Image Releases

ALMA and SMA’s Survey of Exocomet Belts is Transforming our Understanding of Planetary Systems 

17 January, 2025 / Read time: 7 minutes

Scientific Paper

The Edgeworth-Kuiper Belt (or simply the Kuiper Belt) is a huge structure extending 30 to 50 times the distance from the Earth to the Sun. It is full of icy and rocky objects. But where does it come from? 

In its infancy, the Sun was surrounded by a giant rotating cloud of gas and dust. Much like making a pizza from a ball of dough, by rotating over time, the cloud eventually became a flat disc, which we call a protoplanetary disc. 

There, grains of dust collided and fused into increasingly large structures. Once they reached a kilometer-size, their own gravity started to attract more objects, fueling their growth even further. In a very summarized version, this is how we think planets formed. We have seen this process taking place in other planetary systems. However, we also know that not all dust ends up forming planets. 

Astronomers think that the influence of large planets like Neptune could have prevented dust beyond its orbit from creating new planets, leaving us with just a belt of debris. This debris, though, is a hidden space treasure. 

Today, the Kuiper Belt is a collection of dusty and icy space rocks, ranging from dust grains and pebbles to comets and dwarf planets. From a few millimeters to kilometers in diameter, many of these objects have changed little since their formation. Frozen (literally) in time, these are remnants of the early stages of the Solar System and tell us a lot about its initial properties. 

Belts of debris like the Kuiper Belt exist in other planetary systems, too. They are broadly known as “planetesimal belts” since the objects within them have the potential to coalesce to form planets, or “exocomet belts” since they usually hide comets (icy planetesimals) within them. But how can we observe these belts? 

Given their size, finding exocomet belts should be easy at first glance. However, belts have been difficult to observe and image. 

The reason behind this is their temperature. The objects lying within an exocomet belt are very far from their host star, and thus, they are extremely cold. In the Kuiper belt, temperatures range from -250 to -150 degrees Celsius. At these low temperatures, belts only shine at long wavelengths, making them difficult to observe for most — but not all — telescopes. 

One of the telescopes that can observe them is the Atacama Millimeter/submillimeter Array (ALMA). This array of 66 antennas in northern Chile is specifically designed to detect long wavelength radiation from cold astronomical sources, like exocomet belts. 

Using ALMA, the Hawaiian Submillimeter Array (SMA), and archival data, a team led by Luca Matrà, an associate professor at the University of Dublin, has embarked on a mission to image as many exocomet belts as possible in all stages, from newly formed to very mature. The survey, Resolved ALMA and SMA Observations of Nearby Stars (REASONS), is the largest of its kind to date. 

The survey contains images of 74 belts around “nearby” planetary systems located within 500 light-years of Earth. The results, published in Astronomy and Astrophysics, already challenge astronomers' ideas about these structures. 

Not all belts are equal. The REASONS survey revealed that exocomet belts come in all shapes, sizes, and ages, but scientists are starting to see some patterns within this variation. 

One of these patterns is that belts are remarkably larger than expected. Smaller belts are closer to their host star, making them hotter, brighter, and theoretically more straightforward to find. And yet, the new observations indicate that they are infrequent. This means that either most belts form further out or smaller belts are less massive and, in fact, more complex to detect. 

The team also confirmed previous findings: as belts evolve, collisions within them smash their large objects into smaller ones. If this process were to happen faster in belts closer to their stars, it could also explain why the team didn’t find small belts. 

Belts are more extensive than previously thought and extend more widely. Think of a doughnut with a small hole rather than an onion ring. Narrower belts — called “rings” — are uncommon in the survey. 

One possibility is that belts broaden as time passes. The first results from this survey, though, found that older belts are not necessarily broader, indicating that this is probably not the case. Another possibility is that wide belts have gaps within them that would split them into narrower rings, but we can’t see this yet. 

This is not the end of the story of belts, though. Researchers think future telescopes can uncover substructures within belts, like gaps and rings. Belts could even hide dwarf planets, much like Pluto, ready to be discovered. 

But studying these belts is more than just searching for space treasures; it is also learning about the history of our solar system and planet. 

Earth is always watching the Kuiper Belt, a significant source of asteroids and comets. Since an asteroid caused a major extinction 65 million years ago, it is understandable why we would be concerned. However, one theory suggests that some of Earth’s water may have also arrived from the Kuiper Belt, a giant repository of frozen water. Large far-away planets like Neptune or Uranus may have had a crucial role in propelling water-carrying comets toward us, providing an element that would otherwise be rare on the primitive Earth. Only time will tell whether we owe our lives to space rocks. 

As we learn more about exocomet belts, we may finally be able to understand the role that belts play in the formation and evolution of planetary systems.

Additional information

The results of the observation are published in the following scientific article:

Matrà et.al "REsolved ALMA and SMA Observations of Nearby Stars (REASONS): A population of 74 resolved planetesimal belts at millimetre wavelengths". Published in Astronomy & Astrophysics.

The European Southern Observatory (ESO), an ALMA partner on behalf of Europe, published the original press release.

The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of the European Southern Observatory (ESO), the U.S. National Science Foundation (NSF), and the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Republic of Chile. ALMA is funded by ESO on behalf of its Member States, by NSF in cooperation with the National Research Council of Canada (NRC) and the National Science and Technology Council (NSTC) in Taiwan, and by NINS in cooperation with the Academia Sinica (AS) in Taiwan and the Korea Astronomy and Space Science Institute (KASI).

ALMA construction and operations are led by ESO on behalf of its Member States; by the National Radio Astronomy Observatory (NRAO), managed by Associated Universities, Inc. (AUI), on behalf of North America; and by the National Astronomical Observatory of Japan (NAOJ) on behalf of East Asia. The Joint ALMA Observatory (JAO) provides the unified leadership and management of ALMA's construction, commissioning, and operation.

Images