Analysis of meteorite Northwest Africa 12264 suggests that we may have to alter our timeline of the early Solar System, potentially changing our models of planet formation.
In 2018, a small meteorite was found in Northwest Africa. The precise area where it was found is unknown, but in August of that year, the small piece of space rock was sold to an independent researcher in Agadir, Morocco, who allowed it to be studied by other researchers. That appears to have been a very good move, as upon further analysis, the 50.8-gram (1.8-ounce) chunk of rock appears to provide evidence that we have the timeline of the early Solar System a little muddled up.
According to what we know of planet formation, in the early years of the Sun, it was surrounded by a protoplanetary disk. Over time, this disk of gas and dust began to clump together through gravitational interactions, eventually becoming a planet in a process known as accretion.
“The energy from this initial planet forming process causes the planet’s elements to heat up and melt. Upon melting, layers form and separate,” NASA explains of the next stage in the formation of rocky planets.
“The heavier elements sink to the bottom, the lighter ones float to the top. This material then separates into layers as it cools, which is known as ‘differentiation’. A fully formed planet slowly emerges, with an upper layer known as the crust, the mantle in the middle, and a solid iron core.”
This process was thought to take different amounts of time for planets in different areas of the Solar System. Around 4.566 billion years ago, the innermost planets between the Sun and the main asteroid belt were thought to form. Then, around 4.563 billion years ago, the rocky outer planets were thought to differentiate, having their formation slowed by the water and ice within them, slowing down the melting process of their inner core.
But thanks to meteorite Northwest Africa 12264, researchers from the Open University have suggested that this may not be the case.
“We thought that the icy conditions in the outer Solar System delayed the formation of rocky planets,” Dr Rider-Stokes, lead author of the study, said in a statement. “But our findings show they were forming just as fast as those closer to the Sun.”
Using a scanning electron microscope, the team determined that the piece of rock came from the outer Solar System, based on its chromium-oxygen isotopic signatures. Looking at the isotopes of lead contained in the rock, they aged the meteorite at 4.564 billion years, which is more similar to inner rocky planets.
“This sample, Northwest Africa (NWA) 12264, formed on a first–generation differentiated protoplanet in the outer Solar System. We demonstrate that it is the oldest magmatic rock from the outer Solar System analyzed thus far and provides crucial empirical constraints on the timing of differentiation in the most ancient protoplanets that formed beyond the snowline,” the team explains in their paper. “It demonstrates simultaneous accretion and differentiation processes operating in the inner and outer Solar System, challenging the long-held paradigm of delayed planet formation beyond Jupiter.”
“Crucially, the ages recorded by NWA 12264 are older than expected, outside the uncertainty of Al-Mg derived ages of the angrites (4563.31 ± 0.21 Ma34), some of the most ancient basalts from the inner Solar System,” the team adds.
The paper suggests that another meteorite found in Northwest Africa – NWA 7822 – potentially supports the rapid differentiation scenario.
“NWA 7822 displays major differences in chemistry, chromium isotopic compositions and distinct oxygen isotopic compositions,” the team writes. “This implies that NWA 7822 formed on a distinct parent body from NWA 12264 that also experienced core–mantle differentiation, indicating that at least two distinct bodies in the outer Solar System experienced extensive differentiation, supporting the existing evidence found in iron meteorites from the [carbonaceous chondrite] reservoir.”
Though they do not know the parent planetary body, the team suggests that looking at shock metamorphism within the sample could give an estimate of the parent protoplanet’s breakup.
While certainly an interesting study, there are uncertainties in the aging of the sample, which could alter the timeline. Further analysis of this meteorite and others could yield surprises, and maybe even alter our models of planetary formation in the Solar System, or even across the galaxy.
“This study highlights the value of rare meteorites in helping us understand planetary origins,” Rider-Stokes added. “It’s incredibly exciting to challenge the existing models of how planets — and ultimately life — begin.”
The study is published in Communications Earth & Environment.
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