Explained | How A Young Star Dropped The Curtain To Reveal Hidden Chemistry That Forms Planets

Bengaluru: About 350 light-years from Earth, a young star called T Chamaeleontis (T Cha) is quietly forming planets. For years, activity inside its dusty, planet-forming disk remained hidden behind a dense inner wall of material. Then, unexpectedly, that wall partially collapsed—and long-hidden chemistry came into view.

T Cha is surrounded by a circumstellar disk, a rotating ring of gas and dust where planets form. The disk contains a wide gap, likely carved out by a newly forming planet. Within this disk are polycyclic aromatic hydrocarbons (PAHs)—complex, carbon-based molecules made of carbon and hydrogen. These flat, honeycomb-shaped molecules are considered important precursors to life, since life on Earth is built on carbon chemistry.

Why PAHs are hard to spot

Normally, the dense inner disk acts like a curtain, blocking much of the star's ultraviolet (UV) radiation from reaching the colder outer regions. This shielding makes PAHs difficult to detect around low-mass, Sun-like stars, like T Cha, which emit relatively little UV light. While PAHs are common in space, spotting them in planet-forming disks has been a long-standing challenge.

The study at a glance (ETV Bharat Creative)

"In most stars, PAHs appear highly stable, showing little change over decades,” said Arun Roy, formerly a postdoctoral fellow at the Indian Institute of Astrophysics (IIA) and now an assistant professor at CHRIST University, Bengaluru, who led the study. “T Cha followed this pattern until recently.

JWST detected strong PAH emission bands

When the Spitzer Space Telescope observed the system in 2002, PAH emission was extremely weak. However, observations made with the James Webb Space Telescope (JWST) in 2022 revealed a dramatic increase in PAH brightness. Such a large variation over two decades is unusual for a low-mass star. The change was not caused by altered chemistry, but by a structural disruption in the disk. In 2022, a burst of high accretion caused material from the disk to fall onto the star, thinning or partially collapsing the inner disk wall. This allowed stellar light to reach regions of the disk that had previously remained in shadow.

An illustration of the James Webb Space Telescope and its key elements (Credits: NASA)

PAHs can only be detected when they absorb stellar light and re-emit it in the infrared. When shielded from light, they remain invisible. Once illuminated, however, the molecules glow strongly.

"It was like a curtain lifting, revealing chemistry that had been hidden for years. Such a collapse had never been identified before because astronomers did not know that this kind of disk disruption could occur. It was only after comparing the JWST data with earlier observations from the Spitzer Space Telescope that researchers realised the inner wall of the disk had partially collapsed, leading to the observed changes in the disk. In the case of T Cha, this turned out to be a very surprising and unexpected discovery. JWST telescope, almost by accident, caught the moment—and an ancient kind of chemistry lit up in space," said Arun Roy.

All about the James Webb Space Telescope (JWST) (ETV Bharat Creative)

Using JWST's Mid-Infrared Instrument (MIRI), scientists from IIA used archival spectroscopic data to study PAHs. Astronomers detected strong PAH emission bands between 5 and 15 microns, making T Cha one of the lowest-mass stars with a clear PAH detection in its circumstellar disk. The results were published in The Astronomical Journal.

Stable molecules, dynamic disk

To check whether the molecules themselves had changed, Roy examined archival Spitzer data. The comparison showed that while PAH emission became much brighter in 2022, the relative intensities of the features remained nearly the same. This indicates that the molecules' size, charge, and structure remained stable over time. The study also found that the PAHs in T Cha are small molecules, containing fewer than 30 carbon atoms. Their survival through a major disk disruption suggests that complex organic chemistry can persist even in highly dynamic, planet-forming environments.

In terms of light-gathering capability, the most important component of a telescope is its primary mirror. The larger the mirror, the more light it can collect, and the smaller, dimmer, and more distant objects it can detect. Webb’s mirror has an area more than 5 times greater than Hubble’s and 45 times greater than Spitzer’s (Credit: STScI via NASA)

"With JWST, we can now revisit the disk of T Cha at multiple times and watch how PAHs evolve as the disk itself changes," Roy noted.

Talking to ETV Bharat, Arun Roy explained why the detection of PAHs around a low-mass, Sun-like star such as T Cha is important for astronomy. He said that the disk around T Cha represents a system similar to our Sun about four billion years ago. Finding complex molecules around a Sun-like star in a planet-forming disk suggests that the ingredients for life do not need to be delivered from elsewhere in the cosmos. Instead, they may already be present at the very earliest stages of planet formation.

Hidden chemistry around young stars

Speaking about the "hidden chemistry" that may exist around many young stars but remains undetected due to disk geometry or limited ultraviolet exposure, Roy explained that researchers are now trying to uncover these hidden molecules using new analysis techniques and combined datasets. He added that some early results—yet to be published—suggest these molecules can be detected through X-ray–driven processes, which would be different from what was observed in T Cha.

Top panel: The MIR spectrum of T Cha observed by JWST (red) in 2022 along with that observed by Spitzer (red) in 2002, showing that the slope of the spectrum had changed due to the collapse of the inner wall of the stellar disk. Bottom panel: the continuum subtracted MIR data from JWST and Spitzer showing the various spectral emission bands from PAH molecules. The PAH signals are much stronger in 2022 but the relative intensities of the features have stayed nearly the same, evidence that the molecules themselves have remained stable over time. (Credits: IIA)

Highlighting how a disk gap influences the chemistry of the surrounding disk, Roy said, "What happens is that the front of the gap—regions previously shielded from the star's light—becomes exposed. This is where molecules like carbon monoxide or silicates are hidden. When a planet carves a gap, these molecules get excited and can be detected. This allows us to actually see what is happening inside the disk. Before, we could only observe the disk's surface. With gaps, we can now probe more complex chemistry deeper within the disk."

PAHs: Seeds of planet formation

PAHs do not directly form planets, but they play an important role in the chemistry of protoplanetary disks. They can help build more complex carbon-based molecules and contribute to the growth of carbonaceous material in regions where dust grains are forming. Over time, these processes enrich the disk environment and support the growth of larger solid particles—such as silicates and carbon-rich grains—that eventually combine to form comet-like bodies and planetary building blocks.

Key discovery made by IIA researchers (ETV Bharat Creative)

“PAH-like molecules may provide some of the earliest chemical starting points in disks, helping create favourable conditions for planet formation,” Roy noted.

A first for T Cha

Roy highlighted a modelling breakthrough, "PAHs with 30 carbon atoms were characterised for the first time in the T Cha disk. Until now, models used by scientists were based on hot stars and considered PAHs with 50 to hundreds of carbon atoms. Now, researchers have started integrating smaller PAHs into disk models to better match observations and obtain a more accurate picture of the disk."

This study shows how disk geometry—not chemistry—can hide or reveal complex organic molecules, offering new insights into planet formation and the early chemistry that could seed life.