Scientific Paper

By combining ALMA observations and simulations, the ODISEA team traces how planets may form and reshape their disks

Ever since ALMA captured the striking image of HL Tau in 2014, revealing intricate rings and gaps in a disk around a newborn star, astronomers have sought to understand how such complex structures could emerge so early. The surprise deepened in 2018 when the DSHARP survey showed these features were common across many protoplanetary disks, sparking debate over whether planets were behind them.

Now, using data from the Atacama Large Millimeter/submillimeter Array (ALMA) and advanced simulations, a research team led by Santiago Orcajo from the Instituto de Astrofísica de La Plata in Argentina (CONICET and Universidad Nacional de La Plata) in collaboration with researchers from the YEMS Millennium Nucleus (Chile) has presented a new model that traces the evolution of these disks through five distinct stages. The results strongly support a planet-driven origin of these substructures and offer new insights into how planets interact with the disks in which they form.

Protoplanetary disks are the birthplaces of planetary systems, and understanding their evolution is crucial for comprehending planet formation processes. The surprising image of HL Tau captured by ALMA in 2014 prompted astronomers to ask: How could a young protostar system already show such well-defined rings and gaps?

In 2018, the Disk Substructures at High Angular Resolution Project (DSHARP) showed that rings and gaps are widespread in most protoplanetary disks. These findings further challenged our understanding of the planet formation process and generated significant skepticism about their planetary origin.

By using ALMA observational data and PlanetaLP and Radmc-3D simulations, an international scientific team led by Orcajo has now been able to reproduce each one of the stages of the evolutionary sequence proposed by the Ophiuchus Disk Survey Employing ALMA (ODISEA) project in 2021, providing strong evidence in support of the planet formation scenario. This could also confirm the mechanisms by which giant planets affect dust dynamics and the formation of substructures such as gaps and rings.

"In science, we look for patterns and similarities and search for the simplest explanation that might account for many observations. We realized that the disks could be organized in several groups, and each group showed distinct properties that may be linked to distinct stages of a single underlying process: planet formation," said Lucas Cieza leader of the ODISEA project and Full Professor at the Institute of Astrophysical Studies at the Diego Portales University, Chile.

The ODISEA sequence proposes categorizing protoplanetary disks into five distinct stages, each characterized by specific features related to planet formation. Observations indicate that young disks (Stage I¹) exhibit minimal substructure. At the same time, as protoplanets grow, they begin to carve gaps and create rings (Stages II² and III³) due to their gravitational interactions with the surrounding material. These gaps indicate the presence of giant planets, which can form within approximately 1 million years or less at significant distances from their host stars. Large central dust cavities become clear as the disks evolve (Stages IV⁴ and V⁵), marking advanced evolution due to the interactions between the disk and forming planets.

Giant planets significantly influence dust dynamics within protoplanetary disks by creating gaps and pressure bumps that alter the distribution of gas and dust. As a giant planet forms, it generates a deep gap in the disk, redistributing gas density and accumulating millimeter-sized dust at the edges of these gaps. This process drives the evolution of dust within the disk and facilitates the formation of ring-like structures. Simulations using models like PlanetaLP have demonstrated how these gravitational effects lead to observable features in the disk, which can be directly compared with high-resolution ALMA observations.

"Working on this study, we found that the PlanetaLP evolution simulation code allows us to find possible configurations of planets (of different masses and orbits) that form disks with gap and ring structures after thousands of years of evolution, like those we see with ALMA observations. In several tests, we noticed that planets' existence extends the inner disk's lifetime. While the possibilities are endless, planets affect the disk morphology. The first motivation was to recreate the Elias 2-24 disk from simulations. Still, we then realized that our code could reproduce the entire evolutionary sequence," concluded the main author, Santiago Orcajo.

The implications of this work are significant, especially for interpreting the original HL Tau image. "This kind of study is deeply relevant to ALMA because it supports one of the array's most iconic discoveries," said Antonio Hales, an ALMA astronomer and co-author of the study. "By showing that these structures are likely caused by forming planets, we're not just observing disks—we're watching the process of planet formation unfold in real-time. ALMA becomes not just a disk imager, but a powerful tool for planet detection."

The findings also highlight current challenges in explaining how massive planets can form so quickly and far from their host stars. As research continues, detecting minor, rocky planets in fainter disks remains a promising and ambitious goal to understand the origins of planetary systems like our own.

Additional Information

The results of the study are published in the Astrophysical Journal Letters in the following scientific article by Orcajo et al.: "The Ophiuchus DIsk Survey Employing ALMA (ODISEA): A Unified Evolutionary Sequence of Planet-Driven Substructures Explaining the Diversity of Disk Morphologies."

The research team is composed of young researchers from the Institute of Astrophysics of La Plata (CONICET and National University of La Plata, Argentina) and the Millennium Nucleus for the Study of Young Exoplanets and their Moons (YEMS, Chile), a research center funded by the National Agency for Research and Development of Chile (ANID) through its Millennium Scientific Initiative program, and housed at the Diego Portales University, the University of Santiago de Chile, the Pontifical Catholic University of Chile and the University of Concepción.

The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of the European Southern Observatory (ESO), the U.S. National Science Foundation (NSF), and the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Republic of Chile. ALMA is funded by ESO on behalf of its Member States, by NSF in cooperation with the National Research Council of Canada (NRC) and the National Science and Technology Council (NSTC) in Taiwan, and by NINS in cooperation with the Academia Sinica (AS) in Taiwan and the Korea Astronomy and Space Science Institute (KASI).

ALMA construction and operations are led by ESO on behalf of its Member States; by the National Radio Astronomy Observatory (NRAO), managed by Associated Universities, Inc. (AUI), on behalf of North America; and by the National Astronomical Observatory of Japan (NAOJ) on behalf of East Asia. The Joint ALMA Observatory (JAO) provides the unified leadership and management of ALMA's construction, commissioning, and operation.

Images

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Foot Notes

1. Stage I: Very young disks with shallow or no obvious substructures, corresponding to an epoch in which protoplanets are not massive enough to carve noticeable gaps in the disks. ↩︎
2. Stage II: Disks with relatively narrow, but clear gaps and rings, indicating the growth of protoplanets ↩︎
3. Stage III: A rapid widening of the gaps due to the sudden growth in the mass of some planets when they acquire their gaseous envelopes. This stage includes the rapid accumulation of dust at the outer edges of the gaps (the inner rims of the outer disks) due to the strong “pressure bumps” caused by the giant planets that recently formed, which stops the inward drift of dust. ↩︎
4. Stage IV: Dust filtration at the edges of the cavities, resulting in dust-depleted inner disks. The millimeter dust from the outer disks efficiently drifts in and accumulates at the edges of the gaps. ↩︎
5. Stage V: Eventually, the dusty inner disks drain completely onto the stars, and the outer disks become narrow rings (or collections of narrow rings). ↩︎