JWST Reveals Hidden Organic Chemistry in Planet-Forming Disk of Young Star T Cha

T Cha is no ordinary young, Sun-like star. It is surrounded by a circumstellar disk, the dense, rotating structure of gas and dust from which planets form.

 

Astronomers have uncovered a remarkable new insight into the evolution of planet-forming systems by studying the young star T Chamaeleontis (T Cha), located about 350 light-years from Earth. Observations show that a partial collapse of the star’s inner circumstellar disk wall allowed ultraviolet radiation to illuminate previously hidden regions of the disk, revealing the presence of complex organic molecules and offering fresh clues about early planetary environments.

T Cha is no ordinary young, Sun-like star. It is surrounded by a circumstellar disk, the dense, rotating structure of gas and dust from which planets form. This disk contains a wide gap, believed to have been carved out by an emerging protoplanet, making the system a prime laboratory for studying how young planets interact with and reshape their natal environments.

A Collapsing Disk Wall Lifts the Veil

Under normal conditions, the dense inner regions of a circumstellar disk act as a protective wall, blocking much of the star’s ultraviolet (UV) radiation from reaching the cooler, outer disk. This shielding makes it difficult to detect polycyclic aromatic hydrocarbons (PAHs)—flat, honeycomb-shaped molecules made of carbon and hydrogen that are considered key precursors to prebiotic chemistry—around low-mass, Sun-like stars.

In 2022, however, NASA’s James Webb Space Telescope (JWST) captured T Cha during a dramatic phase. A sudden burst of accretion caused material from the disk to plunge onto the star, leading to the partial collapse or thinning of the disk’s inner wall. As a result, ultraviolet photons streamed outward, illuminating regions of the disk that had long remained in shadow.

This sudden exposure excited the PAH molecules, causing them to emit strongly in the mid-infrared wavelength range (5–15 microns)—a glow that JWST’s highly sensitive Mid-Infrared Instrument (MIRI) was able to detect with unprecedented clarity.

Indian Scientists Lead the Discovery

Scientists from the Indian Institute of Astrophysics (IIA), an autonomous institute under the Department of Science and Technology (DST), analysed archival JWST-MIRI spectroscopic data to study these molecules in detail. The findings, published in The Astronomical Journal, represent one of the lowest-mass stars known to host a confirmed detection of PAHs in its circumstellar disk.

“JWST’s MIRI has now revealed them clearly in T Cha, making it one of the lowest-mass stars with PAH detection in a planet-forming disk,” said Arun Roy, post-doctoral fellow at IIA and lead author of the study.

A Rare Look at Chemical Stability Over Time

To better understand the significance of the discovery, Roy revisited archival data from NASA’s Spitzer Space Telescope, which observed T Cha in 2002. Although Spitzer’s sensitivity was limited, faint PAH signatures were present even then. This makes T Cha the first confirmed case where PAHs were retrospectively identified in Spitzer data and later observed in much greater detail by JWST.

The comparison between the two datasets revealed a striking result: while the PAH emission became significantly brighter in 2022 due to increased UV illumination, the relative strengths of the emission bands remained nearly unchanged. This indicates that the intrinsic properties of the molecules—such as their size and charge—remained stable over nearly two decades, despite dramatic changes in the disk environment.

The study suggests that the PAHs in T Cha are relatively small, containing fewer than 30 carbon atoms, and that they are resilient enough to survive dynamic disk evolution.

Implications for Planet Formation and Origins of Life

PAHs are widespread in interstellar clouds but have been notoriously difficult to detect in disks around Sun-like stars because such stars emit relatively little ultraviolet radiation. The T Cha observations demonstrate that disk geometry and episodic events, such as accretion-driven collapses, can play a decisive role in revealing or hiding complex chemistry.

“This was like a curtain lifting, revealing chemistry that had been hidden for years,” Roy explained. “JWST has shown us that these molecules can persist and glow brightly when conditions suddenly change.”

Because T Cha hosts a disk gap likely shaped by a forming planet, the system offers valuable insight into how planet formation, disk evolution, and organic chemistry are interconnected. Such environments may represent early stages of chemical complexity that eventually contribute to habitable conditions.

“With JWST still in its prime, we can revisit the disk of T Cha multiple times and track how PAHs evolve as the disk changes,” Roy added, pointing to future time-domain studies that could further reshape our understanding of planetary system evolution.