

Life-building ingredients found around infant stars and in meteorites

BY EMILY BALDWIN

ASTRONOMY NOW

Posted: March 14, 2008

In two separate reports released this week, scientists have unveiled new discoveries of water vapour and organic gases in the proto-planetary discs of young stars, and unprecedented levels of amino acid concentrations have been detected in meteorites.

Artist impression of a young star and proto-planetary disc of dust and gas. Image: NASA Scientists believe that the chemical composition of gases within the protoplanetary discs of young stars may contain the building materials for planets and moons. John Carr of the Naval Research Laboratory, Washington, and Joan Najita of the National Optical Astronomy Observatory, Arizona used the Spitzer Space Telescope to make in-depth measurements of the gases in the planet-forming disc around the star AA Tauri, which is less than one million years old. By pushing the telescope’s capabilities to a new level, the astronomers were able to detect minute spectral signatures for three simple organic molecules: hydrogen cyanide, acetylene and carbon dioxide, as well as water vapour. Moreover, they found more of these substances in the disc than in the dense interstellar molecular clouds from which the disc originated. “Molecular clouds provide the raw materials from which the protoplanetary discs are created," says Carr. "So this is evidence for an active organic chemistry going on within the disc, forming and enhancing these molecules." In a separate study led by Colette Salyk of the California Institute of Technology in Pasadena, water molecules were detected in the discs of several other young stars. Spitzer was pointed at dozens of stars and two were subsequently selected to observe with high resolution measurements from the Keck II Telescope in Hawaii. "This is one of the very few times that water vapour has been directly shown to exist in the inner part of a protoplanetary disc - the most likely place for terrestrial planets to form," says Salyk. An important missing gap can now be filled. Astronomers know that water and organics are abundant in the interstellar medium but not what happens to them after they are incorporated into a disc. The new findings will allow astronomers to identify and inventory the molecules in order to better understand their origin and evolution. This plot of infrared data shows the signatures of water vapor and simple organic molecules in the disk of gas and dust surrounding AA Tauri. The data on the top line is from Spitzer, compared with model data on the lower line. Matching the observed spectral peaks with the model allows astronomers to determine the chemical details of the observed region. Image: NASA/JPL-Caltech/Naval Research Laboratory. In another study, raw ingredients for life - amino acids - have been uncovered in meteorite samples. Amino acids form the backbone of proteins, which make much of life’s structures and drive chemical reactions in cells. The production of proteins is considered one of the first steps in the emergence of life and although some amino acids could have formed in certain environments in the early Earth, the presence of these compounds in specific meteorites has led many researchers to look to space as a source. The meteorites used for this study were collected from Antarctica and represent some of the rarest types of meteorites - CR chondrites, which are thought to contain the oldest and most primitive organic materials found in meteorites. Two of the meteorites in the collection exhibited over ten times the amount of amino acids compared to any other meteorites - a massive 180 and 249 parts per million respectively. "The amino acids probably formed within the parent body before it broke up," says Conel Alexander of the Carnegie Institution of Washington. "For instance, ammonia and other chemical precursors from the solar nebula, or even the interstellar medium, could have combined in the presence of water to make the amino acids. Then, after the break up, some of the fragments could have showered down onto the Earth and the other terrestrial planets. These same precursors are likely to have been present in other primitive bodies, such as comets, that were also raining material onto the early Earth.”

The new findings, from both studies, raise hopes that the early Solar System was awash with organic material.