The origin of the Solar System is one of the fundamental problems in astrophysics. The general mechanism is well understood–a giant cloud of gas and dust must have collapsed to form the Sun and planets–but the devil is the detail. For example, what could have triggered such a collapse?
There are various clues. The most tantalising come from the study of isotopes inside meteorites. These are important because astrophysicists think these rocks were formed during the collapse and have remained untouched ever since. So their isotopic make-up is a direct reflection of the conditions that existed inside the gas cloud as it condensed.
One puzzle is the amount of aluminium-26 in these rocks when they formed. Al-26 has a half-life of about 700,000 years. So it doesn’t take long for the ratio between Al-26 and its cousin, Al-24, to change.
But the ratio is strangely high in a class of carbonaceous chondrite meteorites called CV-chondrites (the ‘V’ comes about because they are named after a meteorite that fell over Vigarano, Italy). Something must have been injecting freshly forged Al-26 into this gas cloud as it collapsed.
The isotope measurements also time stamp the formation of these meteorites and this raises another puzzle. The measurements indicate that the meteorites must all have formed within 20,000 years of each other–that’s practically simultaneously on these timescales.
So what could have produced this Al-26 and triggered the formation of meteorites so quickly?
There are several possibilities. Various types of stars release Al-26 in the wind that streams away from them. One idea is that our Solar System formed near one of these.
Most astrophysicists favour another idea, however. This is that a supernova occurred nearby, sending a shockwave of hot gases, including Al-26, careering through our progenitor cloud.
The difficulty is in distinguishing between these scenarios.
Today, Matthias Gritschneder at Peking University in Beijing and a few pals unveil a new computer simulation of the formation of the Solar System that clearly favours the supernova hypothesis.
The new model recreates what happens when a shockwave of hot gases from a supernova passes through a progenitor cloud of cold gases.
Not only does the supernova provide exactly the right amount of Al-26, the shockwave also causes our gas cloud to collapse, thereby triggering the formation of the Solar System.
What’s more, the entire process happens very quickly. CV-chondrites probably formed when the temperature of gas cloud dropped below about 1800 degrees C.
The new model shows that this would have occurred over a timescale compatible with the 20,000 years that the evidence suggests.
There are certainly improvements that could be made to this model. It is a 2D simulation, rather than 3D, so some physical processes may not be simulated exactly. There are also other isotope ratios that need to be explained.
But these will have to wait until more advanced simulations are possible with greater computing horsepower.
In the meantime, astrophysicists will be relieved that their most cherished ideas about the formation of the Solar System are bearing up, indeed flourishing, under the scrutiny of the strongest numerical tests possible today.
3 Nov: Edited to change an error in the half-life of Al-24
Ref: arxiv.org/abs/1111.0012: The Supernova Triggered Formation And Enrichment Of Our Solar System