This strange planet pair shouldn’t exist, but it does

Across the Milky Way, about 190 light years from Earth, astronomers have identified a highly unusual planetary pairing. A massive hot Jupiter, a type of giant planet typically found alone, shares its system with a smaller mini-Neptune orbiting even closer to their star. This rare configuration has puzzled scientists since it was first discovered in 2020. Now, researchers at MIT have taken a closer look at the inner planet's atmosphere and uncovered new clues that help explain how this strange system formed. JWST Reveals a Heavy, Water-Rich Atmosphere In a study published in Astrophysical Journal Letters, the team used NASA's James Webb Space Telescope (JWST) to analyze the atmosphere of the mini-Neptune. This marks the first time scientists have measured the atmospheric composition of a mini-Neptune located inside the orbit of a hot Jupiter. The observations show that the planet's atmosphere is surprisingly dense and filled with heavier molecules, including water vapor, carbon dioxide, sulfur dioxide, and traces of methane. This type of atmosphere would be unlikely if the planet had formed close to its star, where lighter gases usually dominate. Instead, the findings suggest a very different origin. Planets Likely Formed Far From Their Star According to the researchers, both the mini-Neptune and the hot Jupiter probably formed much farther away from their star, in a colder region of the system's early disk of gas and dust. In that environment, icy material and volatile compounds could accumulate more easily, allowing the planets to build thicker, heavier atmospheres. Over time, the two planets likely migrated inward together, moving closer to their star while maintaining their atmospheres and their unusual orbital arrangement. The results provide the first clear evidence that mini-Neptunes can form beyond a star's "frost line," the distance at which temperatures are low enough for water to freeze into ice. "This is the first time we've observed the atmosphere of a planet that is inside the orbit of a hot Jupiter," says Saugata Barat, a postdoc in MIT's Kavli Institute for Astrophysics and Space Research and the lead author of the study. "This measurement tells us this mini-Neptune indeed formed beyond the frost line, giving confirmation that this formation channel does exist." The research team includes scientists from institutions around the world, including MIT, the Harvard and Smithsonian Center for Astrophysics, the University of South Queensland, the University of Texas at Austin, and Lund University. Mini-Neptunes are smaller than Neptune and are made mostly of gas surrounding a rocky core. They are actually the most common type of planet found in the Milky Way, even though none exist in our own solar system. In 2020, Chelsea X. Huang, then a Torres Postdoctoral fellow at MIT (now on the faculty at University of South Queensland), identified this unusual system. The mini-Neptune was found orbiting alongside a hot Jupiter, something astronomers rarely see. Using data from NASA's Transiting Exoplanet Survey Satellite (TESS), the team studied a star called TOI-1130 and detected both planets. The mini-Neptune completes an orbit every four days, while the hot Jupiter takes eight days. "This was a one-of-a-kind system," says Huang. "Hot Jupiters are 'lonely,' meaning they don't have companion planets inside their orbits. They are so massive, and their gravity is 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." The discovery led researchers to investigate the planets in more detail using JWST, focusing on the inner world known as TOI-1130b. However, observing the planet was not straightforward. Unlike most planets, which follow predictable orbital schedules, this pair is in what scientists call "mean motion resonance." Each planet's gravity slightly alters the other's orbit, making their movements less regular and harder to predict. To overcome this, a team led by Judith Korth of Lund University compiled previous observations and created a model to determine exactly when the planets would pass in front of their star in a way JWST could observe. "It was a challenging prediction, and we had to be spot-on," Barat says. Once the timing was right, JWST captured detailed data across multiple wavelengths of light. "The beauty of JWST is that it does not observe just in one color, but at different colors, or wavelengths," Barat explains. "And the specific wavelengths that a planet absorbs can tell you a lot about the composition of its atmosphere." The data revealed strong signatures of water, carbon dioxide, and sulfur dioxide, along with smaller amounts of methane. These heavier molecules contrast with the lighter hydrogen and helium typically expected in planets that form close to their stars. This finding challenges previous assumptions

This strange planet pair shouldn’t exist, but it does

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