Primordial Chemistry: Where Do the First Organic Molecules Originate?

What is an Organic Molecule?

An organic molecule is a molecule containing at least one carbon atom bonded to other atoms (often hydrogen, oxygen, nitrogen, sulfur, or phosphorus) by one or more pairs of electrons. These molecules are the basis of the chemistry of life, but they can also be synthesized or present in non-biological environments (such as protoplanetary disks, interstellar molecular clouds, or comets).

Examples of organic molecules:

• Methane (CH₄)
• Ethanol (CH₃CH₂OH)
• Glucose (C₆H₁₂O₆)
• Methanol (CH₃OH)

The Zones of the Disk Where Chemistry Begins

Protoplanetary disks, swirling remnants of gas and dust surrounding young stars, are the cradles of planetary systems. It is also in these structures that the very first organic molecules — precursors to the building blocks of life — come into being. Their appearance strongly depends on local thermodynamic and radiative conditions, such as temperature, density, and exposure to ultraviolet and cosmic radiation.

The chemistry in a protoplanetary disk is far from uniform. Three main zones define the regimes of organic molecule formation:

• Hot inner zone (r < 3 AU): the temperature here exceeds 300 K, preventing the condensation of most volatile compounds. However, it is an environment rich in free radicals favoring rapid reactions in the gas phase, producing, for example, formaldehyde (H₂CO) or hydrogen cyanide (HCN).
• Middle transition zone (~3 to 30 AU): often called the "snow line," this is where temperatures drop below 150 K, allowing the condensation of molecules like H₂O, CO₂, NH₃, or CH₄ on silicate grains. These icy grains, exposed to UV rays, trigger rich surface photochemistry leading to more complex compounds.
• Cold outer zone (beyond 30 AU): in this region, little disturbed by stellar radiation, the temperature falls below 50 K. Chemistry here occurs mainly on the surfaces of icy grains, through slow processes such as recombination chemistry or the successive hydrogenation of CO to CH₃OH (methanol).

Role of Dust Grains

Dust grains play a crucial role: they are solid matrices where molecules "stick" and react. The porosity of the grains, their local temperature, and the nature of the icy mantle strongly influence chemical reactions. Dust grains are catalysts of complexity. From simple species like CO, NH₃, and H₂O, we observe the synthesis of complex organic molecules such as amino acids or nitrogenous bases, through surface chemistry processes, often activated by UV radiation or energetic particles.

Examples of amino acids:

• Glycine C₂H₅NO₂
• Alanine C₃H₇NO₂
• Serine C₃H₇NO₃
• Leucine C₆H₁₃NO₂
• Glutamate C₅H₉NO₄

Several simple amino acids — as well as their chemical precursors — can be formed or assembled in certain zones of the protoplanetary disk, under specific physicochemical conditions, although the complete formation of complex amino acids remains subject to discussion.

The total number of amino acids detected in extraterrestrial samples (meteorites, comets, simulated environment experiments) exceeds 90, but with varying degrees of certainty depending on the detection method, terrestrial contamination, and analysis context.

There are 20 standard amino acids that make up proteins in terrestrial organisms.

Observation and Detection: ALMA and the Traces of Life

Thanks to the ALMA observatory (Atacama Large Millimeter/submillimeter Array), astronomers directly detect the spectral signatures of these molecules in the disks of young stars like TW Hydrae or IRS 48. The emission lines of methanol, formaldehyde, or even ethanol testify to a chemistry already rich well before the formation of planets. This suggests that the organic matter we find on comets or meteorites, bearing prebiotic signatures, comes directly from these primordial environments.

Origin of Life: Seeds Sown Before the Birth of Planets

Thus, primordial chemistry in protoplanetary disks creates a molecular reservoir already complex before planets even agglomerate. These organic compounds, preserved in planetesimals and comets, are then delivered to young planets, providing a potential substrate for the emergence of life.