Imagine a packed orchestra, each musician playing their own part, while you try to pick out the quiet voice of a rare flute: its melody drowns in the sound of violins and trumpets. That's what hunting for molecules in interstellar clouds is like: telescopes pick up the 'radio voices' of countless substances, and picking out the signal you want from the noise is no easy task.
Recently, astronomers claimed they had heard the 'flute'—dithioformic acid (a compound of sulfur, carbon, and hydrogen)—in the protostellar cloud NGC 1333 IRAS 4A2. That would have been significant: sulfur in space seems to be hiding, with far less found than expected. However, a new study shows the excitement was premature. The authors took the same dataset from the ALMA radio telescope (a vast array of antennas in the Chilean desert) and performed an independent analysis. They found no spectral line that could be attributed to this molecule without overlapping with emission from other, well-known species. All the 'notes' turned out to be played by familiar 'instruments'—methanol, formaldehyde, and other cosmic organics.
In the end, the scientists set an upper limit on the abundance: no more than four hundred trillion molecules per square centimeter along the line of sight. Relative to the most abundant species, molecular hydrogen, that's one particle of dithioformic acid per ten billion H₂. Less than one grain of sand in a truckload of sand. Chemical models confirm that complex molecules with two sulfur atoms are extremely rare in hot protostellar cores (so-called 'hot corinos'), so the negative result is quite expected. The work not only closes one dubious discovery but also sets a strict standard for future searches: how to check every peak in the spectrum and not mistake wishful thinking for reality.
So the quiet flute that someone thought they heard in the orchestra's noise turned out to be just an echo of long-familiar instruments. But the search goes on, and now with a keener ear.
More than 200 molecules have been found in the interstellar medium, but only about 20 contain sulfur — even though by cosmic abundance it should rank tenth.