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A lot as changed in the 4.5 billion or so years since the Solar System first came together from a disk-shaped cloud of swirling dust and gas.
The stuff from which everything formed has undergone some serious alterations – packed into planets, blasted by solar radiation and plasma, changed by interactions with other atoms.
The basic components of that initial, early dust disk are therefore difficult to discern. But not, as it happens, entirely impossible.
Preserved inside an ancient rock that fell to Earth from space and was recovered in 2018, an international team of scientists have now identified traces of material that, they say, must have originated in the protoplanetary disk, back when the Solar System was young.
It’s a discovery that can give us new insights into the history of the Solar System, and the basic building blocks from which everything around us, here on Earth and around the Sun, was born, so many eons ago.
The Sun, like all stars, was born in a cloud of dust. A denser knot in the cloud collapsed under its own gravity, spinning, spooling the material around it into a disk that fed into the growing star. When the Sun was finished, what remained of that disk formed everything else in the Solar System: the planets, the moons, the asteroids, the comets, and the icy chunks of rock that make up the spherical Oort Cloud that is thought to encapsulate it all.
That Oort Cloud is made up of icy chunks of rock that sometimes make their way into the inner Solar System, looping around the Sun, shedding gas and dust as they do so. These are the long-period comets, with orbits of hundreds to hundreds of thousands of years.
The Oort Cloud, so far from the Sun, is thought to have remained relatively unaltered since the birth of the Solar System, and thus represents the most pristine example of the primordial material that made up the disk that formed the planets.
But this material has been challenging to study closely. Even when cometary fragments containing that primordial material do make their long journey through the Solar System to enter Earth’s atmosphere, they melt away as they fall.
This brings us to meteorites. Even though space is mostly fairly empty, comets and meteorites do sometimes collide. When this happens, it’s possible that some cometary material can become mixed into the meteorite, trapped inside as fragments called clasts.
If that meteorite enters Earth’s atmosphere, it, too, will be heated – but the cometary clasts contained inside can remain protected and reach the surface intact.
This is what the team of researchers led by cosmochemist Elishevah van Kooten of the University of Copenhagen discovered in a meteorite named Northwest Africa 14250 (NWA 14250).
Using a scanning electron microscope, and spectroscopic analysis, the researchers conducted a very close perusal of the contents of NWA 14250, and the isotopes of various minerals found in clasts therein. The minerals in some clasts, the researchers determined, are most likely to be cometary in origin, which means meteorites like NWA 14250 could represent a tool for studying the composition of the early Solar System.
But there’s more. The clasts, the team found, were very familiar: they resembled clasts found in other meteorites from the outer Solar System near Neptune, as well as samples taken from the asteroid Ryugu.
This suggests, the researchers say, that not only is primordial material relatively common (if a little difficult to access), the composition of the protoplanetary disk was relatively uniform during the formation of the Solar System.
“Contrary to current belief, the isotope signature of the comet-forming region is ubiquitous among outer Solar System bodies, possibly reflecting an important planetary building block in the outer Solar System,” the researchers write.
“This provides the opportunity to determine the nucleosynthetic fingerprint of the comet-forming region and, hence, unravel the accretion history of the solar protoplanetary disk.”
The research has been published in Science Advances.