Tracking Life's Precursors In Protoplanetary Disks

University of Copenhagen/Lars Buchhave. An artist's impression of a protoplanetary disk. Modelling is investigating which of the precursors of life we may be able to detect in places like this.

Modeling suggests that complex organic molecules (COMs) should exist in the disks from which planets are forming around other stars, and that in some cases we should be able to detect them. While still a long way from finding life, observing the presence of COMs should help provide ideas on where to target searches in future.

The disks from which planets form “possess all the material, gas and dust, which may form a planetary system,” a paper in Astronomy and Astrophysics reminds us. So far spectroscopic traces of elements and simple molecules have been found. While it is possible that such simple molecules could convert into more complex predecessors for life after a planet has formed, any world where complex molecules precede its formation presumably starts with an advantage. If we can find more complex molecules, and identify the characteristics of the disks in which they form, we may be able to match these to worlds formed from similar disks.

T Tauri type stars are still in the process of forming, characterized as pres-main-sequence stars that will eventually come to have masses less than double that of the sun. Leiden University's Dr Catherine Walsh  and colleagues modeled the formation of a disk around such a star and report, “We find COMs are efficiently formed in the disk midplane via grain-surface chemical reactions.” Some of these then return to the gas phase where their spectra can be observed. 

The modeled results were compared with observations of T Tauri stars and found “reasonable agreement” with regard to Formeldehyde (H2CO), but poor agreement with HC3N. They also predict that methanol (CH3OH) should be detectable in protoplanetary disks using the Atacama Large Millimeter Array (ALMA), the most sensitive instrument operating in the appropriate wavelengths. 

ALMA started operations last year, and time on it is exceptionally scarce, but Walsh hopes to eventually use it to find organic molecules during planetary formation. "This is really the next big thing in molecular astrophysics, and ALMA will give us orders of magnitude (of improvement) in sensitivity,” Walsh told Astrobiology Magazine.

Even methanol, a molecule with six atoms, falls well short of the sort of larger complex molecules we are hoping to see. After all if the eleven atom molecule ethyl formate can be found in galactic dust clouds there is room to hope for even more complex molecules in protoplanetary disks. 

Nevertheless, Walsh describes methanol as “the first run on the ladder of complexity”. The “holy grail” in Walsh's words, for such projects is to detect glycine, the simplest amino acid. However, Walsh thinks “It's possibly beyond the capabilities of [ALMA]”, though she holds out hope for the Square Kilometer Array when it comes online in the mid 2020s.

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