Published online 22 August 2010 | Nature | doi:10.1038/news.2010.423

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Solar System older than previously thought

Meteorite dating resets solar clock.

the Orion NebulaThe origin of the Solar System may have been within a cloud of gas and dust like that of the Orion nebula.NASA , ESA, M. Robberto (Space Telescope Science Institute) and the Hubble Space Telescope Orion Treasury Project Team

A meteorite found in the Sahara Desert has helped to pin down the age of the Solar System and shed light on how it may have formed.

The new estimate, which comes from measuring the ratios of lead isotopes inside the chondrite — an ancient stony meteorite — suggests that the Solar System is 4.568 billion years old. This is 0.3–1.9 million years older than some previous studies projected. The relatively small revision means that models of the gas and dust that gave rise to the Solar System should have around double the amount of a certain iron isotope, iron-60, than previously suggested.

This high quantity of iron can be traced back to the bellies of massive, short-lived stars, one of the only places in the universe where iron is produced. When these stars explode as supernovae at the end of their lives, they would have seeded the ancient Solar System.

"This suggests that one or more supernovae happened before the Sun's formation, explaining all these elements and their respective abundances," says Audrey Bouvier from the University of Arizona in Tempe, lead author of the study, published online in Nature Geoscience today1.

The team's estimate helps to resolve differing results obtained by two dating techniques that astronomers use to assess the age of meteorites. The first, which is trustworthy but can only point at broad timescales, looks at the ratio between two isotopes of lead (Pb), lead-207 and lead-206. The second, which uses the ratio of aluminium to magnesium, can zoom in on much smaller timescales but has previously given an age 1 million years older than the Pb–Pb chronometer. Bouvier and her team have reconciled the two estimates by pushing back the lead-measured age of the Solar System to match the aluminium–magnesium measurements.

Geophysicist Andrew Davis of the University of Chicago, Illinois, is glad to see that the two timescales now agree. "However, more chondrites need to be measured to see if this holds up and we need to understand why previous meteorites gave younger Pb-Pb ages," he says.

Explosive past

Planetary scientist David Kring of the Lunar and Planetary Institute in Houston, Texas, says the results are exciting because researchers are now able to probe the timescale of events before the Sun's birth with high precision. "We are beginning to paint a very compelling and dramatic picture of a fairly dynamic period in the Solar System's history," he says.

Six years ago, most astronomers studying star formation would probably have said that the Solar System was born in a molecular cloud in which low-mass, Sun-like stars form in relative isolation. Today, the iron abundances discovered in meteorites bolsters the case that our Solar System grew up surrounded by giant, heavy stars and was shaped by their activities.

Before the dawn of our Solar System, massive stars lived within a cloud of gas and dust like that seen in the Orion Nebula2. These giants produced huge amounts of ultraviolet light, whose photons exert a pressure that pushed outward in a sphere, carving out a cavity from the nebular gas and dust. As this cavity expanded, its edge squeezed together the surrounding debris. Areas of debris with increased density had slightly greater mass, which attracted more particles to them because of an increased gravitational pull. This led to a snowball effect of accumulation and, eventually, enough mass collected in one place to collapse into a hot ball called a protostar.

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Even as the massive star's light swept nearby debris away in the ever-expanding cavity, the protostar lived within its own bubble of gas and dust, creating an 'evaporating gaseous globule' or EGG. When the massive stars exploded as supernovae, they sent an element-rich rain to rapidly mix with the materials inside the EGG. After millions of years, our Solar System coalesced from the rocky grains and asteroids now saturated with supernovae iron.

Although astronomers have observed such developments in distant stellar nurseries, this new research brings to light information about the history of our own neighbourhood before the Solar System's birth. Previously, researchers have mainly relied on a chondrite named Allende, which provided the earlier estimates of the Solar System's age. Bouvier's research suggests that more samples would expand astronomer's understanding of early solar nebula processes.

"We need to recover new meteorites and, to the extent possible, go directly to asteroids in space," says Kring. "This research points to the fact that there are more materials out there to study, which means there are more secrets to uncover." 

  • References

    1. Bouvier, A. & Wadhwa, M. Nature Geosci. advance online publication doi: 10.1038/NGEO941 (2010).
    2. Hester, J. J., Desch, S. J., Healy, K. R. & Leshin, L. A. Science 304, 1116-1117 (2004). | Article | PubMed
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