose-Luis Olivares, MIT To understand this puzzle, an international team led by Saugata Barat of MIT pointed the James Webb Space Telescope at the mini-Neptune during its transit in front of its star. By analyzing the starlight filtered through its atmosphere, they discovered a surprising composition: water vapor, carbon dioxide, sulfur dioxide, and methane. These "heavy" elements (heavier than hydrogen and helium) indicate that the mini-Neptune did not form close to its star, but well beyond the "ice line," the boundary where the temperature allows water to remain frozen.
"This measurement confirms that the mini-Neptune formed beyond the ice line." The two planets would therefore have migrated together, held by a gravitational resonance that prevents them from colliding. Currently, the mini-Neptune completes its orbit in 4 days, while the hot Jupiter takes 8 days, in a 2:1 ratio.
Observing this duo was not easy. Their mutual attractions cause transit variations, making predictions of their passages tricky. The team had only one shot with JWST. "We had to aim just right," Barat recounted. Thanks to a model developed by Judith Korth of Lund University, based on past observations, they were able to perfectly adjust their measurements.
This discovery, published in
The Astrophysical Journal Letters, could explain the existence of other mini-Neptunes close to their star. It shows that planets formed beyond the ice line can survive and retain an atmosphere rich in heavy elements, even in the vicinity of a hot Jupiter.