Planets Can Form Even in Harsh Stellar Environments, New ALMA Observations Suggest

Using the Atacama Large Millimeter/submillimeter Array (ALMA), astronomers have captured high-resolution images of eight protoplanetary disks in Sigma Orionis, a star cluster irradiated by intense ultraviolet light from a massive star. To their surprise, they’ve found evidence of gaps and rings in most of the disks — substructures commonly associated with the formation of giant exoplanets.

“We expected the high levels of radiation in this cluster to inhibit planet formation in the outer regions of these disks,” said Columbia University astronomer Jane Huang.

“But instead, we’re seeing signs that planets may be forming at distances of tens of astronomical units from their stars, similar to what we’ve observed in less harsh environments.”

Previous studies had focused on disks in regions with low ultraviolet (UV) radiation.

The current study provides the ALMA’s highest resolution look at disks in a more extreme environment.

“These observations suggest that the processes driving planet formation are quite robust and can operate even under challenging circumstances,” Dr. Huang said.

“This gives us more confidence that planets may be forming in even more places throughout the galaxy, even in regions we previously thought were too harsh.”

The findings have implications for understanding the formation of our own Solar System, which likely evolved in a similarly high-radiation environment.

They also motivate future studies of disks in even more extreme stellar neighborhoods.

The astronomers used ALMA’s most extended antenna configuration to obtain unprecedented detail in their disk images, achieving a resolution of about 8 AU (astronomical units).

This allowed them to resolve multiple distinct gaps and rings in several of the disks.

While the exact nature of these disk structures is still debated, they are thought to be either conducive to planet formation or a consequence of interactions between forming planets and the disk material.

“Our observations suggest that substructures are common not only in disks in the mildly irradiated nearby star-forming regions but also in disks exposed to intermediate levels of external UV radiation,” the researchers said.

“If these substructures trace planet-disk interactions, ice and gas giants may still be forming on solar system scales in Sigma Orionis, but giant planet formation at significantly larger semimajor axes (50-100 AU) may be rarer compared to nearby star-forming regions.”

“These observations motivate high-resolution imaging of disks in more extreme UV environments to investigate the universality of disk substructures.”

The findings appear this week in the Astrophysical Journal.

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Jane Huang et al. 2024. High-resolution ALMA Observations of Richly Structured Protoplanetary Disks in σ Orionis. ApJ 976, 132; doi: 10.3847/1538-4357/ad84df