Credit: ALMA (ESO/NAOJ/NRAO), T. Shimonishi et al. (Niigata Univ.)
Scientists have discovered two enigmatic icy objects in deep space that defy explanation.
Unlike typical interstellar ice formations, these objects lack surrounding dust, emit an unusual energy signature, and contain unexpectedly high levels of silicon monoxide. Their isolated nature suggests a previously unknown environment for the formation of life’s key molecules.
Unraveling the Mystery of Life’s Building Blocks
Organic molecules, the fundamental building blocks of life, are thought to form in space. However, where exactly they originate and how they reach planets remains a major question in astronomy and planetary science. A crucial piece of this puzzle is the presence of ice in interstellar space. In the cold, dense, and shielded regions of the galaxy, atoms and molecules attach to tiny dust particles, forming interstellar ices — a process similar to how snowflakes form in Earth’s clouds.
Interstellar Ices and a Puzzling Discovery
To investigate this, astronomers from Niigata University and The University of Tokyo used the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile to observe two mysterious interstellar objects. These objects were first identified in 2021 by the Japanese infrared satellite AKARI and are known to contain interstellar ices rich in water and organic molecules. However, their exact nature remained unclear. Unlike most interstellar ices, which are typically found in dense star-forming regions, these two objects exist outside any known star-forming areas, making them particularly intriguing.
A Deep Dive with ALMA Telescope
Using ALMA, the research team observed the objects at a wavelength of approximately 0.9 mm. Infrared observations are useful for studying solid materials like ice, but radio observations, such as those from ALMA, provide better insights into the motion and composition of surrounding gases. If these objects were forming stars, ALMA’s high-resolution imaging would have detected molecular emissions associated with star formation. Similarly, if a previously unknown molecular cloud were present near these objects, it would have appeared as an extended region of gas emission, particularly in carbon monoxide.
Unexpected Observations Challenge Theories
However, the observations revealed something different from either of these expectations. At the positions of the two icy objects, only molecular emission lines of carbon monoxide and silicon monoxide were detected, exhibiting a very compact distribution of less than one arcsecond. Using the ALMA data, the team analyzed the distance, motion, size, and chemical composition of the molecular gas associated with these objects.
For example, based on the analysis of their line-of-sight velocities, it was suggested that the two objects are located approximately 30,000 to 40,000 light-years away from Earth. Additionally, the significant difference in their velocities indicates that these objects are kinematically independent and situated at different distances, despite being separated by only about 3 arcminutes on the celestial sphere and exhibiting similar colors, brightness, and interstellar ice features.
An Unusual Energy Signature Emerges
Interstellar objects with ices are usually embedded in large amounts of dust, causing them to shine brightly in the far-infrared to submillimeter wavelengths. However, the ALMA observations in this study did not detect submillimeter radiation from the two icy objects, revealing an unusual energy distribution that does not match the characteristics of previously known interstellar icy objects.
A Surprising Chemical Composition
Furthermore, the ALMA observations revealed that the ratio of silicon monoxide to carbon monoxide in the two objects is significantly higher than what is typically observed in normal molecular clouds. Such abundant silicon monoxide is usually found only in regions where interstellar dust is being destroyed by intense shock waves, suggesting that the two objects are associated with an energy source that is strongly disturbing the gas.
A New Class of Interstellar Objects?
The unique properties of the mysterious icy objects revealed by ALMA cannot be explained by the characteristics of any known objects associated with interstellar ices, such as newly formed stars, young stars with protoplanetary disks, evolved stars that exhibit intense mass loss, or bright stars located behind the dense molecular clouds.
Future Exploration and Scientific Hopes
“They may represent a new class of interstellar objects that provide an environment conducive to the formation of ices and organic molecules,” says Takashi Shimonishi, an astronomer at Niigata University, Japan, and the lead author of the paper. “Future high-resolution observations of the associated gas using the ALMA telescope, along with more detailed studies of ices and dust with the James Webb Space Telescope, would shed light on the nature of these mysterious icy objects,” hopes Takashi Shimonishi.
The findings were published in The Astrophysical Journal on February 25, 2025.
Reference: “ALMA Observations of Peculiar Embedded Icy Objects” by Takashi Shimonishi, Takashi Onaka and Itsuki Sakon, 25 February 2025, The Astrophysical Journal.
DOI: 10.3847/1538-4357/ada4ad
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