Bennu closed the nucleobase question. Six amino acids are still unaccounted for.

Live Science has published a stock-take of which building blocks of life have actually been found off Earth, and the counts are now specific enough to argue with ^[1]. The asteroid Bennu, whose sample was returned in 2023, contained all five nucleobases used by DNA and RNA along with 14 of the 20 amino acids used by living organisms ^[2]; its sibling asteroid Ryugu also yielded all five nucleobases, including uracil, which is found in RNA ^[3].

One side of that ledger is closed and the other is not. Five out of five nucleobases, from two separate asteroids, is a complete set ^[2][3]. Fourteen of twenty is not: six of the biological amino acids do not appear on the Bennu list ^[4], and the Live Science piece does not say which six ^[5]. That absence is the more useful number, because it is a constraint. Abundance is not.

The remote-sensing side of the inventory is coarser but larger. Of the 350 molecules of any kind detected in space so far, about 180 qualify as complex organic molecules, meaning carbon-containing molecules with six or more atoms, according to Sergio Ioppolo, an astrochemist at Aarhus University, who notes that the six-atom cutoff is an operational convention rather than a fundamental definition of complexity ^[6][7]. On his working definition of a prebiotic candidate, a confident detection that could plausibly form and survive without life already existing and has a documented role in some proposed pathway to life, Ioppolo estimates roughly 30 have been detected, with the caveat that the exact number depends on where the line is drawn ^[8][9]. That is about half of all detected molecules qualifying as complex organics ^[10] and roughly one in six of those making the prebiotic cut ^[11]. Izaskun Jimenez-Serra of the Center for Astrobiology in Spain and her team have found more than a dozen prebiotic molecules in molecular clouds at the center of the Milky Way, including erythrulose, a four-carbon sugar and the first true sugar found in space ^[12].

The reason those two ledgers should not be added together is the detection ceiling. Astronomers identify molecules by spectral lines, and Ioppolo says that for larger species the absorption bands overlap to the point where it is basically impossible to distinguish one from another ^[13]. So the astronomical list is bounded by what spectroscopy can resolve, not by what interstellar chemistry makes. The sample-return numbers come from laboratory analysis of material physically brought back, which is why a specific gap like six missing amino acids in the Bennu inventory carries more information than a gap in a radio survey.

Ioppolo also pushes back on the idea that ubiquity makes each new find boring: it remains unknown whether inherited material survives the formation of stars and planets, since high temperatures, cosmic rays, X-rays and ultraviolet light can all destroy complex molecules, and he says he would still be impressed by amino acids on the surface of Mars ^[14][15]. Jimenez-Serra's framing is the one operators should keep: "These are very small molecules" ^[16].

Watch for a published list of which amino acids the Bennu sample lacks, and whether that pattern repeats in Ryugu. Upcoming missions are aimed at searching for these building blocks ^[17], and a repeated, named absence would be a harder result than another completed set.