Released: 25th June, 2025, Academia Sinica Institute of Astronomy & Astrophysics (ASIAA), Taiwan 

Our galaxy, the Milky Way, is not only filled with shining stars and planets—it’s also filled with dust grains. These tiny particles are the raw material for forming stars and planets, and they influence how light travels through space. Yet, many questions remain about where this dust comes from, what it’s made of, and how it evolves. To explore these mysteries, astronomers launched a research program called Webb Investigation of Silicates, Carbons, and Ices (WISCI). Using the powerful James Webb Space Telescope (JWST), together with the Hubble Space Telescope (HST) and major ground-based observatories like the Very Large Telescope (VLT), the team set out to study dust in the diffuse interstellar medium across a broad range of wavelengths—from ultraviolet to mid-infrared.

WISCI targets 12 bright O- and B-type stars that serve as ideal background light sources for detecting the “fingerprints” of dust along the line of sight. In the first phase of the study, the team focused on two stars: GSC 08152-02121 and CPD-59 5831. They discovered faint absorption features in the infrared, revealing the presence of carbon-rich dust, silicates, and even signs of water molecules. “Surprisingly, the water wasn’t in the form of ice, but appears to be tightly bound within dust grains, suggesting a stronger interaction between water and dust than previously thought,” noted Sascha Zeegers of ESA-ESTEC, first author of the study, and Principal Investigator of the WISCI project. Sascha is a former postdoctoral fellow at the Academia Sinica Institute of Astronomy and Astrophysics (ASIAA), where she undertook the research that led to her role in leading the WISCI collaboration and being awarded JWST time.

The team also detected vibrational signatures from both carbon–carbon and carbon–hydrogen bonds. The presence of C–H bonds is unexpected, as such bonds are normally destroyed by ultraviolet radiation in diffuse environments. In every line of sight, silicates were clearly observed, but the subtle differences in their spectral profiles suggest variations in the composition or structure of these dust grains across different regions of space.

One of WISCI’s major strengths lies in its multi-wavelength observational strategy. “By combining infrared spectra from JWST with visible light from the VLT and ultraviolet data from HST, we are able to trace the same dust grains across a wide range of wavelengths. This allows us to compare the behavior of carbon-based and silicate dust in detail and uncover potential connections between them,” said Jonathan Marshall, second author of the study and NSTC research scholar at ASIAA.

These early findings show that even in regions once considered relatively “clean,” the universe is filled with complex and diverse dust. As more data from WISCI is analyzed, astronomers hope to deepen our understanding of how this dust evolves—and how it ultimately contributes to the birth of stars, planets, and perhaps even life itself.

These figures show the full spectrum of the two WISCI stars from the ultraviolet (UV, observed with the HST) to the optical, near infrared (NIR; with the XSHOOTER instrument on the VLT) and infrared (with JWST). The star on the left side is less obscured by dust than the one on the right side. The insets show the main features we encounter that arise from absorption of the stellar light by dust in the interstellar medium, including carbon and silicate dust features and possibly water trapped in the dust grains. The background shows the interstellar medium near the WISCI stars, observed by the MIRI instrument on JWST. Image Credit: Gilles Otten (ASIAA) and Sascha Zeegers (ESA). 

More Information:

This research was presented in a paper "Investigating Silicate, Carbon, and Water in the Diffuse Interstellar Medium: The First Shots from WISCI" by Zeegers et al., Astrophysical Journal, June 24, 2025.

Media Contact:

Dr. Sascha Tamara Zeegers, sascha.zeegers@esa.int 

Dr. Jonathan Marshall, +886-2-2366-5463, jmarshall@asiaa.sinica.edu.tw 

Dr. Mei-Yin Chou, +886-2-2366-5415, cmy@asiaa.sinica.edu.tw