Planets' birthplace harbours chemical seeds of life

13 March 2008

Astrochemists have for the first time directly observed both organic molecules and water vapour in the region around a young star where planets form. If a similar mix is found around other stars, it could mean our own solar system - with planets harbouring the chemical precursors of life - may not be unusual.

Planetary formation around young stars is believed to occur from disks composed of gas and dust - called 'protoplanetary disks' - but characterizing the gas composition of these disks is challenging. Until recently, the information scientists had about the chemical composition of the early solar system came from studying comets and meteorites that were billions of years old.

Now, a team in the US has reported spectroscopic observations of water and simple organic molecules in the protoplanetary disk around the star AA Tauri - thought to resemble many other young stars, including our early sun.  Using the infrared spectrograph on NASA's Spitzer space telescope, John Carr of the Naval Research Laboratory Remote Sensing Division in Washington DC and Joan Najita of the National Optical Astronomy Observatory in Tucson, Arizona obtained high-sensitivity spectra of the disk around AA Tauri at mid-infrared wavelengths (10-40 micrometres).

'This is the first time that both water vapour and simple organic molecules have been measured in the planet formation region of a disk', Najita says. 'The results open a new window on the physical and chemical conditions in planet formation environments.'

Studies of molecules in space have mostly relied on spectroscopy measurements at radio wavelengths but these techniques have lacked the resolution and sensitivity to probe the region of the disks where planets are found (within 10AU of the central star - an AU, or astronomical unit, is the distance from Earth to the Sun - about 150 million kilometres). 

Birthplace of the planets

Models predict that water ice boulders drifting into the warm inner region of the disk, where terrestrial planets are expected to form, supply this region with water vapor that would otherwise be lost. Organic compounds could be delivered in a similar manner.

© Nature

The species they studied  - water, OH, HCN, C2H2, and CO- are particularly important because they're precursor molecules in reactions that produce the chemical building blocks of life. The high abundance of the detected molecules suggests that they were created in the disk itself.  'Over the long term, we hope that studies like ours will tell us whether the chemical precursors of life are commonly created in planet-forming disks,' says Najita.

The scientists say their observations demonstrate the potential to measure the distribution of water vapour throughout the terrestrial planet region of disks (2 to 3AU) - information that was previously restricted to hot water within 0.3AU - which is important for understanding the origin and evolution of water in our solar system.

Fred Ciesla, a researcher at the Carnegie Institution of Washington's Department of Terrestrial Magnetism, says that the only information that could previously be inferred about the material present in the terrestrial planet region of other protoplanetary disks was the size and amount of silicate dust. 'The observations of Carr and Najita are now providing information about the abundances of water and organic materials in the gas phase in this same region', he says. 'This information is critical for developing our understanding of the chemical evolution of primitive materials in protoplanetary disks and how materials of great astrobiological interest may have been incorporated into planets.'

Kira Welter




J S Carr and J R Najita, Science, 2008, 319, 1504. 

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