The Solar System’s First Solid Objects Formed at a Stunning Speed

The first solids in the solar system formed precipitously

Rather than condensing slowly over millions of years, the first building blocks of Earth and other planets may have formed quickly in a chaotic disk at the dawn of the solar system.

By Javier Barbuzano edited by Lee Billings

About 4.6 billion years ago, when the solar system was born from a vast cloud that collapsed to form the sun and a surrounding disk of swirling gasno planet yet orbits our star. At the time, apart from stardust, no solid matter drifted through this native disk. Only as the disk cooled did mineral grains condense from the gas to become the building blocks of space rocks that would eventually form Earth and other planets.

Scientists have long suspected that this was a relatively peaceful process, with showers of primordial solids slowly escaping from the disk as it cooled over millions of years. However, a study published today in Nature challenges this calm view, suggesting instead that the solar system’s first solids appeared much more quickly because of abrupt temperature changes in the disk’s turbulent maelstrom.

“It’s a real paradigm shift,” says Alessandro Morbidelli, an astronomer at the Observatoire de la Côte d’Azur in France, who was not involved in the new study. “It’s a good idea and the result was quite surprising.”

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The idea of ​​a calm start for the solids of the solar system has prevailed over the last half century. In the late 1960s, researchers studying meteorites discovered that some contained small granules called calcium and aluminum rich inclusions (CAI). These amalgams of minerals are considered the first solids in the solar system and formed when the disk’s temperature dropped just enough for them to condense out of the cooling gas. Based on the composition of IACs, the researchers assumed that their condensation reactions occurred over millions of years. This would allow sufficient time for these reactions to reach chemical equilibrium, meaning that with each successive stage of the disk’s chemical evolution, the distribution of elements in the gas and mineral phases would stabilize.

But this model, called equilibrium condensation, has limits. This cannot explain the clear variations in the composition of the most primitive types of meteorites, called chondrites. Chondrites are divided into three families: ordinary, enstatites and carbonaceous, the main difference being the degree of oxidation of their ferrous minerals, much like the difference between a shiny, unoxidized iron nail and a nail rusted from heavy oxidation. Enstatite chondrites are the least oxidized chondrites, carbonaceous chondrites are the most oxidized, and ordinary chondrites have an intermediate level of oxidation.

Experts have long assumed that this disparity between chondrites meant that each variety came from a different, chemically distinct area of ​​the solar disk, but the details of exactly how this might yield the three known types have remained obscure.

Now, a team led by Sébastien Charnoz, a planetary scientist at the Institute of Planetary Physics in Paris, offers a radically different explanation, derived from computer simulations modeling how minerals condense from a chemically uniform disk over a wide range of pressures and cooling rates. Simulations suggest that if the disk were turbulent instead of placid, parts of it could cool so quickly that the resulting chemistry would not be in equilibrium at all. Rather than raining elements in the form of minerals in majestic succession due to gradual cooling, the rapid drop in temperature would outpace chemical reaction rates in the disk. This would leave some elements temporarily trapped in gaseous form, allowing for more mixing and the simultaneous emergence of multiple minerals. More importantly, Charnoz and colleagues’ results clustered into three mineralogical families that closely resemble the composition of the three main types of chondrites.

To better explain these dizzyingly complex processes, Charnoz compares the minerals falling from a cooling disk to hungry guests at the table. When cooling is slow, the first minerals to condense “eat” the elements from the gas disk, sequestering them and sweeping them off the “table”, so that subsequent minerals that form at lower temperatures are starved. But when cooling is rapid and reservoirs of elements trapped in the gas emerge, many different minerals can compete to eat the different elements at the same time. It’s as if they “were all eating from the same plate,” says Charnoz. “They’re trying to take what they can.”

Oxygen proved to be a particularly powerful arbiter of the disk’s chemical evolution in the simulations, as its fluctuating levels dictate the oxidation state of the resulting minerals, ultimately giving rise to the three families that reflect the three varieties of chondrites. The resemblance between the real chondrites and the model results is not exact, however. But Charnoz says this could simply reflect how the cooling process sets the basic mineralogy of these primitive meteorites, followed by later processes, such as heating, evaporation or water circulation, putting the finishing touches on their mineralogy.

“I think this paper will be really useful to inspire the community and see if we can fit our data into this framework,” says Sara Russell, a planetary scientist at the Natural History Museum in London, who was not involved in the new study.

Charnoz’s model also suggests that the first solids may have formed much earlier than previously thought: before the existence of a disk, during the initial collapse of the giant gas cloud that gave birth to our star. “We’re talking here maybe about the first 10,000 or 100,000 years” of the solar system’s history, Charnoz says, compared to the millions of years of previous dominant models. Recent observations from the James Webb Space Telescope showing episodes of rapid cooling and mineral formation flakes around baby stars suggest that the same process is happening elsewhere in the universe.

The cascading implications of this new discovery could cause radical changes in our understanding of the early history of the solar system. This changes in particular where and how water could have formed. Reshuffling the order in which minerals emerged would create new opportunities for oxygen and hydrogen, the constituents of water, to combine, more easily forming hydrated minerals that, unlike ice, can withstand proximity to the blazing sun. This implies that the inner rocky planets – including Earth – could have originated with built-in water reserves rather than having most of their water imported via ice-rich asteroids and comets from the outer solar system.

“It’s a fairly complex study with many results,” says Charnoz. These results, he admits, do not definitively resolve the problems, but open new avenues for further research. “This is a big exploration,” adds Charnoz, “and we are just beginning to see where these new pathways might take us in understanding the origin of our solar system.”