Cold crystals shed light on the formation of our planetary system

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An international team of astronomers found that the building blocks for planets in a baby brother of our solar system probably formed under circumstances that closely resemble the circumstances in our early Solar System. The discovery suggests that a planetary system like ours can form around very different stars. The results are published tomorrow in Nature.

Olivine crystals in a meteorite found on Earth. The yellow and millimeter to centimeter sized olivine crystals are held together by an alloy of iron and nickel. These meteorites, called pallasites, come from our Solar System and probably originate from heated and partly melted asteroids. In contrast to olivine crystals found in the extra-solar comets of Beta Pictoris and comets in our Solar System these crystals found in pallasites are iron-rich (Jonas Debosscher/Ben L. de Vries).

The astronomers base their findings on their study on olivine crystals in the dusty disk around the young star Beta Pictoris, 63 lightyears from Earth. The dust in the disk around this star originates from catastrophic collisions between thousands of small, icy comet-like bodies orbiting the star. Using the Herschel space telescope of the European Space Agency (ESA) the astronomers were able to study the dust. They discovered olivine crystals with a composition strikingly similar to olivine crystals found in comets of our Solar System.

This suggest that these crystals were formed under similar circumstances. This is a surprise because Beta Pictoris outweighs our Sun with a factor 1,5, is eight times as luminous and its planetary system is still under construction.

Primitive building blocks
Olivines form just before or during the formation of a planetary system. The Earth’s mantle contains large amounts of this mineral, which plays an important role in the formation of stone. Therefore the exact composition of olivine crystals gives us information on the circumstances under which planets form. The olivines in the dust disk around Beta Pictoris are magnesium-rich, just like the olivines in comets of our Solar System. Asteroids (and likewise the Earth) contain iron-rich olivines.

It appears that the cold, magnesium-rich crystals that have been discovered by the Herschel telescope do not come from colliding asteroids (or bigger celestial bodies) but from collisions between large numbers of icy comets. They closely resemble the most primitive building blocks of our Solar System.

Heat treatment
The team of astronomers was led by PhD student Ben L. de Vries of the Institute for Astronomy at the KU Leuven. De Vries, first author of the article that is published tomorrow in Nature: “When larger bodies form in a planetary system, the internal temperature increases to such a degree that the olivine crystals are enriched with iron. Olivines in for example asteroids are subjected to a ‘heat treatment’. Olivine crystals with small amounts of iron probably form in small icy comets, similar to the comets that formed when our Solar System took shape.”   

Forensic research
“With the infrared space telescope Herschel astronomers can observe planetary systems at great distances,” says co-author Rens Waters, Science Director of SRON Netherlands Institute for Space Research. Herschel – with on board SRON’s molecule hunter HIFI – has very accurately measured the weak infrared light which is emitted from the dust disk around Beta Pictoris. In this light we find the ‘fingerprints’ of olivines.”

“It is fascinating stuff to be able to deduct information on the origin of our Solar System from the amount of iron in these olivines. Forensic research in space you might say. It shows that our home around the sun is not so unique after all.”

The results will be published in Nature on 4 October 2012; the full article Comet-like mineralogy of olivine crystals in an extrasolar proto-Kuiper belt can then be found on and