UV Radiation from Massive Stars Shapes Nearby Planetary Systems

For decades, astronomers have known that the powerful light emitted by massive stars can disrupt the swirling planetary disk of dust and gas around young stars, the very cradle from which planets are born. But a crucial question remained unanswered: how quickly does this process occur, and does it leave enough material to form planets? Utilizing the NASA/ESA/CSA James Webb Space Telescope and the Atacama Large Millimeter Array (ALMA), astronomers have now studied a stellar nursery, the Orion Nebula, specifically a protoplanetary disk named d203-506, where the planet-forming material, normally confined to a tiny region, has been blown up to an unusually large size. This allowed them to measure the rate of material loss with unprecedented accuracy.

Young low-mass stars are often surrounded by relatively short-lived protoplanetary disks of dust and gas, which provide the raw materials from which planets form.

As such, gas giant planet formation is limited by processes that remove mass from protoplanetary disks, such as photoevaporation.

Photoevaporation occurs when the upper layers of protoplanetary disks are heated by X-ray or ultraviolet protons, which increases the gas temperature and causes it to escape from the system.

Since most low-mass stars form in clusters also containing massive stars, protoplanetary disks are expected to be exposed to external radiation and experience ultraviolet-driven photoevaporation.

Theoretical models predict that far-ultraviolet radiation produces photodissociation regions — areas where ultraviolet photons cast by nearby massive stars strongly influence gas chemistry on the surfaces of protoplanetary disks. However, direct observation of these processes has been elusive.

Dr. Thomas Haworth from Queen Mary University of London and colleagues combined infrared, submillimeter, and optical observations from Webb and ALMA of the protoplanetary disk d203-506 in the Orion Nebula to determine the effect of ultraviolet irradiation.

By modeling the kinematics and excitation of the emission lines detected within the photodissociation region, they found that d203-506 is losing mass at a high rate due to far-ultraviolet-driven heating and ionization.

According to the team, the rate at which this mass is being lost from d203-506 indicates that the gas could be removed from the disk within a million years, suppressing the ability for gas giants to form within the system.

“This is a truly exceptional case study,” said Dr. Haworth, co-author of a paper published in the journal Science.

“The results are stark: the young star is losing a staggering 20 Earth masses of material per year, suggesting that no Jupiter-like planets could possibly form in this system.”

“The rate we measured aligns perfectly with our theoretical models, giving us confidence to understand how different environments shape planet formation across the Universe.”

“Unlike other known cases, this young star is only exposed to one type of UV radiation from the nearby massive star.”

“The lack of a ‘hot cocoon’ created by the more energetic UV radiation allows the planet-forming material to become much larger and easier to study.”

_____

Olivier Berné et al. 2024. A far-ultraviolet-driven photoevaporation flow observed in a protoplanetary disk. Science 383 (6686): 988-992; doi: 10.1126/science.adh2861