Using the SPHERE instrument on ESO's Very Large Telescope, astronomers have created an extraordinary set of images showing debris disks in a wide range of exoplanetary systems. These dusty structures reveal where small bodies orbit their stars and provide rare insights into the earliest stages of planetary development. Gaël Chauvin (Max Planck Institute for Astronomy), project scientist for SPHERE and co-author of the study, explains: "This data set is an astronomical treasure. It provides exceptional insights into the properties of debris disks, and allows for deductions of smaller bodies like asteroids and comets in these systems, which are impossible to observe directly."
In our own solar system, once you look past the Sun, the planets, and dwarf planets such as Pluto, an enormous variety of smaller ("minor") bodies comes into view. Scientists pay particular attention to objects ranging from about a kilometer to several hundred kilometers in size. Those that occasionally release gas and dust to form visible features like a tail are called comets, while those that do not show such activity are labeled asteroids.
These small bodies preserve clues to the solar system's earliest days. During the long process in which tiny grains grew into planets, intermediate objects known as planetesimals formed. Asteroids and comets are remnants of that transitional phase, planetesimals that never developed into full-size planets. In this sense, they are (somewhat) altered traces of the same ingredients that once built Earth.
Searching for small bodies in exoplanetary systems
Astronomers have identified more than 6000 exoplanets (that is, planets orbiting stars other than the Sun), giving us a clearer picture of how planetary systems vary throughout the galaxy. Directly imaging these worlds is still extremely difficult. Fewer than 100 exoplanets have been photographed so far, and even the largest ones appear only as featureless points of light.
This challenge becomes even greater when searching for small bodies. As Dr. Julien Milli, astronomer at the University Grenoble Alpes and co-author of the study, notes: "Finding any direct clues about the small bodies in a distant planetary system from images seems downright impossible. The other indirect methods used to detect exoplanets are no help, either."
Dust provides the key to detecting hidden planetesimals
The breakthrough comes not from the small bodies themselves, but from the dust created when they collide. Young planetary systems are especially active. Planetesimals frequently crash into each other, sometimes merging into larger bodies and sometimes fragmenting into smaller ones. These events release vast amounts of fresh dust.
The physics behind dust visibility is surprisingly intuitive. Breaking an object into many tiny pieces preserves its total volume, but dramatically increases its surface area. For example, if a one kilometer wide asteroid were crushed into dust grains just one micrometer across (a millionth of a meter), the overall surface area would increase by a factor of one billion. More surface area means far more light reflected from the star, which makes the dust easier to detect. By observing that dust, astronomers can infer details about the unseen small bodies producing it.
How debris disks evolve over time
Debris disks do not remain bright forever. As a young system matures, collisions become rarer. Dust can be pushed outward by radiation pressure from the central star, swept up by planets or planetesimals, or spiral inward and fall into the star.
Our solar system provides a late-stage example. After billions of years, two major planetesimal belts remain: the asteroid belt between Mars and Jupiter and the Kuiper belt beyond the giant planets. A population of smaller dust grains also persists, creating zodiacal dust. Under especially dark skies, sunlight scattered by this dust can be seen shortly after sunset or before sunrise as a faint glow called zodiacal light.
For observers studying our solar system from afar, these faint leftovers would be hard to detect. The new research, however, shows that similar dusty structures around younger systems should be visible for roughly the first 50 million years of a debris disk's lifetime. Capturing these images is extremely challenging. The task has been compared to photographing a thin cloud of cigarette smoke beside a blinding stadium floodlight from several kilometers away. SPHERE, which began operating on one of ESO's Very Large Telescopes (VLT) in spring 2014, was created specifically for such situations.
How SPHERE blocks starlight to reveal faint features
The fundamental idea behind SPHERE is familiar from everyday experience. If the Sun is shining directly into your eyes, you might raise a hand to shield the glare so you can see what lies around it. SPHERE uses a coronagraph to achieve the same effect when imaging exoplanets or debris disks. By inserting a small disk into the path of the star's light, the instrument blocks most of the glare before the image is captured. This method only works if the optical system remains extremely stable and precise.
To maintain this stability, SPHERE relies on a highly advanced version of adaptive optics. Turbulence in Earth's atmosphere distorts incoming starlight, and SPHERE continually monitors these distortions and corrects them in real time using a deformable mirror. An optional component can also isolate "polarized light," which is characteristic of light reflected by dust rather than emitted directly from a star. This additional filtering enhances SPHERE's ability to detect faint debris disks.
A major survey reveals 51 debris disks in sharp detail
The new study presents a unique set of debris disk images created by analyzing starlight scattered by tiny dust particles. "To obtain this collection, we processed data from observations of 161 nearby young stars whose infrared emission strongly indicates the presence of a debris disk," says Natalia Engler (ETH Zurich), the lead author of the research. "The resulting images show 51 debris disks with a variety of properties -- some smaller, some larger, some seen from the side and some nearly face-on -- and a considerable diversity of disk structures. Four of the disks had never been imaged before."
Working with such a large sample makes it possible to find broader patterns. The analysis revealed that more massive young stars tend to host more massive debris disks. Systems where dust is concentrated farther from the star also show a tendency toward more massive disks.
Rings, belts, and hints of unseen planets
One of the most compelling aspects of the SPHERE results is the wide range of structures inside the disks. Many show rings or band-like patterns, with material clustered at specific distances from the star. This arrangement resembles our own solar system, where small bodies gather in the asteroid belt (asteroids) and the Kuiper belt (comets).
These structures are thought to be shaped by planets, especially large ones that clear out paths as they orbit. Some of the planets responsible have already been detected. In other cases, sharp edges or asymmetries in the disks strongly suggest the presence of planets that have not yet been directly observed. Because of this, the SPHERE survey provides a valuable set of targets for upcoming facilities. Instruments on the James Webb Space Telescope (JWST) and the Extremely Large Telescope (ELT) under construction by ESO should be capable of directly imaging at least some of the planets that are sculpting these dusty rings and gaps.
Study authors and publication details
The results described here have been published as Natalia Engler et al., "Characterization of debris disks observed with SPHERE," in the journal Astronomy and Astrophysics.
The MPIA researchers involved are Gaël Chauvin, Thomas Henning, Samantha Brown, Matthias Samland, and Markus Feldt, in collaboration with Natalia Engler (ETH Zürich), Julien Milli (CNRS, IPAG, Université Grenoble Alpes), Nicole Pawellek (University of Vienna), Johan Olofsson (ESO), Anne-Lise Maire (CNRS, IPAG, Université Grenoble Alpes), and others.
