ALMA observes a magnetic field in a protoplanetary disk for the first time 

6 February, 2025 / Read time: 5 minutes

Based on the Atacama Large Millimeter/submillimeter Array (ALMA) multi-band observations of dust polarization, the magnetic field structure in a protoplanetary disk has been measured for the first time. A research team led by Project Assistant Professor Satoshi Ohashi (NAOJ) investigated the dust polarization in the protoplanetary disk around a young star, HD 142527, by analyzing ALMA 0.9, 1-, 2-, and 3-mm observation data. As a result, they have discovered intense dust polarization primarily due to the magnetic field. Furthermore, they successfully estimated the magnetic field's strength and three-dimensional structure. This research can help understand magnetic fields' role in planet formation.  

To study the origin of life, it is important to understand how Earth-like planets form. It has been thought that a planet forms by gradually accumulating interstellar dust and gas within a protoplanetary disk surrounding a young star. However, there are still many mysteries about the environment of planet formation and physical conditions inside the protoplanetary disk.  

One of those mysteries is the magnetic field. It is thought to be a crucial factor in determining the turbulence and the movement of matter in a protoplanetary disk. Nevertheless, observing the magnetic field in a protoplanetary disk has been challenging. The magnetic field is an invisible vector field produced by a magnet or electric current. The magnetic field influences matter and energy like Earth's magnetic field. This invisible force exists everywhere in the Universe and has much to do with star and planet formation.  

HD 142527 is a protoplanetary disk with many dust particles gathering in the north part (the upper side in Figure 1), where planets are forming. 

Ohashi and his research group carried out ALMA 3 mm observations of dust polarization in the protoplanetary disk around HD 142527. They analyzed the polarization mechanisms by combining their new data with the archived data of ALMA 0.9, 1-, and 2-mm observations of HD 142527. They found, in the south part (the lower side in Figure 1), the same polarization pattern in all the observed wavelengths, suggesting that this characterized polarization pattern (the polarization vector is the same as the radial direction of the disk and has intense polarization over 10%) is caused by the magnetic field.  

In addition, the research group found that the direction of the magnetic field varies slightly along the rotation direction of the disk. They thought that the magnetic field's three-dimensional structure caused these changes in direction. They estimated that the strength of the magnetic field is about 0.3 milligauss along the rotation direction and about 0.1 milligauss in both the vertical direction and the direction toward the central star. The team suggests that this three-dimensional structure might create a strong turbulence field within the disk.  

Many protoplanetary disks have been found, but this is the first time the magnetic field's detailed structure can be observed. Interestingly, it was not theoretically expected that the magnetic field would have a certain level of strength toward the central star and in the vertical direction of the disk around a young star like HD 142527; therefore, further investigations are needed.  

Satoshi Ohashi says, "We have found the conditions to observe the magnetic field in a protoplanetary disk and provided a method to measure the three-dimensional structure and the strength of the magnetic field. Therefore, our findings can be a huge help to future observations of the magnetic fields of the protoplanetary disks. On the other hand, during the formation of planets like the Earth, in highly turbulent conditions, interstellar dust grains collide with each other at high speeds, destroying themselves, which might prevent planets from forming. While we observed the magnetic field relatively far from the central star this time, it will be important in the future to observe the magnetic fields at the formation places of planets close to the central stars, like the Earth. Our findings will pave the road to future research on how the magnetic field influences planet formation".  

Additional Information 
 
This research was published in Nature Astronomy on February 5, 2025, as "Observationally derived magnetic field strength and 3D components in the HD 142527 disk" (DOI: 10.1038/s41550-024-02454-x).  

This project is also supported by Grants-in-Aid from the Japan Society for the Promotion of Science (KAKENHI: Nos. JP20K14533, JP20K04017, JP20H00182, JP22H01275, JP23K22546, JP23K03463, JP24K07097).  
 
The original press release was published by the National Astronomical Observatory of Japan (NAOJ), an ALMA partner on behalf of East Asia. 

The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of the European Organization for Astronomical Research in the Southern Hemisphere (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 the construction, commissioning, and operation of ALMA. 

Images