Which building blocks of life have been found in space, and which haven't?

Scientists have uncovered the fundamental components for life in asteroids, meteorites, Mars, and regions surrounding youthful stars. These discoveries have been made in recent years, with future missions set to continue searching for these essential life constituents. However, the question remains: how many life-building blocks have been identified in outer space so far?

To understand the term "building blocks of life," scientists seek molecules similar to those found in Earth's life, such as proteins, RNA, and lipids that form cell membranes. Numerous such molecules have been discovered in sample-return missions. Samples from asteroids Bennu and Ryugu, both retrieved in 2023, contained all five nucleobases essential for DNA and RNA, as well as 14 of the 20 amino acids necessary for life as we know it.

Radio astronomy is another technique used to search for these life-building blocks in distant regions of space. Astronomers classify the molecules they discover based on their complexity, starting with complex organic molecules (COM), which contain six or more carbon atoms. Out of the 350 molecules detected in space thus far, approximately 180 of them are considered COMs.

Certain COMs are prebiotic molecules, which scientists believe could have played a role in the origins of life. A prebiotic molecule doesn't have a strict definition, but it is a molecule whose existence can be confidently detected, can plausibly form and survive without life, and has documented involvement in some proposed life formation pathways. By this definition, Ioppolo estimates roughly 30 prebiotic candidates, though the exact number varies depending on the chosen threshold.

Researchers have detected prebiotic molecules in molecular clouds, dense regions of gas and dust where molecules can form. Using radio astronomy, Izaskun Jiménez-Serra and her team have identified over a dozen prebiotic molecules in the central region of the Milky Way, including the first sugar discovered in space, erythrulose. However, the size and complexity of molecules that can be detected from space are limited, as scientists rely on spectral lines to identify each molecule.

The detection of larger species becomes challenging due to overlapping absorption bands, making it difficult to distinguish between species with the same spectrum.

Despite these limitations, the discovery of life-building blocks in various locations in space does not diminish the significance of such findings, particularly in places like Mars. Ioppolo emphasizes that the presence of amino acids on Mars's surface remains impressive, as these molecules could have been preserved throughout the formation of stars and planets, even under harsh conditions like high temperatures and cosmic radiation.

The question remains whether the abundance of these ingredients in space implies that life itself is common in the universe. While the molecules themselves are tiny, astrochemists are excited by the possibility of discovering more complex molecules, like ribose, which is a part of RNA, under extreme conditions in interstellar space.

This would suggest that the essential ingredients for life could form even before stars and planets are born. Ongoing radio astronomy surveys and upcoming sample-return missions continue to seek these fundamental components of life in the cosmos.