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ALMA Discovers New Clues to Planet Formation

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The Atacama Large Millimeter/submillimeter Array (ALMA) has achieved a groundbreaking milestone in astronomy by detecting a concentrated accumulation of planet-forming dust grains in a protoplanetary disk around a young star, PDS 70. This discovery provides new insights into how planets interact with their surrounding environments and influence the formation of subsequent planets, shedding light on the intricate processes that lead to the development of planetary systems like our solar system.

This remarkable observation was conducted by an international research team led by Kiyoaki Doi, who was a Ph.D. student at the National Astronomical Observatory of Japan (NAOJ) and the Graduate University for Advanced Studies, SOKENDAI, at the time of the study. Now a postdoctoral fellow at the Max Planck Institute for Astronomy, Doi and his team utilized ALMA to capture high-resolution images of the PDS 70 system at a wavelength of 3 millimeters. The findings were recently published in Astrophysical Journal Letters under the title “Asymmetric Dust Accumulation of the PDS 70 Disk Revealed by ALMA Band 3 Observations.” The paper is also accessible on the arXiv preprint server.

PDS 70 is a unique celestial system and the only one currently known to host already-formed planets within a protoplanetary disk. These planets, confirmed through optical and infrared observations, are surrounded by a dense disk of gas and dust—the raw materials from which planets are born. Observing the distribution of these dust grains is crucial for understanding how young planets interact with their environments and potentially stimulate the formation of additional planets.

Planet formation begins in protoplanetary disks, where micron-sized dust grains coalesce over time to form larger structures, eventually giving rise to planets. However, the exact mechanisms by which dust grains accumulate locally to initiate this process remain elusive. While over 5,000 exoplanets have been identified to date, many of which exist in multi-planet systems, the details of their origins from these dust grains are still being unraveled.

Earlier ALMA observations of PDS 70 at a wavelength of 0.87 millimeters revealed ring-shaped emissions of dust grains beyond the planetary orbits. However, these observations were potentially optically thick, meaning the emissions might have obscured the true distribution of dust. To address this limitation, Doi’s team turned to longer-wavelength observations at 3 millimeters, which are optically thinner and provide a more transparent view of the dust distribution.

The high-resolution images captured by ALMA revealed something extraordinary: the dust emissions at 3 millimeters were concentrated in a specific direction within the outer dust ring, forming a localized clump. This distribution differs significantly from previous observations, highlighting the advantages of multi-wavelength studies. The localized clumping of dust grains suggests that the already-formed planets in the system are interacting with the surrounding disk, concentrating the dust into a narrow region at the outer edge of their orbits. Over time, these clumped dust grains may coalesce to form a new planet.

This sequential planet formation process—where already-formed planets pave the way for the next generation of planets by accumulating and concentrating dust—is a compelling explanation for the development of planetary systems. In the case of our solar system, this process may have contributed to the orderly arrangement of planets from the Sun outward.

The findings from PDS 70 provide the first observational evidence of this sequential formation process. They also underscore the importance of multi-wavelength observations in astronomy. By observing the same system across different wavelengths, researchers can gain a more comprehensive understanding of its components. For instance, in PDS 70, the planets were discovered using optical and infrared wavelengths, while the protoplanetary disk was studied using millimeter wavelengths. Each wavelength range offers unique insights, highlighting different aspects of the system.

Doi emphasizes this interdisciplinary approach, stating, “A celestial object is made up of multiple components, each emitting radiation at different wavelengths. Thus, observing the same object at multiple wavelengths offers a unique perspective on the target. This work shows that the disk exhibits different morphologies, even within the observation wavelength range of ALMA. Observing multiple components of a target with various observational settings with different telescopes is necessary for a comprehensive understanding of the entire system.”

The study of PDS 70 is a prime example of how cutting-edge technology and collaboration among international teams are advancing our understanding of planetary formation. ALMA, one of the world’s most powerful radio observatories, continues to play a vital role in unveiling the secrets of the universe. By capturing detailed images of protoplanetary disks and their interactions with young planets, ALMA enables researchers to test and refine theories about how planetary systems evolve.

This discovery not only enhances our knowledge of PDS 70 but also provides a framework for studying other planetary systems. As astronomers identify more systems with protoplanetary disks and young planets, the methods used in this study will serve as a blueprint for future investigations. Understanding the dynamics of dust accumulation and planet formation is key to piecing together the complex puzzle of how planets and planetary systems come to be.

The research conducted by Doi and his team represents a significant step forward in the field of astrophysics. By combining high-resolution observations with innovative analysis techniques, they have captured a snapshot of the dynamic processes that shape planetary systems. These findings not only deepen our understanding of PDS 70 but also offer a glimpse into the universal mechanisms that govern the formation of planets and, ultimately, the potential for life beyond our solar system.