Astronomers utilizing NASA’s James Webb Space Telescope (JWST) have identified the most chemically rich disk ever observed surrounding a brown dwarf, a type of celestial object sometimes referred to as a “failed star.” This discovery was made around Cha Hα 1, a young brown dwarf encircled by a swirling disk of gas and dust, which could potentially be the birthplace of new planets.
Brown dwarfs do not sustain hydrogen fusion like true stars, but they and their surrounding disks provide crucial insights into the formation of planetary systems. The detection of this unique chemical composition suggests that even these less luminous objects might harbor the essential ingredients for planet formation. Unlike more massive stars, brown dwarfs emit less radiation and heat, resulting in cooler and less turbulent disks. These conditions influence how dust grains and molecules behave, affecting the potential for planet formation.
Kamber Schwarz, a postdoctoral researcher at the Max Planck Institute for Astronomy (MPIA) in Heidelberg, Germany, explained, “In the disks around low-mass stars and brown dwarfs, water-rich dust grains move quickly and are accreted by the star, leaving behind the more carbon-rich dust.” This indicates that planets forming in these disks are likely to possess distinct chemical compositions compared to those around sun-like stars.
The research team observed Cha Hα 1 with JWST’s Mid-Infrared Instrument (MIRI) in August 2022. Their findings align closely with data collected nearly twenty years earlier by the now-retired Spitzer Space Telescope. This consistency verifies that the rich chemistry observed by Webb is not merely a transient phenomenon but a stable characteristic of the brown dwarf’s disk. While Spitzer hinted at this complexity in 2005, JWST’s advanced capabilities now reveal a comprehensive inventory of molecules.
Cha Hα 1’s disk is abundant in hydrocarbons such as methane, acetylene, ethane, and benzene, alongside molecules like water, hydrogen, carbon dioxide (CO2), and large silicate dust grains. Schwarz noted the intriguing presence of both hydrocarbons and oxygen-bearing molecules in the JWST data. The absence of oxygen in the hydrocarbons indicates they formed in an oxygen-poor region of the disk, separate from where water and CO2 originate.
Typically, older disks either favor oxygen-rich environments that produce abundant water and silicates or carbon-rich environments conducive to hydrocarbons. The simultaneous presence of both types suggests a complex chemical environment, potentially influenced by temperature variations, turbulence, or the age of the disk. Schwartz remarked that this disk is likely younger than those surrounding other brown dwarfs.
The MIRI data also revealed emissions from large silicate dust grains in the upper layers of the inner disk, indicating that dust grains are beginning to coalesce even at this early stage. “Dust creates a solid surface in space, which is essential for the formation of complex molecules,” explained Thomas Henning, a professor at MPIA. The presence of dust grains of varying sizes facilitates the rapid growth of cores for giant planets, contrasting with a scenario where all dust is uniform in size.
The relative absence of simpler molecules like carbon dioxide and hydroxide (-OH), paired with the presence of larger, more complex molecules, suggests the disk is already undergoing significant chemical evolution. Schwartz added, “Comparing disks at different evolutionary stages enables us to test our theories regarding what drives this evolution and enhances our understanding of the materials available for planet formation.”
The research team also identified spectral features in Cha Hα 1’s disk that do not correspond to any known molecules studied in Earth-based laboratories. This raises the possibility of previously unobserved or poorly understood molecules requiring further investigation. Henning emphasized that while the team has characterized the gas and dust properties separately, understanding how these elements interact is crucial for comprehending the disk’s evolution.
The discovery of this chemically rich disk offers a rare opportunity to study how chemical processes influence planet formation. Insights gained from examining these molecular reservoirs could shed light on the types of planets that may eventually emerge around brown dwarfs, enhancing our understanding of planetary systems beyond our own.
