Astrophysicists reveal huge variation in the shape of exocomet belts

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Exocomets
Shape shifter: the images reveal a large variation in the shape of exocomet belts (courtesy: Prof Luca Matra, Trinity College Dublin)

Astrophysicists have released a series of images of exocomet belts and the tiny “pebbles” that reside in them.

Exocomets are boulders of rock and ice, at least 1 km in size, that exist outside our solar system. Exocometary belts – regions containing many such icy bodies – are found in at least 20% of planetary systems. When the exocomets within these belts smash together they can also produce small pebbles.

The belts in the latest study orbit 74 nearby stars that cover a range of ages – from those that are have just formed to those in more mature systems like our own Solar System. The belts typically lie tens to hundreds of astronomical units (the distance from the Earth to the Sun) from their central star.

At that distance, the temperature is between -250 to -150 degrees Celsius, meaning that most compounds on the exocomets are frozen as ice.

While most exocometary belts in the latest study are disks, some are narrow rings. Some even have multiple rings/disks that are eccentric, which provides evidence that yet undetectable planets are present and their gravity affects the distribution of the pebbles in these systems.

According the Sebastián Marino from the University of Exeter, the images reveal “a remarkable diversity in the structure” of the belts.

Indeed, Luca Matrà from Trinity College Dublin says that the “power” of such a large survey is to reveal population-wide properties and trends. “[The survey] 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,” Matrà adds. “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.”

The researchers took the images using the Atacama Large Millimeter/submillimeter Array – an array of 66 radio telescopes in the Atacama Desert of northern Chile – as well as the eight-element Submillimeter Array based in Hawaii.

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