An international team of astrophysicists has imaged a large number of exocomet belts around nearby stars, and the tiny pebbles within them.
The crystal-clear images show light being emitted from these millimetre-sized pebbles within the belts that orbit 74 nearby stars of a wide variety of ages - from those that are just emerging to those in more mature systems like our own Solar System.
The REASONS (REsolved ALMA and SMA Observations of Nearby Stars) study, led by Trinity College Dublin and involving researchers from the University of Cambridge, is a milestone in the study of exocometary belts because its images and analyses reveal where the pebbles, and the exocomets, are located. They are typically tens to hundreds of astronomical units (the distance from Earth to the Sun) from their central star.
In these regions, it is so cold (-250 to -150 degrees Celsius) that most compounds are frozen as ice on the exocomets. What the researchers are therefore observing is where the ice reservoirs of planetary systems are located. REASONS is the first programme to unveil the structure of these belts for a large sample of 74 exoplanetary systems. The results are reported in the journal Astronomy & Astrophysics.
This study used both the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile and the Submillimeter Array (SMA) in Hawai'i to produce the images that have provided more information on populations of exocomets than ever before. Both telescope arrays observe electromagnetic radiation at millimetre and submillimetre wavelengths.
"Exocomets are boulders of rock and ice, at least one kilometre 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 lead author Luca Matrà from Trinity College Dublin. "Exocometary belts are found in at least 20% of planetary systems, including our own Solar System."
"The images reveal a remarkable diversity in the structure of belts," said co-author Dr Sebastián Marino from the University of Exeter. "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."
Some systems have multiple rings/disks, some of which are eccentric, providing evidence that yet undetectable planets are present and their gravity affects the distribution of pebbles in these systems.
"The power of a large study like REASONS is in revealing population-wide properties and trends," said Matrà.
For example, the study 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 objects as large as 140 km across and even Moon-size objects are likely present in these belts.
"We have been studying exocometary belts for decades, but until now only a handful had been imaged," said co-author Professor Mark Wyatt from Cambridge's Institute of Astronomy. "This is the largest collection of such images and demonstrates that we already have the capabilities to probe the structures of the planetary systems orbiting a large fraction of the stars near to the Sun."
"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 co-author Dr David Wilner from the Center for Astrophysics | Harvard & Smithsonian "The REASONS survey required a large community effort and has an incredible legacy value, with multiple potential pathways for future investigation."
Reference:
L. Matrà et al. 'REsolved ALMA and SMA Observations of Nearby Stars. REASONS: A population of 74 resolved planetesimal belts at millimetre wavelengths.' Astronomy & Astrophysics (2025). DOI: 10.1051/0004-6361/202451397
Adapted from a Trinity College Dublin media release.
