Dusty shock waves generate planet ingredients
BY DR EMILY BALDWIN
ASTRONOMY NOW

Posted: 12 November, 2008

Using NASA’s Spitzer Space Telescope, astronomers have shown that shock waves around dusty young stars could be generating the raw materials needed for planet formation.

Astronomers detected tiny crystals, similar in make-up to quartz, around young stars just beginning to form planets. The cristobalite and tridymite crystals are already known to reside in comets, volcanic lava flows on Earth, and in some meteorites collected on Earth. They are thought to form in short-lived heating events followed by rapid cooling, conditions that are generated in shock waves.

Shock waves formed by colliding gas and dust in young planet-forming discs may force the creation of raw materials needed for planet growth. Image: ESO.

It is well known that planets are born out of swirling discs of dust and gas that surround young stars, building up from tiny grains into progressively larger planetisimals and eventually fully fledged planets in just a few millions of years. Forrest and colleagues used Spitzer to examine five young planet-forming discs around stars 400 light years away, and detected the high-temperature forms of silica, that is, cristobalite and tridymite, in planet forming discs for the first time. Silica is made of only silicon and oxygen and is the main ingredient in glass.

"Cristobalite and tridymite are essentially high-temperature forms of quartz," says Ben Sargent, one of the co-authors of the paper that describes the results that will appear in a future edition of the Astrophysical Journal. "If you heat quartz crystals, you'll get these compounds."

But these specific crystals require temperatures as high as 1,220 Kelvin to form, and the young planet-forming discs are only about 100 to 1,000 Kelvin, presenting something of a paradox. Since the crystals require a heating event that is followed by rapid cooling for their genesis, astronomers theorised that shock waves could be the culprit, creating violent, high speed collisions between the clouds of gas swirling around a young planetary disc and elevating the temperatures there.

"By studying these other star systems, we can learn about the very beginnings of our own planets 4.6 billion years ago," says William Forrest of the University of Rochester, New York.
"Spitzer has given us a better idea of how the raw materials of planets are produced very early on."

The findings are also in agreement with local evidence from our own Solar System. Spherical pebbles, called chondrules, found in ancient meteorites that have fallen to Earth are also thought to have been crystallised by shock waves in our Solar System's young planet-forming disc.