Gerald Eichstäd / NASA/JPL-Caltech/SwRI/MSSS.
Amalthea’s shadow cast on Jupiter’s red spot, captured by the Juno spacecraft in March 2024
Hydrogen sulfide detected for the first time on distant gas giant exoplanets. The discovery resolves an identity crisis that has been going on for decades regarding the massive gas giants that lie on the fuzzy boundary between planets and brown dwarfs.
No one is expecting the famous hydrogen sulfide (H₂S) smell good. The molecule, which is responsible for the characteristic odor of rotten eggshardly suggests revolutionary science.
However, your detection in the atmosphere of four distant gas giants has just answered one of the most fundamental questions in planetary science: what makes a planet a planet?
The discovery, last week reviewed “Nature Astronomy“, points out the first time that hydrogen sulfide has been identified on exoplanets beyond our Solar System.
Most importantly, solves an identity crisis that has been going on for decades regarding the massive gas giants that are located on the diffuse border between planets and brown dwarfsfailed stars that never actually triggered nuclear fusion.
The four planets orbit HR 8799a young star located 133 light-years away in the constellation Pegasus. They are absolutely huge. The smallest weighs 5 times more than Jupiter, while the largest weighs 10 Jupiter masses. They orbit at vast distances from their mother star; the closest is fifteen times further away from the Sun than Earth.
Jason Wang
The star HR 8799 (center) with HR 8799e (right), HR 8799d (bottom right), HR 8799c (top right), HR 8799b (top left)
“For a long time, it hasn’t been clear whether these objects are actually planets or brown dwarfs. The problem comes from the way we define these objects. Astronomers have traditionally used a threshold of 13 Jupiter masses as a dividing line”, explains Jerry Xuanresearcher at UCLA and first co-author of the study, in .
“Above this mass, deuterium fusion may occura light nuclear process that makes brown dwarfs glow dimly like faint stars. Below this threshold, we have a planet”, details Xuan.
But reality isn’t that tidy. There are brown dwarfs smaller than thirteen Jupiter masses, while some planet candidates exceed that limit. The mass alone does not tell us how these objects actually formed nor what they are made of, leaving their true nature ambiguous.
Then comes the hydrogen sulfidedetected through a thorough analysis of spectral data from the James Webb Space Telescope.
Jean-Baptiste Ruffioa scientific researcher at the University of California and first co-author of the study, developed new data analysis techniques to extract the incredibly faint signals of planets that are about 10,000 times fainter than their host star.
Xuan created after detailed atmospheric models which could be compared with JWST observations to confirm the presence of sulfur.
Sulfur detection is irrefutable proofl. Unlike carbon and oxygen, which can be incorporated into a planet either as gas or as ice and solid matter, sulfur at the distances these planets orbit can only exist in solid form.
There is simply no way these planets could have accumulated their sulfur as a gas. It had to come from solid material in the disk of dust and rock around the young star.
The extreme heat in their cores and atmospheres then evaporated these solids in the currently detected hydrogen sulfide gas. This definitely proves that if It’s about planets, not brown dwarfss. They formed through planetary accretion processes, swallowing solid matter from the protoplanetary disk rather than collapsing directly from the gas, as a star would.
A proportion of sulfur to hydrogen in these distant worlds reproduces a intriguing pattern found closer to us.
Jupiter and Saturn present an unexpectedly high enrichment in heavy elements compared to the Sun: more carbon, oxygennitrogen, and sulfur than would be expected if they had simply condensed from the same nebula. Now we see this same signature in a completely different planetary system 133 light years away.
The study also advances the search for Earth-like exoplanets. The technique that allowed researchers to visually and spectrally separate these planets from their star will eventually be refined to study smaller, rocky worlds, notes the .
Probably decades away until we get the first spectrum of a true Earth analogue, but when that day comes, astronomers will be looking for biosignatures like oxygen and ozone in its atmosphere.
