ALMA’s Surprising Discovery Enhances Protoplanet Search

In the constellation Sagittarius, a young star called HD 169142 has astounded scientists with an unexpected find—a massive protoplanet nestled within its gas-rich protoplanetary disk. This artist’s depiction illustrates the interaction between the Jupiter-like planet and the neighboring molecular gas, resulting in visible outflows detected in various emission lines, including shock-tracing molecules like SO and SiS, as well as the commonly observed 12CO and 13CO. Image Credit: ALMA (ESO/NAOJ/NRAO), M. Weiss (NRAO/AUI/NSF)

By utilizing the ALMA observatory, researchers investigating the protoplanetary disk surrounding the young star HD 169142 have made a remarkable breakthrough in detecting protoplanets, presenting a novel approach to their study. The identification of silicon monosulfide, a strong indicator of a gas giant protoplanet, offers valuable insights into protoplanetary chemistry.

Employing the Atacama Large Millimeter/submillimeter Array (ALMA), scientists have uncovered the most compelling chemical evidence to date regarding protoplanet formation within a protoplanetary disk encircling a youthful star. This discovery introduces an alternative means for astronomers to detect and characterize protoplanets when direct observations or imaging prove challenging. The findings will be published soon in an upcoming edition of The Astrophysical Journal Letters.

HD 169142, a young star positioned in the Sagittarius constellation, has attracted considerable attention from astronomers due to its significant circumstellar disk, rich in dust and gas, which is observed almost face-on. Over the past decade, numerous protoplanet candidates have been identified. Earlier this year, scientists from the University of Liège and Monash University confirmed that one such candidate, HD 169142 b, is indeed a massive Jupiter-like protoplanet.

The recent analysis of archival data from ALMA, an international collaboration that includes the National Science Foundation’s National Radio Astronomy Observatory (NRAO), has unveiled exciting new discoveries. These findings may facilitate the detection, confirmation, and characterization of protoplanets in the process of formation around young stars.

“When we observed HD 169142 and its disk at submillimeter wavelengths, we identified several compelling chemical signatures associated with this recently confirmed gas giant protoplanet,” explained Charles Law, an astronomer from the Center for Astrophysics | Harvard & Smithsonian and the lead author of the study. “We now have confirmation that chemical signatures can guide us in determining the types of planets that may be forming within disks surrounding young stars.”

The team focused on the HD 169142 system with the expectation that the presence of the giant protoplanet HD 169142 b would be accompanied by detectable chemical signatures, and their hypothesis proved correct. Law’s team detected carbon monoxide (both 12CO and its isotopologue 13CO) and sulfur monoxide (SO), which had been previously observed and associated with protoplanets in other disks.

However, the team made an unprecedented discovery by detecting silicon monosulfide (SiS). This finding surprised researchers since, for ALMA to detect SiS emission, silicates must be released from nearby dust grains through massive shock waves caused by rapidly moving gas—a phenomenon typically generated by outflows driven by massive protoplanets.

“SiS was a molecule that we had never seen before in a protoplanetary disk, let alone in the vicinity of a giant protoplanet,” Law stated. “The detection of SiS emission caught our attention because it indicates that this protoplanet is generating powerful shock waves in the surrounding gas.”

This newfound chemical approach to detecting young protoplanets offers scientists a fresh perspective on the universe and deepens their understanding of exoplanets. Protoplanets, particularly those still embedded within their parental circumstellar disks like those in the HD 169142 system, establish a direct connection with the known exoplanet population.

“Exoplanets exhibit tremendous diversity, and by utilizing chemical signatures observed with ALMA, we gain a new method for comprehending the development of various protoplanets over time, ultimately linking their properties to those of exoplanetary systems,” Law explained.

“In addition to serving as a new tool for planet hunting with ALMA, this discovery unveils a plethora of exciting chemistry that we have never encountered before. As we continue to survey more disks around young stars, we are bound to discover other intriguing and unforeseen molecules, much like SiS. Such discoveries imply that we have only scratched the surface of the vast chemical diversity associated with protoplanetary environments.”

Reference: “SO and SiS Emission Tracing an Embedded Planet and Compact 12CO and 13CO Counterparts in the HD 169142 Disk” by Charles J. Law, Alice S. Booth, Karin I. Öberg, Accepted, The Astrophysical Journal Letters.
arXiv:2306.13710

The National Radio Astronomy Observatory (NRAO) is a significant facility of the National Science Foundation (NSF) operating under a cooperative agreement with Associated Universities, Inc.

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