Complex organic molecules found in protoplanetary disk

For the first time, astronomers have found complex organic molecules in a protoplanetary disk. Using the ALMA telescope, the discovery of methyl cyanide (CH3CN),  cyanoacetylene (HC3N) and hydrogen cyanide (HCN) in the dusty, gas-rich disk surrounding the young star MWC 480, show that the building blocks of life are not unique to our Solar System.

Artist impression of the protoplanetary disk around young star MWC 480, in which astronomers have detected complex organic cyanides using the ALMA telescope. The organics were detected in the outer part of the disk where comets reside. These findings show that the building blocks of life are common in the Universe and that the conditions for life are not unique to our Solar System. (Credit: B. Saxton, NRAO/AUI/NSF)

These complex organic molecules were detected in the cold outer regions of the disk surrounding MWC 480, in the neighbourhood of its Kuiper Belt equivalent where we expect comets and icy planetesimals to reside.  Water and complex organics have been found in both asteroids and comets in our Solar System, and organic molecules have been detected in giant molecular clouds where stars form.   Simple molecules like water (H2O) and hydrogen cyanide (HCN)  have also been found in protoplanetary disks, indicating that some volatile molecules can either survive during the formation of disks or that they quickly form in young disks. But what about more complex organics? It was not known whether complex organic molecules could survive the energetic shocks and intense radiation that result when molecular cloud cores gravitationally collapse to form young protostars and their surrounding protoplanetary disks.

This new discovery, lead by Karin Öberg of the Harvard-Smithsonian Center for Astrophysics, shows that the conditions that produce complex organic cyanide molecules must also be common around young planet-forming disks with a range of conditions.  MWC 480 is a 1.8 solar mass star which hosts a massive disk (about 0.2 solar masses) in the constellation of Taurs.  Being a massive Herbig Ae star, MWC 480 exposes the disk material to much higher levels of ultraviolet radiation compared to stars the mass of our Sun and the disk will also be much hotter, 2-3 times warmer at a given radius that our Solar System.

Öberg et al. not only found complex organics in the outer disk of MWC 480, but determined that their abundance ratios are similar to those found in the comets of our Solar System. This implies that rich organic chemistry of that existed in the solar nebula that formed our Solar System was not unique.

ALMA detection of simple and complex cyanides in the disk surrounding MWC 480. (a) 1.1 mm continuum emission from the dust in the disk, and integrated line emission from the gas component of the disk from (b) H13CN, (c) HC3N and (d) CH3CN. Panels (e-g) show the velocity spread around the mean velocity of the star, showing the disk rotation. (Credit: Oberg et al. 2015, Nature)

ALMA detection of simple and complex cyanides in the disk surrounding MWC 480. (a) 1.1 mm continuum emission from the dust in the disk, and integrated line emission from the gas component of the disk from (b) H13CN, (c) HC3N and (d) CH3CN. Panels (e-g) show the velocity spread around the mean velocity of the star, showing the disk rotation. (Credit: Oberg et al. 2015, Nature)

Laboratory experiments indicate that the chemistry which produces CH3CN also produces simple sugars and amino acids. If complex cyanides and other rich organics are common in icy bodies around young stars, and if as in our Solar System planetary migration brings this icy material to the surface of the inner rocky planets, then the conditions for life are not unique to our Solar System. The results are published in the April 9 issue of the journal Nature.

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