Free Astronomy Magazine July-August 2016

SPACE CHRONICLES tion and the distribution of cold gas in nearby discs. These unique capabilities have now been exploit- ed by a group of astronomers led by Catherine Walsh (Leiden Obser- vatory, the Netherlands) to investi- gate the chemistry of the TW Hy- drae protoplanetary disc. The ALMA observations have revealed the fin- gerprint of gaseous methyl alcohol, or methanol (CH 3 OH), in a proto- planetary disc for the first time. Methanol, a derivative of methane, is one of the largest complex or- ganic molecules detected in discs to date. Identifying its presence in pre-planetary objects represents a milestone for understanding how organic molecules are in- corporated into nascent planets. Furthermore, meth- anol is itself a building block for more complex species of fundamental pre- biotic importance, like ami- no acid compounds. As a result, methanol plays a vital role in the creation of the rich organic chemi- stry needed for life. Catherine Walsh, lead au- thor of the study, explains: “Finding methanol in a pro- toplanetary disc shows the unique capability of ALMA to probe the complex or- ganic ice reservoir in discs and so, for the first time, al- lows us to look back in time to the origin of chemical complexity in a planet nurs- ery around a young Sun-like star.” Gaseous methanol in a protoplane- tary disc has a unique importance in astrochemistry. While other species detected in space are formed by gas-phase chemistry alone, or by a combination of both gas and solid- phase generation, methanol is a complex organic compound which is formed solely in the ice phase via surface reactions on dust grains. The sharp vision of ALMA has also al- lowed astronomers to map the gas- eous methanol across the TW Hydrae disc. They discovered a ring-like pat- tern in addition to significant emis- sion from close to the central star. A ring of methanol between 30 and 100 astronomical units (au) repro- duces the pattern of the observed methanol data from ALMA. The identified structure supports the hypothesis that the bulk of the disc ice reservoir is hosted primarily on the larger (up to millimetre-sized) dust grains, residing in the inner 50 au, which have become decoupled from the gas, and drifted radially in- wards towards the star. The observa- T his artist’s impression video shows the mol- ecule methanol, or methyl alcohol (CH 3 OH). This organic compound has been found by ALMA in the closest known protoplanetary disc, around the star TW Hydrae. This is the first such detection of the compound in a young planet- forming disc. Its detection helps astronomers understand the chemical processes that occur during the formation of planetary systems and that ultimately lead to the creation of the ingre- dients for life. [ESO/M. Kornmesser] tion of methanol in the gas phase, combined with information about its distribution, implies that methanol formed on the disc’s icy grains, and was subsequently released in gas- eous form. This first observation helps to clarify the puzzle of the methanol ice–gas transition, and more generally the chemical proces- ses in astrophysical environments. Ryan A. Loomis, a co-author of the study, adds: “Methanol in gaseous form in the disc is an unambiguous indicator of rich organic chemical processes at an early stage of star and planet formation. This result has an impact on our understanding of how organic matter accumulates in very young planetary systems.” This successful first detection of cold gas-phase methanol in a protoplan- etary disc means that the production of ice chemistry can now be explor- ed in discs, paving the way to future studies of complex organic chemistry in planetary birthplaces. In the hunt for life-sustaining exo- planets, astronomers now have ac- cess to a powerful new tool. n