Webb telescope detects water vapor on a planet outside the solar system smaller than Neptune

Researchers announced that NASA’s James Webb space telescope captured an unusual snapshot of a small, gassy planet known as TOI-421 B. They found strong signs of water vapor but no traces of carbon dioxide or methane, hinting that this planet is more hospitable to specific chemical mixtures than expected.

Principal investigator Eliza Kempton of the University of Maryland, College Park notes that TOI-421 B’s atmosphere belongs to a class of planets called sub-neptunes, which are planets between the size of Earth and Neptune.

Many of these smaller worlds are shrouded in haze, but this one seems to have clear skies and to be unusually hot.

Sniffing out a hidden atmosphere

TOI-421 B sits several times closer to its star than earth does to the sun. The planet’s atmosphere reaches around 1,340°F, which is intense even by exoplanet standards.

Astronomers using the Webb Telescope used a method known as transmission spectroscopy to spot water in the planet’s upper layers.

They observed light passing through the planet’s atmosphere during a transit, then checked for chemical signatures in the spectrum.

Unusual temperature on TOI-421 B

Its scorching temperature has likely cleared out much of the haze seen on cooler sub-neptunes. Observations indicate these conditions prevent heavy clouds from forming high in the atmosphere.

Scientists were surprised to detect possible sulfur dioxide in such a fierce environment. This molecule is more common in cooler or more chemically complex planets, so it raises new questions about chemical interactions at high temperatures.

The results suggest TOI-421 B has a hydrogen-dominated envelope rather than a mixture that is heavy in carbon or oxygen.

Such a configuration leads to a lighter overall atmosphere, different from sub-neptunes observed around cooler stars.

Researchers confirmed this by comparing various wavelength signatures across near-infrared ranges. They matched small dips in starlight to the molecular fingerprints of water, hydrogen, and other trace gases.

No methane or carbon dioxide

Scientists were expecting to find methane or carbon dioxide, since these gases are often linked to the chemistry of small, gassy exoplanets. But neither molecule showed up in TOI-421 B’s spectrum.

That absence supports the idea that methane breaks down at high temperatures and that these types of planets may not follow the same chemical rules as their cooler counterparts.

Without methane, the formation of hazes is unlikely, which helps explain why the atmosphere is so transparent.

Hot sub-neptunes like TOI-421 B appear rare among the objects studied so far with advanced tools. Many smaller planets in the same size range showed hazy, high-metallicity atmospheres that flattened out the chemical signals.

In contrast, this planet’s composition seems closer to what astronomers expect for primordial hydrogen envelopes.

That discovery may improve theories on how these types of planets form and keep their light gases under punishing heat.

How TOI-421 B compares

Although TOI-421 B is a sub-neptune, there’s nothing quite like it in our solar system. Neptune and Uranus are colder, farther from the sun, and wrapped in thick clouds that block direct views of their deep atmospheres.

TOI-421 B’s hot, hydrogen-rich envelope and clear skies make it more chemically similar to Jupiter, but it’s much smaller and far hotter.

Its strange combination of size, temperature, and clarity gives astronomers a new benchmark to study planets that form outside our own solar neighborhood.

Some investigators suggest that intense radiation from a star usually strips away lighter molecules, leaving behind heavier ones.

Yet TOI-421 B seems to have dodged that fate, or it reshaped its atmosphere over time in a different way than previously thought.

“I had been waiting my entire career for Webb so that we could meaningfully characterize the atmospheres of these smaller planets,” said Kempton. She and her team plan to look at more hot sub-neptunes to see if this planet’s profile is unique or part of a broader pattern.

Planets beyond our solar system

The clear, hydrogen-rich atmosphere of TOI-421 B suggests that not all sub-neptunes follow the same evolutionary path.

It challenges previous assumptions that most of these planets have murky atmospheres full of heavy molecules or cloud layers that block observation.

Understanding why this planet avoided those conditions could unlock new clues about how stars, temperature, and distance shape a planet’s chemistry.

With more examples like this, scientists may be able to piece together a larger picture of how common or rare clear-skied, hydrogen-rich sub-neptunes really are.

The study is published in The Astrophysical Journal Letters.

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