A giant planet was supposed to have the place to itself. Instead, in a star system 190 light-years away, a hot Jupiter shares space with a much smaller world, a mini-Neptune tucked even closer to the star. That pairing is rare enough on its own. Now, a fresh look at the smaller planet’s atmosphere is giving astronomers a clue to how both worlds may have ended up there. Using the James Webb Space Telescope, a team led by MIT examined the atmosphere of TOI-1130 b, the inner planet in the unusual two-planet system. What they found was not a light, hydrogen-and-helium envelope of the kind often expected for a close-in planet. Instead, the atmosphere appears rich in heavier molecules, including water vapor, carbon dioxide, and sulfur dioxide, along with a tentative hint of methane. That mix matters because TOI-1130 b now circles very close to its star, completing an orbit in a little more than four days. The outer planet, TOI-1130 c, a hot Jupiter, circles every 8.35 days. The team argues that the smaller …