Astronomers have been identifying protoplanetary disks for many decades—structures that evolve into solar systems like our own—and they have been found by the hundreds throughout the Milky Way, primarily in relatively calm regions of the galaxy.

Now, scientists have discovered more than 500 of these cores thanks to observations made with the ALMA telescope in Chile, but surprisingly, they are located in the so-called Central Molecular Zone (CMZ), the region near Sagittarius A*, the supermassive black hole at the center of the Milky Way where high pressures and extreme densities make the finding a challenge to conventional theories of planet formation.

The study, led by researchers from the Kavli Institute for Astronomy and Astrophysics (Beijing), the Shanghai Astronomical Observatory, and the University of Cologne, found that over 70% of these cores exhibit an unexpectedly “red” spectral signature, indicating the presence of hidden protoplanetary disks.

A Puzzle 27,000 Light-Years Away

Located about 17 billion astronomical units from Earth (approximately 27,000 light-years), the CMZ is a region where molecular gas is compressed to densities thousands of times higher than those in our cosmic neighborhood.

center milky way protoplanetary disks solar systems
Images from the ALMA telescope of the central molecular zone of the Milky Way. The research team suspects that protoplanetary disks are forming in the clouds there. Credit: Fengwei Xu (PKU) / Laura Pérez (NRAO) / ALMA Partnership

Until recently, observing its structures in detail was technologically impossible due to its being shrouded in thick interstellar dust. But thanks to the exceptional angular resolution of ALMA’s interferometer, scientists managed to observe details as small as a thousand astronomical units—approximately the size of the Kuiper Belt.

With this tool, we were able to distinguish structures that, at such great distances, measure just a thousand astronomical units, explained Professor Xing Lu, a researcher at the Shanghai Observatory and leader of the project. The team used an innovative dual-band technique, capturing two different wavelengths simultaneously with identical spatial resolution, which allowed them to extract crucial information about the temperature, dust composition, and morphology of the cores.

The Mystery of the “Red” Cores

The surprise came during data analysis, as more than 350 of the 500 cores emitted a signal much redder than expected. After ruling out instrumental errors, the researchers proposed two hypotheses—and both lead to the conclusion that these are protoplanetary disks, in other words, solar systems in formation.

The first hypothesis explains that these cores are not homogeneous and transparent spheres, as previously believed, but rather conceal compact protoplanetary disks within. The self-absorption of these disks at short wavelengths could explain the observed reddening, they noted. This finding calls into question our assumptions about what dense cores are like in extreme environments, added Professor Ke Wang.

The second hypothesis has to do with the accelerated growth of dust grains. While in normal interstellar environments these particles don’t exceed a few microns, some cores may contain grains up to a millimeter in size—precisely the size typically found only inside protoplanetary disks.

Both hypotheses therefore agree on an unprecedented fact: that the CMZ and the galactic center may harbor hundreds, and perhaps thousands, of protoplanetary disks in formation. In fact, just within the three clouds now studied, over 300 of these systems may already have formed. It’s extraordinary to have detected possible protoplanetary disks in a region so different from our solar neighborhood. This gives us the opportunity to study how planets form in conditions that, until now, we could only simulate, said Professor Peter Schilke, also from the University of Cologne.

The next step will be to carry out multi-band observations to precisely determine the physical properties of these objects and confirm their nature. This will rewrite the models of star and planet formation, Lu concluded.

SOURCES

Universität zu Köln

Fengwei Xu, Xing Lu, et al., Dual-band Unified Exploration of three CMZ Clouds (DUET), Astronomy & Astrophysics, Volume 697, May 2025. doi.org/10.1051/0004-6361/202453601

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