Astrophysicists led by a team from Trinity College Dublin have — for the first time — 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 from birth to those in more mature systems like our own Solar System.
The REASONS (REsolved ALMA and SMA Observations of Nearby Stars) study marks such a significant milestone in the study of exocometary belts because its images and analyses reveal where the pebbles, and hence the exocomets, are located. They are typically tens to hundreds of au (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 including water are frozen as ice on these exocomets. What the astrophysicists are therefore observing is where the ice reservoirs of planetary systems are located. REASONS is the first program to unveil the structure of these belts for a large sample of 74 exoplanetary systems.
The Atacama Large Millimeter/submillimeter Array (ALMA) is an array of 66 radio telescopes in the Atacama Desert of northern Chile, while the Submillimeter Array (SMA) is a similar eight-element array in Hawaii. Both observe electromagnetic radiation at millimetre and submillimetre wavelengths. This study used both to produce the images that have provided more information on populations of exocomets than ever before.
“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, and senior author of the research article that has just been published in thel journal Astronomy and Astrophysics.
Dr Sebastián Marino, Royal Society University Research Fellow at the University of Exeter, and coauthor in this study, added: “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.”
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.
“The power of a large study like REASONS is in revealing population-wide properties and trends,” explained Prof. Matrà.
“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, 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.”