Scientists have discovered two mysterious icy objects in deep space that are difficult to explain. Unlike typical interstellar ice, these objects are surrounded by no dust, emit an unusual energy signature, and contain unexpectedly high concentrations of silicon monoxide. Their isolation points to an unknown environment where life's key molecules were formed.
Organic molecules are the building blocks of life and are thought to have formed in space. However, exactly where they originate and how they get to the planets remains a major question in astronomy and planetary science. The presence of ice in interstellar space is the key to this puzzle. In cold, dense and obscured regions of the Milky Way, atoms and molecules attach to tiny dust particles to form interstellar ice - a process similar to how snowflakes form in Earth's clouds.
To study this issue, astronomers from the University of Niigata and the University of Tokyo observed two mysterious interstellar objects using the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile. The Japanese infrared satellite AKARI first discovered these two objects in 2021, and they are known to contain interstellar ice rich in water and organic molecules. However, their exact nature remains unclear. Unlike most interstellar ices, which typically exist in dense star-forming regions, these two objects are particularly fascinating because they exist outside any known star-forming regions.
The research team used ALMA to observe objects with a wavelength of about 0.9 millimeters. Infrared observations are useful for studying solid matter like ice, but radio observations like ALMA provide a better understanding of the motion and composition of surrounding gases. If these objects are forming stars, ALMA's high-resolution imaging could detect the molecular radiation associated with star formation. Likewise, if there were previously unknown molecular clouds near these objects, it would also appear in the form of extended regions of gas emission, especially carbon monoxide.
However, the observed results were different from any of the above expectations. At the positions of these two icy objects, only the molecular emission lines of carbon monoxide and silicon monoxide were detected, and their distribution was very compact, less than an arcsecond. The team used ALMA data to analyze the distance, motion, size and chemical composition of the molecular gas associated with these objects.
For example, based on analysis of their line-of-sight velocities, the two objects are approximately 30,000 to 40,000 light-years away from Earth. Furthermore, their significant difference in speed suggests that the two objects are kinematically independent and located at different distances, even though they are only about 3 arcminutes apart on the celestial sphere and exhibit similar color, brightness, and interstellar ice characteristics.
Interstellar objects with ice usually have large amounts of dust embedded in them, 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 these two icy objects, revealing an unusual energy distribution that is inconsistent with previously known characteristics of interstellar icy objects.
In addition, ALMA's observations show that the ratio of silicon monoxide to carbon monoxide in these two objects is significantly higher than that observed in ordinary molecular clouds. Such abundance of silica is usually only found in regions where interstellar dust has been destroyed by strong shock waves, suggesting that both objects are related to energy sources that strongly disturb the gas.
The unique properties of the mysterious icy objects revealed by ALMA cannot be explained by any of the characteristics of known objects related to interstellar icy objects, such as newly formed stars, young stars with protoplanetary disks, evolved stars showing strong mass loss, or bright stars located behind dense molecular clouds.
Takashi Shimonishi, an astronomer at Niigata University in Japan and the first author of this article, said: "They may represent a new class of interstellar objects that provide a favorable environment for the formation of ice and organic molecules. In the future, high-resolution observations of related gases using the ALMA telescope and more detailed studies of ice and dust using the James Webb Space Telescope will reveal the nature of these mysterious icy objects."
These findings were published in the February 25, 2025 issue of The Astrophysical Journal.
Compiled from /scitechdaily