Planets form in dusty, gas-rich circumstellar disks around young stars, while at the same time, the planet formation process alters the physical and chemical structure of the disk itself. Embedded planets will locally heat the disk and sublimate volatile-rich ices, or in extreme cases, result in shocks that sputter heavy atoms such as silicon from dust grains. This should cause chemical asymmetries detectable in molecular gas observations. Using archival data from the Atacama Large Millimeter/submillimeter Array (ALMA), astronomers detected sulfur monoxide and silicon monosulfide emissions coincident with the position of a giant Jupiter-like protoplanet in the circumstellar disk around the young star HD 169142.
This artist’s conception shows the Jupiter-like protoplanet HD 169142b interacting with and heating nearby molecular gas, driving outflows seen in several emission lines, including those from shock-tracing molecules like SO and SiS, and the commonly seen 12CO and 13CO. Image credit: ALMA / ESO / NAOJ / NRAO / M. Weiss / AUI / NSF.
HD 169142 is a young star located 375 light-years away in the constellation of Sagittarius.
The star is of significant interest to astronomers due to the presence of its large, dust- and gas-rich circumstellar disk that is viewed nearly face-on.
Otherwise known as PDS 514 and TIC 51077087, it hosts at least one protoplanet: HD 169142b.
“When we looked at HD 169142 and its disk at submillimeter wavelengths, we identified several compelling chemical signatures of this recently-confirmed gas giant protoplanet,” said Dr. Charles Law, an astronomer at the Harvard & Smithsonian’s Center for Astrophysics.
“We now have confirmation that we can use chemical signatures to figure out what kinds of planets there might be forming in the disks around young stars.”
Dr. Law ad his colleagues focused on the HD 169142 system because they believed that the presence of the giant protoplanet was likely to be accompanied by detectable chemical signatures, and they were right.
They detected carbon monoxide (both 12CO and its isotopologue 13CO) and sulfur monoxide (SO), which had previously been detected and were thought to be associated with protoplanets in other disks.
But for the first time, the astronomers also detected silicon 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 protoplanetary disk, let alone in the vicinity of a giant protoplanet,” Dr. 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 surrounding gas.”
With this new chemical approach for detecting young protoplanets, scientists 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 connection with the known exoplanet population.
“There’s a huge diversity in exoplanets and by using chemical signatures observed with ALMA, this gives us a new way to understand how different protoplanets develop over time and ultimately connect their properties to that of exoplanetary systems,” Dr. Law said.
“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 molecules, 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.”
The findings will be published in the Astrophysical Journal Letters.
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Charles J. Law et al. 2023. SO and SiS Emission Tracing an Embedded Planet and Compact 12CO and 13CO Counterparts in the HD 169142 Disk. ApJL, in press; arXiv: 2306.13710
