This Far-Away Mini Neptune Next to a Hot Jupiter Shouldn’t Exist. Its Atmosphere May Explain Why

The planetary odd couple has puzzled astronomers since their 2020 discovery.

Approximately 190 light-years from Earth, two planets circle the star TOI-1130 in a setup astronomers rarely see. One is a hot Jupiter, a giant planet skimming close to its star. The other is a smaller gas-rich mini-Neptune traveling even closer in.

However, the pairing is a bit strange as hot Jupiters generally live alone. Their gravity is so strong that nearby planets often get scattered, swallowed or pushed into unstable paths. Yet in this system, the inner mini-Neptune survived, and it has puzzled astronomers since its discovery in 2020.

Now, astronomers using NASA’s James Webb Space Telescope say they may have found out why. Their observations suggest both planets probably formed farther from their star, beyond a cold region known as the frost line, and then slowly moved inward together. The findings appear in Astrophysical Journal Letters.

“Hot Jupiters are ‘lonely,’ meaning they don’t have companion planets inside their orbits,” said Chelsea X. Huang, who discovered the system in 2020 using NASA’s Transiting Exoplanet Survey Satellite. “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.”

Mini-Neptunes are usually thought to contain rocky cores wrapped in thick atmospheres. While our own Solar System does not have one, they are common around other stars. Finding one tucked inside a hot Jupiter’s orbit is rare.

Reading the Air of a Distant World

To investigate the system, the team used Webb to study the atmosphere of the mini-Neptune, TOI-1130b. It was an uneasy task. The two planets are in what astronomers call a mean motion resonance, meaning they tug on each other’s orbits in a regular pattern. Those tugs make the timing of each transit slightly irregular.

The team had to model years of earlier observations to predict when Webb could catch the mini-Neptune passing in front of its star.

During that crossing, a small amount of starlight filtered through the planet’s atmosphere. Webb measured which wavelengths of light were absorbed. Different molecules absorb different wavelengths, leaving chemical fingerprints.

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“The beauty of JWST is that it does not observe just in one color, but at different colors, or wavelengths,” said lead author Saugata Barat, a postdoc at MIT’s Kavli Institute for Astrophysics and Space Research. “And the specific wavelengths that a planet absorbs can tell you a lot about the composition of its atmosphere.”

The atmosphere contained several heavier molecules, including water vapor, carbon dioxide, sulfur dioxide and traces of methane. That chemical mix gave the team an important clue.

If TOI-1130b had formed close to its star, the researchers would have expected a different atmospheric pattern. Instead, the abundance of heavier molecules points to material gathered in a colder region farther out in the system.

A Journey from the Cold

The evidence suggests both planets likely formed beyond the star’s frost line. Past that boundary, temperatures are low enough for water vapor to freeze onto dust grains, creating icy solids that young planets can collect.

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As TOI-1130b grew, it may have swept up those icy materials. Later, after the planet moved closer to its star, that ice would have warmed and turned into vapor, leaving behind the chemical fingerprints Webb detected.

The same slow inward migration could explain why the mini-Neptune survived beside the hot Jupiter. A sudden gravitational shove might have scattered the smaller planet or thrown the system into chaos. A slower drift inward would have allowed both planets to move together while keeping their orbits stable.

“This is the first time we’ve observed the atmosphere of a planet that is inside the orbit of a hot Jupiter,” Barat said. “This measurement tells us this mini-Neptune indeed formed beyond the frost line, giving confirmation that this formation channel does exist.”

The result adds a new wrinkle to how scientists think about close-in mini-Neptunes. Many astronomers had assumed these planets formed near where they are found today. TOI-1130b suggests that at least some may have taken a longer route, forming in colder regions before migrating inward over millions of years.

“This system represents one of the rarest architectures that astronomers have ever found,” Barat said.

Some of the galaxy’s most common planets, it turns out, may have begun in some of its coldest places.