Study reveals the structure of exocomet belts orbiting nearby stars
ALMA images of a subsample of 30 exocomet belts studied in REASONS displaying a great diversity of morphologies - credit Luca Matrà,
Scientists have gained a fascinating new insight into the structure of numerous exocomet belts found around nearby stars.
A team of international researchers led by Prof. Luca Matrà at Trinity College Dublin, including Dr Sebastián Marino from the University of Exeter, has produced images of more than 70 exocomet belts, and the tiny pebbles within them. The study, called REASONS (REsolved ALMA and SMA Observations of Nearby Stars), heralds a significant landmark in the study of exocometary belts.
The astonishing images show light being emitted from these millimetre-sized pebbles within the belts orbiting 74 nearby stars. The study is able to show where the pebbles, and hence the exocomets, are located – typically tens to hundreds of au (the distance from Earth to the Sun) from their central star.
These regions are typically extraordinarily cold – with temperatures ranging from -250 to -150 degrees Celsius. At these temperatures, most compounds including water are frozen as ice on these exocomets. Due to this, researchers are able to observe where the ice reservoirs of planetary systems are located.
Producing these images and characterising the structure of these belts was only possible by using the most advanced radio interferometers. These are the 66 radio telescopes that form the Atacama Large Millimeter/submillimeter Array (ALMA) in the Atacama Desert of northern Chile and the eight-element Submillimeter Array (SMA) in Hawaii. Both observe electromagnetic radiation at millimetre and submillimetre wavelengths.
Dr Sebastián Marino, Royal Society University Research Fellow at the University of Exeter, and coauthor in this study, said: “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. We still don’t know the origin of this diversity, but this is something that we will be investigating over the next years at Exeter. ”
Some systems have multiple rings/disks, some of which are eccentric, which provides evidence that yet undetectable planets are present and their gravity affects the distribution of pebbles in these systems.
“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 Matrà, Associate Professor in Trinity’s School of Physics.
“The power of a large study like REASONS is in revealing population-wide properties and trends. 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 & Smithsonian and co-author in this study, underlined: “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.
“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.”
The upcoming study ARKS led by Dr. Sebastian Marino is the next frontier as he adds “REASONS has enabled a population study of exocomet belts and has led the foundations of many future studies. For example, the undergoing ARKS program is using ALMA to image a subsample of these belts at a much higher resolution to reveal the detailed structures in these belts. We will use this detailed information to understand if planets shape exocomet belts with their gravity. ”
