Rotten Egg Smell Solves Exoplanet Mystery

Astronomers detect hydrogen sulfide gas in atmospheres of distant gas giants, revealing their true nature.

Published on Feb. 16, 2026

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Astronomers using the James Webb Space Telescope have detected hydrogen sulfide gas, which gives off the smell of rotten eggs, in the atmospheres of four distant gas giant exoplanets orbiting the star HR 8799. This discovery helps resolve a long-standing debate over whether these massive planets are true planets or failed stars known as brown dwarfs.

Why it matters

The detection of hydrogen sulfide, which can only exist in solid form at the distances these planets orbit, definitively proves they formed through planetary accretion processes rather than collapsing directly from gas like a star. This provides key insights into how planetary systems form and evolve. The findings also advance the search for Earth-like exoplanets by demonstrating techniques to separate and analyze the atmospheres of distant worlds.

The details

The four planets orbiting HR 8799 are absolutely massive, ranging from 5 to 10 times the mass of Jupiter. For a long time, it was unclear whether they were true planets or brown dwarfs, a type of failed star. The key was analyzing the composition of their atmospheres, which revealed the presence of hydrogen sulfide gas. This gas can only exist in solid form at the distances these planets orbit, proving they formed by accumulating solid material from the protoplanetary disk rather than collapsing directly from gas like a star.

  • The discovery was published in the journal 'Nature Astronomy' in February 2026.

The players

Jerry Xuan

A postdoctoral researcher at UCLA and first co-author of the study.

Jean-Baptiste Ruffio

A research scientist at UC San Diego and first co-author of the study.

HR 8799

A young star located 133 light years away in the constellation Pegasus, with four massive gas giant exoplanets orbiting it.

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What they’re saying

“For a long time, it was kind of unclear whether these objects are actually planets or brown dwarfs. The problem stems from how we define these objects. Astronomers traditionally use a mass threshold of about thirteen Jupiter masses as the dividing line. Above that mass, deuterium fusion can occur, a lightweight nuclear process that makes brown dwarfs glow faintly like dim stars. Below that threshold, you have a planet.”

— Jerry Xuan, postdoctoral researcher at UCLA (Nature Astronomy)

What’s next

The research techniques used to separate and analyze the atmospheres of these distant gas giants will be further refined to eventually study smaller, rocky exoplanets and search for potential signs of life.

The takeaway

The detection of hydrogen sulfide gas in the atmospheres of these massive exoplanets provides definitive proof that they formed through planetary accretion processes, rather than collapsing directly from gas like failed stars. This resolves a long-standing debate over their true nature and provides key insights into how planetary systems form and evolve.