Around young stars, flat clouds of gas and dust swirl, like pizza dough from which planets will eventually be shaped. This cloud contains various chemical elements, and sulfur is one of the most elusive: it's hard to catch with telescopes because it hides in solid grains or escapes as gas.
Astronomers pointed the highly sensitive ALMA telescope at the disk around the star HD 163296—it's 300 light-years away, quite close by cosmic standards. And right in the center of the disk, at a distance roughly from the Sun to Jupiter, they saw light from hydrogen sulfide molecules. This gas smells like rotten eggs, but in space it doesn't smell; instead, it glows in the radio range. Nearby, they found two other sulfur compounds, SO and SO₂. All three substances orbit the star at tremendous speeds, and from how their spectral lines are broadened, scientists realized they are very close to the star, where temperatures exceed 90 degrees Celsius. That's where water and sulfur ice melts, releasing these gases.
But there's a puzzle: the total amount of sulfur in the gas is still less than expected. Some of it might be locked in salts and minerals that telescopes can't yet see. If sulfur ends up in rocky planets, it will affect their chemistry and even the potential for life — since sulfur is part of proteins and other vital molecules.
The discovery shows that even in the innermost regions of the disk, where Earth-like planets are born, sulfur isn't lost but bides its time. Further observations will help us understand how much of it will end up in future worlds.
Hydrogen sulfide is the gas that smells like rotten eggs. In space, its molecules glow in the radio range, and telescopes 'sniff' them without any sense of smell.