🧬 The molecules of life may form even before planets

A team of researchers has just demonstrated that essential molecular chains, peptides, can form spontaneously on cosmic dust grains, meaning in space. This result changes our understanding of the origin of life.

To simulate the hostile conditions of the interstellar medium, scientists cooled glycine, a simple amino acid, to extreme temperatures close to -436°F (-260°C). They then exposed this frozen sample to a bombardment of energetic protons, mimicking the effect of cosmic rays. This laboratory experiment allowed for the observation of the creation of glycylglycine, the smallest possible peptide chain.


Contrary to what was thought, this chemical reaction does not require liquid water. Indeed, the energy provided by ionizing radiation is enough to break and reform bonds between amino acids, even in such a cold and inhospitable environment. Consequently, cosmic rays act as a true engine for assembling more elaborate molecules.

These discoveries considerably expand the places where the precursors of life can appear. For example, the clouds of gas and dust that give birth to stars and planets could already contain these peptides. Later, when this material aggregates to form a stellar system, these essential molecules settle onto planetary surfaces.

If a rocky planet has liquid water, these molecular building blocks from space could then participate in the emergence of life. Nevertheless, the transition from peptides to the first living cells remains a process that science is still seeking to elucidate.

In addition to glycylglycine, the experiment also led to the formation of normal water and heavy water, as well as other elaborate organic molecules. According to Sergio Ioppolo, a researcher at Aarhus University, this diversity shows that active chemical processes take place well before the formation of stars, in interstellar clouds previously thought to be inert.

The study, published in Nature Astronomy, opens new avenues for understanding the distribution of the ingredients of life in the Universe. The next steps will involve verifying if other, longer peptides can form via the same mechanism in space.

Cosmic rays, artisans of space chemistry

In the interstellar void, temperatures are so low that most chemical reactions are normally impossible. Yet, very energetic radiation, called cosmic rays, constantly traverse space. These charged particles, often protons accelerated to speeds close to that of light, interact with the matter they encounter.

When a cosmic ray strikes an icy dust grain, it transfers a part of its energy to the molecules trapped in the ice. This energy can break existing chemical bonds, releasing atoms and highly reactive molecular fragments. These unstable chemical species then quickly seek to bind to other atoms or molecules to regain a more stable state.

In the case of amino acids like glycine, this agitation caused by the radiation allows two molecules to come together and form a peptide bond. It is this bond that links amino acids together to create chains, the first steps towards proteins. This process occurs without needing the heat or liquid water found on planets.

Thus, far from being a chemically dead environment, interstellar space is the stage for active chemistry driven by radiation. This mechanism explains how increasingly elaborate molecules can assemble in the deep cold, long before the birth of stars and planets.

From the interstellar cloud to the habitable planet

Giant molecular clouds, composed of gas and dust, are the cradles of stars. Under the effect of gravity, certain regions of these clouds collapse upon themselves, forming a rotating protoplanetary disk around a young star. All the matter from the cloud, including the organic molecules formed on the ice grains, is incorporated into this disk.

Within this disk, dust and ices clump together to form larger and larger bodies: pebbles, planetesimals, and finally planets. Elaborate molecules like peptides, present from the beginning in the cloud, are therefore integrated into the planetary building materials. They survive the journey and end up on the surfaces of newly formed worlds.

For a planet to be considered habitable, it must have conditions allowing water to be liquid. The pre-existence of peptides and other organic molecules then provides a sort of chemical starter kit.

The presence of these molecular building blocks does not guarantee the appearance of life, but it considerably facilitates its first steps. This phenomenon implies that the fundamental ingredients could be widely distributed throughout the Galaxy, increasing the chances of finding environments conducive to the emergence of living things beyond Earth.