For the first time, astronomers have imaged dozens of belts around nearby stars where comets and tiny pebbles within them are orbiting.
This result reveals regions around 74 stars spanning a wide range of ages – from those recently formed to others billions of years old – showing how comets play a role in the formation of stars and planetary systems.
To find evidence for comets outside our Solar System (called “exocomets”), astronomers turned to two facilities that detect particular bands of radio waves. Because of the size of the dust and rocks in these belts, this type of light is particularly good at finding and imaging these structures.
The Submillimeter Array (SMA) is an eight-dish array of radio telescopes near the summit of Maunakea in Hawaii, which is operated by the Smithsonian Astrophysical Observatory as part of the Center for Astrophysics | Harvard & Smithsonian (CfA). The Atacama Large Millimeter/submillimeter Array (ALMA) is a 66-dish array in the Atacama Desert of northern Chile.
A joint program between SMA and ALMA dubbed REASONS (REsolved ALMA and SMA Observations of Nearby Stars) marks a significant milestone in the study of exocometary belts because its images and subsequent analysis reveal where the pebbles and the exocomets are located.
In these regions, it is so cold (-250 to -150 degrees Celsius) that most compounds including water are frozen as ice on these exocomets. The astrophysicists are, therefore, observing where the ice reservoirs of planetary systems are located.
“Exocomets are boulders of rock and ice, at least a kilometer in size, which smash together within these belts to produce the pebbles that we observe here with the ALMA and SMA arrays of telescopes,” said Luca Matrà of Trinity College Dublin in Ireland, and previously a Submillimere Array Postdoctoral Fellow at the CfA, who led the study. “Exocometary belts are found in at least 20% of planetary systems, including our own Solar System.”
The Kuiper Belt is an example of a cometary belt in our own Solar System. Located far beyond the orbit of Pluto, some scientists think the Kuiper Belt is the source of water for the inner Solar System where Earth is located, delivered through comets billions of years ago.
The new gallery shows a remarkable diversity of structure in the belts. Some are narrow rings, while others are wider and could be categorized more as “disks” than “belts”.
Moreover, some of the 74 exocomet systems have multiple rings or disks and some of those are “eccentric,” meaning not a circular orbit but more like an oval. This provides evidence that yet undetectable planets or perhaps moons are present and their gravity affects the distribution of pebbles in these systems.
“Arrays like the ALMA and SMA used in this work are extraordinary tools that are continuing to give us incredible new insights into the universe and its workings,” said Smithsonian astrophysicist David Wilner, a co-author from the CfA. “The REASONS survey required a large community effort and has an incredible legacy value, with multiple potential pathways for future investigation.”
The REASONS dataset of belt and planetary system properties will enable studies of the birth and evolution of these belts, as well as follow-up observations across the wavelength range, from JWST to the next generation of Extremely Large Telescopes and upcoming ALMA’s plans to zoom even further into the details of these belts.
A paper describing these results appeared recently in the January 2025 issue of Astronomy & Astrophysics journal and is available at https://arxiv.org/abs/2501.09058
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