From organic soup to planets

ALMA reveals fine-grain differences in the chemistry of protoplanetary ingredients

Protoplanetary discs provide the raw materials that planets are assembled from. Within this swirling skirt of dust and gas surrounding a newly forming star, grains collide and coalesce together to create pebbles, which themselves amalgamate into large boulders, then planetesimals, and finally entire, fully grown planets. The exact composition of the local protoplanetary neighbourhood therefore determines the composition of each planet; and is ultimately why, for example, Earth turned out so different from Mars or Neptune.

The problem for scientists who are trying to understand the factors affecting the final outcome of planetary systems is just how much variation there might be in the chemical composition – not only between different protoplanetary discs, but also within the same disc as you move outwards from the central star. However, a huge jump has been made in how to understand these questions now a large partnership of astronomers has used the ALMA (the Atacama Large Millimeter/submillimeter Array) to scrutinise systems in the process of forming planets. ALMA offers high resolution radio observations of the distribution of molecules in protoplanetary discs.

The results of this ambitious programme are being published as a whole series of 20 separate papers in a special edition of The Astrophysical Journal. The study I’ve picked out here was led by Charles Law, a PhD student in the Harvard and Smithsonian Center for Astrophysics. Law’s team created high-resolution maps of the distribution of 18 different compounds within the protoplanetary discs of five newly forming stars. These compounds included carbon-rich compounds such as hydrocarbons like c-C ₃H ₂, oxygen-rich species like carbon monoxide CO, and molecules containing nitrogen, such as nitriles likeHC ₃N – some of which are thought to be important in the origins of life. The raw data, collected by ALMA from 2018 to 2019, required a 100 terrabyte hard drive to store it all. Law then began the task of processing and analysing all this interferometer data to generate the separate maps of each different compound.

Different mixes of ingredients

Law’s team discovered that the chemicals within the discs are not stirred uniformly throughout – there is a variation in the concentrations of different compounds. His molecular maps showed a diversity of distribution patterns: some compounds were present throughout the disc except for a clear gap, while others existed in concentric rings around the star, and others still were present close to the star but dropped off further out. Such structures within the disc could be due to differences in the dust particles within the disc, or compounds freezing onto the surface of dust grains, or being created or destroyed by ultraviolet radiation from the star.

What these maps make abundantly clear is that different planets form in molecular soups with varying ingredients, depending on their location within the disc. This has huge implications for what sorts of planets will form at what distance from their star, and what their composition will be – all factors that will have a profound impact on our ideas of what a planetary system ‘should’ look like, and how habitable to life they might be.

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