The analysis of a meteorite reveals secrets about the birth of the Solar System
Research led by the University of Arizona in collaboration with IEEC-UPC has led to the discovery of a meteorite grain forged during the final phases of a star that disappeared a long time ago.

The study has been carried out by an international team of cosmochemists and astrophysicists, including researcher Jordi José, from the Institute of Space Studies of Catalonia (IEEC) and the Department of Physics of the Universitat Politècnica de Catalunya - BarcelonaTech (UPC). Encapsulated in a meteorite collected in Antarctica, the tiny grain – only a few microns in size – has shed new light on the terminal phases of the star’s life and how stars sow the Universe with the building blocks for new stars, planets and life.

Called LAP-149, the small grain was confined to a rocky, non-metallic meteorite found in Antarctica and represents the only meteor grain composed of both graphite and silicate discovered to date. Its origin can be traced back to a specific type of stellar explosion known as a nova. Surprisingly, as Jordi José, professor of physics at the East Barcelona School of Engineering (EEBE), points out, "the grain was incorporated and mixed with the nebular material that, shortly afterwards, would give rise to the Solar System, about 4,500 million years ago, and later went on to be embedded in a primitive meteorite.” The discovery challenges some of today's ideas about how dying stars sow the Universe with raw materials, from which planets will form and, ultimately, the precursor molecules of life.

Pierre Haenecour, researcher at the University of Arizona, has led the investigation and is the leading author of the article published in the journal Nature Astronomy. As he points out, "like the real star's dust, these grains from the presolar era give us an idea of the building blocks from which our Solar System formed. They also provide a direct snapshot of the conditions of the star at the moment this grain formed.”

Novas are binary stellar systems composed of a compact star, which is called a white dwarf, and a slightly more massive companion, from the main sequence or a red giant. The white dwarf accretes material from the companion star. Once it accumulates enough stellar material, the white dwarf burns again in periodic outbursts, violent enough to forge new chemical elements and eject them into space. These and other stellar explosions create the elements that form the basis of life on Earth. As Jordi José explains, "stellar explosions allow us to understand how the Universe, which during the first hundreds of millions of years after the Big Bang was chemically very poor, presenting only hydrogen, helium and a little lithium, has progressively enriched itself with the majority of chemical elements found today in the periodic table. These elements  form the basis of our planet and our bodies."

The team of researchers at the University of Arizona has analysed the small – about the size of a microbe – messenger from outer space down to the atomic level. LAP-149 has been analysed with a multitude of first-level experimental techniques and has turned out to be really strange: when studied with a technique called secondary ion mass spectrometry, which makes it possible to distinguish between different varieties of atoms called isotopes, LPA-149 was highly enriched in the carbon isotope 13C. The isotopic composition of carbon (i.e., the ratio 12C/13C) measured on any planet or object in our Solar System typically varies by a factor of the order of 5. However, the 13C found in LAP-149 is enriched more than 50,000 times. LAP-149 provides evidence that the novas have contaminated the gas from which the Solar System formed with grains rich in carbon and oxygen.

Although their progenitor stars no longer exist, the isotopic composition and microstructure of the stellar dust grains identified in meteorites provide unique data on their formation and on the thermodynamic conditions in the material ejected by the stars, according to the authors. The detailed analysis of LAP-149 revealed even more unexpected secrets: unlike other similar grains extracted from meteorites forged into dying stars, this is the first known grain to contain graphite (the material of which a pencil mine is made) and a central inclusion rich in silicates.

"Our finding provides a vision of a process we could never witness on Earth," lead author Pierre Haenecour adds. "It tells us how meteoric grains form in material expelled by a nova. We now know that grains rich in carbon and oxygen can form simultaneously in the same material ejected by a nova, possibly in regions of different chemical composition, something that had already been predicted by simulations of nova explosions, but had never been observed in the laboratory before.”

According to Jordi José, LAP-149 has provided a lot of information about the condensation processes in the material ejected by the novas and its contribution to the chemical composition of the Solar System. "LAP-149 has had a truly hazardous life: it was born in space, from the material expelled in a stellar explosion; it wandered through the interstellar medium surviving cosmic rays and high energy radiation, until it became trapped in the cloud of gas and dust that would become our Solar System", adds the IEEC-UPC researcher. Thus, the meteorite grain roamed for an unknown time in the primitive Solar System, before becoming part of an asteroid that later fell on Earth and from which it was extracted by scientists at the University of Arizona.

Figure: Artistic recreation of a stellar explosion. The ejected material forms dust grains (enclosed box) with isotopic compositions not found in our Solar System. Credit: University of Arizona (USA).
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Generalitat de CatalunyaUniversitat de BarcelonaUniversitat Autònoma de BarcelonaUniversitat Politècnica de CatalunyaConsejo Superior de Investigaciones CientíficasCentres de Recerca de Catalunya