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BY KEITH COOPER ASTRONOMY NOW Posted: 14 May, 2009 Since comets formed out in the cold depths of the Solar System, the existence of materials in them that must have been created in high temperatures has been a real puzzle, until now. NASA’s Spitzer Space Telescope has observed the infrared signature of tiny silicate crystals, of the type found in comets, being created in the planet-forming disc around a young star called EX Lupi, in the constellation of Lupus. Back in 2005 a team of European astronomers studied EX Lupi, which is believed to be a lot like our Sun was 4.5 billion years ago, they saw amorphous grains of silicate dust in the disc. When they came back to EX Lupi three years later, after a major outburst on the star, they found those dust grains had been crystallised. The question is, how?
An artist’s impression of the planet-forming dust disc around EX Lupi, where silicate crystals have been discovered. Image: NASA/JPL-Caltech. The stellar outbursts occur when the growing young star accumulates a large amount of mass from the dusty, gaseous disc that is spinning around it. Each outburst sends a flash of heat permeating through the disc. At the distance the crystals were seen at, the temperature reached 725 degrees Celsius (about 1,000 kelvin), enough to thermally ‘anneal’ the silicate dust. Annealing describes the process of heating a material to the point that its chemical bonds break and then re-form to fundamentally change the material’s properties. In this case, the dust was annealed into crystals of a material called forsterite, which can form part of olivine. Turbulent convection currents running through the planet-forming disc then mixes up the forsterite so some of it ends up in the outer reaches of the disc where icy comets form. Consequently, forsterite and olivine are commonly found in comets and meteorites.
The two lines on this graph indicate the change in the spectra of EX Lupi’s disc between 2005 and 2008, revealing the crystallising event. Image: NASA/JPL–Caltech/P Abraham (Konkoly Observatory, Hungarian Academy of Sciences). “This is a completely new scenario about how this material could be created,” says Attila Juhasz of the Max Planck Institute for Astronomy in Heidelberg, Germany, who participated in the research that is published in the 14 May edition of Nature. Two previous explanations had been put forward: one, that exposure to heat from a young star could, over a long period, anneal silicate dust; and two, shockwaves produced by a growing planet moving through the disc could suddenly heat the grains sufficiently to anneal them. “We concluded that this is a third way… one not considered before,” says lead researcher Peter Abraham of the Hungarian Academy of Sciences’ Konkoly Observatory in Budapest. The implication is that silicate crystals in comets, such as those returned to Earth in sample–return expeditions like NASA’s Stardust mission, formed when our own Sun was undergoing frequent outbursts during its dim and distant formative years, thus providing new insight into the early times of our Solar System. Readers may be interested in an animation of the outburst and the formation of the crystals on NASA’s Spitzer website.
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