The study, known as REASONS (REsolved ALMA and SMA Observations of Nearby Stars), provides unprecedented insights into the locations and structures of these exocomet belts. Typically found tens to hundreds of astronomical units (au) from their host stars, these regions are exceptionally cold, with temperatures ranging from -250 to -150 degrees Celsius. In such conditions, compounds like water freeze into ice, forming reservoirs within these planetary systems.

This extensive survey is the first to map the structure of exocomet belts across a broad sample of 74 exoplanetary systems. The findings rely on advanced imaging from the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile and the Submillimeter Array (SMA) in Hawaii. These powerful telescopes capture electromagnetic radiation at millimetre and submillimetre wavelengths, enabling detailed observations of exocomet populations.

"Exocomets are boulders of rock and ice, at least 1 km in size, which smash together within these belts to produce the pebbles that we observe here with the ALMA and SMA arrays of telescopes. Exocometary belts are found in at least 20% of planetary systems, including our own Solar System," said Luca Matra, Associate Professor in Trinity's School of Physics and senior author of the research article published in Astronomy and Astrophysics.

Dr Sebastian Marino, Royal Society University Research Fellow at the University of Exeter and co-author of the study, noted: "The images reveal a remarkable diversity in the structure of belts. Some are narrow rings, as in the canonical picture of a 'belt' like our Solar System's Edgeworth-Kuiper belt. But a larger number of them are wide, and probably better described as 'disks' rather than rings."

Certain systems exhibit multiple rings or disks, some of which are eccentric, suggesting the presence of unseen planets whose gravitational forces shape the distribution of pebbles.

"The power of a large study like REASONS is in revealing population-wide properties and trends," explained Prof. Matra. "For example, it confirmed that the number of pebbles decreases for older planetary systems as belts run out of larger exocomets smashing together, but showed for the first time that this decrease in pebbles is faster if the belt is closer to the central star. It also indirectly showed-through the belts' vertical thickness-that unobservable objects as large as 140 km to Moon-size are likely present in these belts."

Dr David Wilner, Senior Astrophysicist at the Center for Astrophysics | Harvard and Smithsonian, emphasized the importance of the technology used: "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. The REASONS survey required a large community effort and has an incredible legacy value, with multiple potential pathways for future investigation."

Wilner added, "For example, 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 ALMA's upcoming ARKS Large Program to zoom even further onto the details of these belts."

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