Planets Can Form in Even the Harshest Conditions

According to the most widely held astronomical model (the Nebular Hypothesis), new stars are born from massive clouds of dust and gas (aka. a nebula) that experience gravitational collapse. The remaining dust and gas form a protoplanetary disk that encircles the new star, which slowly accretes to form systems of planets. For the past decade, astronomers have relied on the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile to study young stars and their disks and learn more about how this process occurs.

In a recent study, an international team of astronomers used ALMA to capture high-resolution images of eight protoplanetary disks in the Sigma Orionis cluster, a group of stars located in the constellation Orion. During their observations, the team found evidence of gaps and rings in most of the disks, which are potential indications that giant planets are forming. This was surprising, seeing as how these disks are irradiated by intense ultraviolet (UV) radiation from a massive star in the cluster. Their findings suggest that planet formation can occur in conditions that were previously thought to be inhospitable.

The study was led by Jane Huang, an assistant professor in the Department of Astronomy at Columbia University. She was joined by Shangjia Zhang, a NASA Sagan Fellow from Columbia University and the Nevada Center for Astrophysics, and Feng Long (also a NASA Sagan Fellow) from the Lunar and Planetary Laboratory (LPL). The team also included researchers from the Ludwig Maximilian University of Munich (LMU), the University of St. Andrews, the University of Hawaii at Manoa, and NASA Headquarters. Their research was recently published in The Astrophysical Journal.

The research team used the most extended configuration of ALMA’s 12-meter antennas, which provided a zoom lens effect, allowing them to achieve a resolution of about eight astronomical units (eight times the distance between the Sun and Earth). This allowed them to resolve multiple gaps and rings in images of five of the disks, comparable to what astronomers have observed in other systems where giant planets were forming. The most impressive of these was the disk known as SO 1274, which features five gaps that could be a system of planets in the making.

Whereas previous studies have focused on disks in regions with low ultraviolet radiation, this research provides ALMA’s highest-resolution observations of disks in a more extreme environment. While stars are typically born in much harsher UV environments, astronomers understanding of substructures is primarily based on observations of nearby star-forming regions with mild UV environments. These findings could have implications for our understanding of how the Solar System formed, which may have evolved in a similarly high-radiation environment. As Huang noted in an NRAO press release:

“We expected the high levels of radiation in this cluster to inhibit planet formation in the outer regions of these disks. 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. These observations suggest that the processes driving planet formation are quite robust and can operate even under challenging circumstances. 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.”

However, the team acknowledges that these structures could also result from interactions between planets in formation and the disk material. Their findings, therefore, illustrate the need and potential for follow-up studies of disks in even more extreme stellar environments. It also demonstrates the ability of ALMA to probe protoplanetary disks in diverse environments throughout the galaxy.

Further Reading: NRAO, The Astrophysical Journal