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For decades, astronomers have discovered hundreds of protoplanetary disks—structures believed to represent the early stages of our own solar system. However, most of these discoveries lie within our neighborhood, which may not reflect the extreme conditions found in other parts of the Milky Way.

Among the most dynamic and turbulent regions is the Central Molecular Zone (CMZ) near the Milky Way galactic center, where high pressure and density may shape star and planet formation in fundamentally different ways. Studying protoplanetary systems in the CMZ provides a rare opportunity to test and refine our theories of solar system formation.

An international team of researchers have conducted the most sensitive, highest-resolution, and most complete survey to date of three representative molecular clouds in the Milky Way's CMZ. Their observations revealed over five hundred dense cores—the sites where stars are being born.

The results have been in the journal Astronomy & Astrophysics under the title "Dual-band Unified Exploration of three Central Molecular Zone Clouds (DUET)." The team includes researchers from the Kavli Institute for Astronomy and Astrophysics at Peking University (KIAA, PKU), the Shanghai Astronomical Observatory (SHAO), and the Institute of Astrophysics of the University of Cologne (UoC), along with several collaborating institutions.

Detecting such systems in the CMZ is exceptionally challenging. These regions are distant, faint and deeply embedded in thick layers of interstellar dust. To overcome these obstacles, the team utilized the Atacama Large Millimeter/submillimeter Array (ALMA) in the Chilean Atacama Desert, an interferometric telescope that combines signals from antennas spread over several kilometers to achieve extraordinary angular resolution.

"This allows us to resolve structures as small as a thousand astronomical units even at CMZ distances of roughly 17 billion AU away," said Professor Xing Lu, a researcher at Shanghai Astronomical Observatory and the Principal Investigator of the ALMA observing project.

By reconfiguring the array and observing at multiple frequencies, the team performed "dual-band" observations—capturing two different wavelengths at the same spatial resolution. Just as human vision relies on color contrast to interpret the world, dual-band imaging provides critical spectral information about the temperature, dust properties and structure of these remote systems.

To their surprise, the researchers found that more than 70% of the dense cores appeared significantly redder than expected. After carefully ruling out observational bias and other possible explanations, they proposed two leading scenarios—both suggesting the widespread presence of protoplanetary disks.

"We were astonished to see these 'little red dots' cross the whole molecular clouds," said first author Fengwei Xu, who is currently conducting research at the University of Cologne's Institute of Astrophysics in the context of his doctoral work. "They are telling us the hidden nature of dense star-forming cores."

One possible explanation is that these cores are not transparent, homogeneous spheres as once thought. Instead, they may contain smaller, optically thick structures—possibly protoplanetary disks—whose self-absorption at shorter wavelengths results in the observed reddening. "This challenges our original assumption of canonical dense cores," said Professor Ke Wang, Fengwei Xu's doctoral supervisor at the Kavli Institute.

Another possibility involves the growth of dust grains within these systems. "In the diffuse interstellar medium, dust grains are usually just a few microns in size," explained Professor Hauyu Baobab Liu at the Department of Â鶹ÒùÔºics of National Sun Yat-sen University, who led the radiative transfer modeling in the study. "But our models indicate that some cores may contain millimeter-sized grains, which could only form in protoplanetary disks and then be expelled—perhaps by protostellar outflows."

Regardless of which scenario proves dominant, both require the presence of protoplanetary disks. The findings suggest that over three hundred such systems may already be forming within just these three CMZ clouds.

"It is exciting that we are detecting possible candidates for protoplanetary disks in the galactic center. The conditions there are very different from our neighborhood, and this may give us a chance to study planet formation in this extreme environment," said Professor Peter Schilke at the University of Cologne, Fengwei Xu's doctoral co-supervisor. Computing resources and technical support at the UoC's Institute of Astrophysics contributed to the result.

Future multi-band observations will help to further constrain their physical properties and evolutionary stages, offering a rare glimpse into the early processes that give rise to planetary systems like our own, even in the most extreme corners of the Milky Way.