New Images Reveal Exocomets Around 74 Nearby Stars – Sky & Telescope

Astronomers have obtained new, high-resolution images of 74 exocometary belts — rings of comet-size bodies surrounding nearby stars. This treasure trove represents the largest and most detailed set of images of these systems to date, enabling scientists to investigate the belts’ large-scale properties and evolution in unprecedented detail.

Exocomets are icy bodies at least one kilometer in size orbiting stars other than our Sun. While they are too small to observe directly, these bodies occasionally collide with one another, releasing copious amounts of dust and pebbles. The exocomets and the debris they shed tend to orbit stars in belts, akin to our solar system’s Kuiper Belt, and those belts are within reach of modern-day telescopes.

For the new survey (called Resolved ALMA and SMA Observations of Nearby Stars, or REASONS), astronomers used the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile and the Submillimeter Array (SMA) in Hawai‘i to observe these dusty belts. By combining new observations with reanalysis of archival data, the REASONS survey significantly expanded the number of well-resolved images of exocometary disks. The team, led by Luca Matrà (Trinity College, Dublin), has published these results in Astronomy and Astrophysics.

While 74 samples might not sound like a lot, “It’s the largest sample available to this day,” says team member Carlos del Burgo (University of La Laguna, Spain). He notes that acquiring these images “has been hard work.”

The existence of exocomets was first inferred in 1984, around the nearby star Beta Pictoris — well before the discovery of the first exoplanet in 1995. As the comet’s tails  passed in front of Beta Pictoris, spectra of the star’s light pick up the chemical signatures of evaporating ices.

Even then, astronomers knew disks of debris leftover from planet formation were common around newborn stars; exocometary belts are a type of debris disk containing abundant volatile molecules that evaporate easily when near the star’s warmth. Researchers estimate that about 20% of planetary systems harbor exocomet belts.

But while the presence of hundreds of exocometary disks has been confirmed, only a few have been resolved in sufficient detail to study their internal structure. “This is the first time we can make a statistical analysis of what’s going on in these disks,” says Isabel Rebollido (European Space Astronomy Centre, Spain), who wasn’t involved in the new study.

Imaging these faint and cold disks presents significant challenges because they are relatively small, cold, and faint objects. However, observatories such as ALMA and SMA, with their arrays of radio antennas, are uniquely suited for this task. The radio wavelengths these observatories detect come from the faint glow of warm dust within the disks. Moreover, by combining data from several dishes, the observatories can make out fine details in the disk structures.

Belts and Disks

The ages of REASONS’ disks range from around 10 million years old, in which the protoplanetary phase has just ended, to around 1 billion years old. That makes them older than protoplanetary disks, the gas-and-dust remnants of star formation in which planets are thought to coalesce. (The solar system represents an even later stage than the oldest REASON disk, when stellar radiation has depleted our Kuiper Belt of almost all its dust and gas, and the remaining volatiles are trapped within solid objects.)

The newly observed disks are located in the outer regions of their systems, more than 10 astronomical units (au) from their host stars. (For reference, Saturn’s orbit is out at 9.5 au). At these distances, temperatures are extremely cold, ranging from -250 to -150°C, so that volatiles, including water, remain frozen. Nevertheless, these disks are not static, and bodies in them interact gravitationally, both amongst themselves and with the planets in the system. As a result, comets can be hurled around, both toward the star or outward into elongated orbits.

This reshuffling plays an important role in distributing mass and chemical elements throughout a planetary system, researchers think. For example, this process could be linked to how Earth obtained water from the outer solar system.

Emerging Trends

The REASONS images reveal a surprising variety in the structure of cometary belts. While some conform to the classic image of a narrow ring, similar to the Kuiper Belt, many are significantly wider. A few systems have multiple rings, some of them with elongated shapes, suggesting the presence of undetected planets. Those larger worlds’ gravitational influence likely shapes the distribution of dust and pebbles within the disk.

While exocomet belts are thought to contain significantly less mass than planet-forming disks, “these belts are more massive than we expected,” del Burgo says.

The strength of a large-scale study like REASONS lies in its ability to uncover population-wide trends. For instance, the reseachers confirmed that disks evolve with age, losing both mass and surface area. They also confirmed that when a disk is closer to its central star, it tends to disappear faster, since the star’s radiation dispenses gas and disintegrates dust. The team also found that while the disks doesn’t necessarily thin out with age, over time they seem to hold less gas overall.  

Some of the researchers that instigated REASONS are already working in a follow-up program called The ALMA survey to Resolve exoKuiper belt Substructures (ARKS), which will image 18 additional systems at even higher resolution, putting an extra emphasis in the gaseous components of these disks. With these observations researchers expect to be able get a better view of their inner structures and the dynamics — or planets — that could be sculpting them.

“Knowing these other systems,” del Burgo says, “we can get a more complete interpretation of the formation of the solar system.”