Rings of comets have been discovered around 74 alien star systems, revealing not only the existence of trillions of icy exocomets, but also the source of most of the water in these systems.
The discovery of what are being termed 'planetesimal belts' was made in joint observations by the Atacama Large Millimeter Array (ALMA) in Chile and the Submillimeter Array (SMA) on Mauna Kea in Hawaii. Because the belts are so far away from their central star, they are cold, with temperatures ranging between –418 to –238 degrees Fahrenheit (–250 to –150 degrees Celsius). As such, they mostly radiate light at long wavelengths, hence the need for ALMA and SMA. Although to us a millimeter is small, it is much larger than the nanometer wavelength of optical light and the micron-sized wavelengths of infrared light seen by other observatories like the James Webb Space Telescope.
