Free Astronomy Magazine September-October 2023

earlier this year, scien- tists at the University of Liège and Monash Uni- versity confirmed that one such candidate — HD 169142 b— is, in fact, a giant Jupiter-like protoplanet. The dis- coveries revealed in a new analysis of archival data from ALMA may now make it easier for scientists to detect, confirm, and ultimately characterize, proto- planets forming around young stars. “When we looked at HD 169142 and its disk at submillimeter wave- lengths, we identified several compelling chemical signatures of this recently-confirmed gas giant protoplanet,” said Charles Law, an as- tronomer at the Center for Astrophysics | Har- vard & Smithsonian, and the lead author of the new study. “We now have confirmation that we can use chemi- cal signatures to figure out what kinds of planets there might be forming in the disks around young stars.” The team focused on the HD 169142 system because they believed that the presence of the HD 169142 b giant protoplanet was likely to be accompanied by detectable chemical signatures, and they were right. Law’s team detected carbon monoxide (both 12 CO and its isotopologue 13 CO) and sulfur monoxide (SO), which had previ- ously been detected and were 17 SEPTEMBER-OCTOBER 2023 ASTRO PUBLISHING the presence of its large, dust- and gas-rich circumstellar disk that is viewed nearly face-on. Several pro- toplanet candidates have been iden- tified over the last decade, and thought to be associated with pro- toplanets in other disks. But for the first time, the team also detected sil- icon monosulfide (SiS). This came as a surprise because in order for SiS emission to be detectable by ALMA, silicates must be released from nearby dust grains in massive shock waves caused by gas traveling at high velocities, a behavior typically resulting from outflows that are driven by giant protoplanets. “SiS was a molecule that we had never seen before in a protoplane- tary disk, let alone in the vicinity of a giant protoplanet,” Law said. “The detection of SiS emission popped out at us because it means that this protoplanet must be producing powerful shock waves in the sur- rounding gas.” With this new chemical approach for detecting young protoplanets, scien- tists may be opening a new window on the Universe and deepening their understanding of exoplanets. Protoplanets, especially those that are still embedded in their parental circumstellar disks such as in the HD 169142 system, provide a direct con- nection with the known exoplanet population. “There’s a huge diver- sity in exoplanets and by using chemical signatures observed with ALMA, this gives us a new way to understand how different proto- planets develop over time and ulti- mately connect their properties to that of exoplanetary systems,” said Law. “In addition to providing a new tool for planet-hunting with ALMA, this discovery opens up a lot of exciting chemistry that we’ve never seen before. As we continue to survey more disks around young stars, we will inevitably find other interesting but unanticipated mole- cules, just like SiS. Discoveries such as this imply that we are only just scratching the surface of the true chemical diversity associated with protoplanetary settings.” L ocated in the constellation Sagit- tarius, the young star HD 169142 is host to a giant protoplanet em- bedded within its dusty, gas-rich protoplanetary disk. This artist’s conception shows the Jupiter-like planet interacting with and heating nearby molecular gas, driving out- flows seen in several emission lines, including those from shock-tracing molecules like SO and SiS, and the commonly seen 12 CO and 13 CO. (Not to scale) [(ESO/NAOJ/NRAO), M. Weiss (NRAO/AUI/NSF)] !