In 2011, planetary researchers using samples of the solar wind collected by NASA’s Genesis spacecraft found that solar system planetary bodies have a lower concentration of the oxygen-16 isotope than does the Sun. Early in the history of the Solar System, its primordial building blocks had been hit with a hefty dose of ultraviolet light (UV), which can explain this difference. But where did it come from? According to new research from Washington University in St. Louis, stellar neighbors of the Sun — likely O- and B-type stars in a massive-star-forming region — irradiated the edge of its parent molecular cloud and left the impression on the Solar System’s primordial material.
Hubble’s image of the Orion Nebula with proplyd highlights. Image credit: NASA / ESA / M. Robberto, Space Telescope Science Institute & ESA / Hubble Space Telescope Orion Treasury Project Team / L. Ricci, ESO.
“We knew that we were born of stardust: that is, dust created by other stars in our Galactic neighborhood was part of the building blocks of the Solar System,” said co-author Dr. Ryan Ogliore, a researcher in the Department of Physics at Washington University in St. Louis.
“But this study showed that starlight had a profound effect on our origins as well.”
In the study, Dr. Ogliore and colleagues examined Acfer 094, a carbonaceous chondrite meteorite found in Algeria in 1990.
Acfer 094 is the only known meteorite that contains cosmic symplectite, an intergrowth of iron-oxide and iron-sulfide with extremely heavy oxygen isotopes.
“This is one of the most primitive meteorites in our collection,” said lead author Dr. Lionel Vacher, a postdoctoral researcher in the Department of Physics at Washington University in St. Louis.
“It was not heated significantly. It contains porous regions and tiny grains that formed around other stars. It is a reliable witness to the Solar System’s formation.”
Cosmic symplectite in the meteorite Acfer 094. Image credit: Washington University in St. Louis.
The Sun contains about 6% more of the lightest oxygen isotope compared with the rest of the Solar System. That can be explained by UV light shining on the Solar System’s building blocks, selectively breaking apart carbon monoxide gas into its constituent atoms. That process also creates a reservoir of much heavier oxygen isotopes.
Until cosmic symplectite, however, no one had found this heavy isotope signature in samples of solar system materials.
“That’s when we came up with the idea of sulfur isotopes,” Dr. Vacher said.
The sulfur isotope measurements of Acfer 094’s cosmic symplectite were consistent with UV irradiation from a massive star, but did not fit the UV spectrum from the young Sun.
The results give a unique perspective on the astrophysical environment of the Sun’s birth 4.6 billion years ago.
Neighboring massive stars — likely of O- and B-types — were close enough that their light affected the Solar System’s formation.
“We see nascent planetary systems, called proplyds, in the Orion Nebula that are being photoevaporated by UV light from nearby massive O- and B-type stars,” Dr. Vacher said.
“If the proplyds are too close to these stars, they can be torn apart, and planets never form.”
“We now know our own Solar System at its birth was close enough to be affected by the light of such stars. But thankfully, not too close.”
The findings were published in the journal Geochimica et Cosmochimica Acta.
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Lionel G. Vacher et al. Cosmic symplectite recorded irradiation by nearby massive stars in the Solar System’s parent molecular cloud. Geochimica et Cosmochimica Acta, published online June 25, 2021; doi: 10.1016/j.gca.2021.06.026
