🪐 Planets form from incandescent gas: how is it possible?

How, from the incandescent gas surrounding a young star, do the first solid materials at the origin of planets emerge? This transition, from gas to solid, called "condensation," constitutes one of the great open questions of the formation of the Solar System. It took place 4.5 billion years ago.

A study published in the journal Nature by an international team led by the Paris Institute of Global Physics (Institut de Physique du Globe de Paris/CNRS/Université Paris Cité), in collaboration with the Institute of Mineralogy, Materials Physics and Cosmochemistry (IMPMC) (CNRS/MNHN/Sorbonne Université), the Institute of Geochemistry and Petrology (IGP) (ETH Zürich) and the Centre for Petrographic and Geochemical Research (CRPG) (CNRS/Université de Lorraine), now sheds new light on this founding moment.

Sudden and extreme cooling, driving material diversity...

For decades, models have described the formation of the first minerals as a slow condensation process governed by chemical equilibrium: by cooling slowly, the gas of the solar nebula would have given rise to well-ordered mineral assemblages. But this view struggles to account for the diversity of meteorites, those ancient fragments that bear witness to the first stages of planetary formation.

The researchers explored another hypothesis. Using a new model describing the condensation of solar gas out of equilibrium, they show that, in an environment where heating is strong and cooling is rapid, matter does not have time to follow the laws of thermodynamic equilibrium. It freezes into transient states... thus minerals that should not appear at equilibrium naturally emerge out of equilibrium.

This framework allows only three main types of mineralogical assemblages to emerge, in agreement with the three main families of meteorites known in the Solar System. The diversity of planetary materials would therefore not necessarily result from large-scale compositional variations in the solar nebula, but could be explained, in large part, by local formation conditions — in particular the rapidity of cooling episodes, which testifies to their formation in a solar nebula agitated by violent movements and intense heating episodes in the first hundred thousand years.

... And early incorporation of oxygen into the first solids

These results also shed new light on another major question: the origin of oxygen and water in terrestrial planets. In classical models, the formation of oxidized or hydrated minerals from a gas of solar composition remains difficult to explain without invoking external inputs.

Here, on the contrary, the researchers show that during rapid cooling, certain elements remain available at low temperatures and can be incorporated into the forming solids. This mechanism thus offers a natural pathway for integrating oxygen — and potentially water — from the earliest stages of planetary material formation.


The emerging picture is that of a young solar nebula far from calm. Rather than a homogeneous environment evolving slowly, it appears as a dynamic medium, punctuated by episodes of intense heating and rapid cooling. Recent observations of protoplanetary disks, notably thanks to the James Webb Space Telescope, reveal that these phenomena are frequent in forming stellar systems, thus supporting this new interpretation.

By reproducing both the mineralogical diversity and the oxidation states of meteorites from a single initial gas, this work proposes an important shift in perspective. It suggests that the composition of planets depends not only on their position in the protoplanetary disk, but also on the physical and dynamical conditions — in particular the heating and cooling rhythms — that presided over the formation of their first constituents.

Carried out by teams from IPGP and its partners, with the support of the CNRS, this study thus opens a new path for understanding the first stages of the history of the Solar System, and more broadly, those of the formation of planetary systems.