Observations with the SPHERE instrument on ESO's Very Large Telescope (VLT) have yielded one of the most detailed galleries of debris disks around young stars to date. These are thin but surprisingly complex bands of dust surrounding stars like a kind of cosmic archaeological layers. Hidden within these disks is the trace of collisions of billions of tiny bodies – sort of "cousins" of our asteroids and comets – which carry information about how planets form and how an entire planetary system matures. Astronomers point out that the new collection of images, created with the help of extreme adaptive optics and coronagraphs, is a true scientific goldmine of data because it systematically links the appearance of disks, stellar properties, and the hidden population of small bodies for the first time.

Small bodies as fossils of planet formation

To understand the importance of these observations, it is good to start from our own backyard – the Solar System. Besides the Sun, planets, and several dwarf planets, a vast population of so-called small bodies orbits around us. These are objects ranging from a kilometer to several hundred kilometers in diameter, composed of rock, ice, or a mixture thereof. When such an object occasionally releases gas and dust creating a coma and tail, we call it a comet; when there is no such activity, we speak of an asteroid. These bodies are not "leftovers without order", but fossils of the early days of the Solar System. Planet formation begins with microscopic dust grains in the protoplanetary disk; collisions and sticking of these grains lead to a transitional phase of so-called planetesimals, and a portion of these bodies never grows to the size of a planet. It is precisely this halted step in the evolution of material that we recognize today as asteroids and comets.

Small bodies are particularly precious because they preserve the chemical composition and structure of the original building material for planets. Unlike planets, their interiors have not undergone intense melting and mixing, so they are in many ways closer to the initial conditions in the disk. When we study them in the Solar System, we gain insight into the conditions under which Earth, Mars, or the ice giants formed. But the real question is: how universal are such processes and do similar belts of small bodies exist around other stars?

Exoplanets and limitations of direct imaging

The answer to that question comes from the explosion of exoplanet discoveries in the last three decades. By December 2025, more than six thousand planets outside the Solar System have been confirmed, and statistics show that a planetary system is the rule rather than the exception in the Milky Way. Most of these worlds were discovered by indirect methods – changes in star brightness during planetary transits or tiny wobbles in the star's speed due to the gravity of an invisible companion. Direct images of exoplanets, however, remain rare: less than a hundred planets can be seen as small, blurry dots next to a dazzling host star. From these images, it is difficult to directly obtain information about the population of small bodies like asteroids and comets in the same system.

Dust as a light amplifier

That is precisely why astronomers turn to even smaller particles – to dust. In younger planetary systems, planetesimals collide constantly. Sometimes they merge and continue to grow towards planet size, but often they simply crumble into clouds of tiny particles. Every time a larger object breaks into a multitude of smaller ones, the total volume remains the same, but the surface area increases dramatically. If we were, for example, to grind one asteroid with a diameter of one kilometer into dust grains with a diameter of a micrometer, the total surface area would increase by about a billion times. Increased surface area also means much more space on which starlight can scatter, so a disk of such dust is seen much more easily than the original large bodies.

Dust in debris disks is visible in two ways. In the infrared range, it radiates heat because it heats up to several tens or hundreds of kelvins, depending on the distance from the star. In the visible range of light, as registered by SPHERE, dust predominantly scatters and polarizes the star's radiation. The younger the disk and the more frequently collisions occur, the more fresh dust there is and the brighter the disk is. Over time, radiation pressure, stellar winds, and the gravitational effects of planets disperse the dust, so the disk fades. Estimates show that debris disks in the young phase, in which they are easily noticeable in scattered light, are mostly younger than about fifty million years, after which they become increasingly difficult to detect.

The Solar System as a reference

Our Solar System is an example of a system where the dust-rich phase has long passed, but planetesimal belts are still present. Between Mars and Jupiter lies the main asteroid belt, while beyond the orbits of the giant planets extends the Kuiper belt, a vast reservoir of icy bodies that we recognize as long-period comets. Additionally, the space between planets is filled with fine dust that creates zodiacal light – a faint, triangular phenomenon in the night sky visible under very dark conditions after sunset or before sunrise. If a distant astronomer were observing the Solar System with technology comparable to today's, they would probably barely register our debris disks. That is exactly why young systems around nearby stars are an ideal laboratory: in them, dust production is still in full swing and disk structures are much clearer.

Technological challenge: how to "extinguish" a star

Imaging such a disk, however, is technically extremely demanding. Visualizing the situation is often compared to trying to photograph a cloud of cigarette smoke next to a football stadium floodlight, from a distance of several kilometers. The brightness of the star manifoldly surpasses the faint glimmer of dust, so the telescope's first task is to dim the starlight without damaging the faint light of the disk. It is precisely at this task that the SPHERE instrument excels, which has been installed on one of the four VLT telescopes at Cerro Paranal in Chile since 2014. SPHERE is a specialized system for very high contrast around bright stars and combines extreme adaptive optics, an exceptionally stable optical path, and a series of coronagraphs.

The heart of the instrument is extreme adaptive optics. Earth's atmosphere constantly distorts light waves coming from space, which causes flickering and image blurriness even at the calmest locations. SPHERE uses a deformable mirror with a large number of actuators that adjusts hundreds of times per second to cancel out turbulence in real time. Additionally, a small disk – a coronagraph – is inserted into the optical path, which blocks the strongest part of the stellar glare. The principle is similar to shielding the Sun with a palm to see the surroundings better: by eliminating the glare, a view opens up to much fainter structures around the star. An additional advantage of SPHERE is the possibility of polarimetric measurements; dust in the disk scatters and polarizes light differently than the star itself, so such detection further emphasizes the disk signal relative to the background.

Three faces of the SPHERE instrument

SPHERE actually encompasses three scientific subsystems. The Integral Field Spectrograph (IFS) allows imaging of a small field around the star in a range of wavelengths, giving scientists a three-dimensional "cube" set of data where every point in the image is accompanied by a spectrum. IRDIS, the infrared dual-band imager and spectrograph, offers a wider field of view and the possibility of imaging in two different colors simultaneously or in two mutually perpendicular polarization directions. The third subsystem, ZIMPOL, is optimized for the visible range of the spectrum and is particularly sensitive to polarized light scattered on tiny particles. The combination of these techniques turns SPHERE into an exceptionally powerful "microscope" for studying the building material of planetary systems.

Statistical study of 161 young stars

In a new study, a team led by Natalia Engler from ETH Zurich analyzed observations of 161 nearby young stars for which infrared radiation had already previously indicated the presence of debris disks. The data were collected during a series of different observing programs at the VLT, and now they have been consolidated and processed by the same procedures for the first time, enabling a true statistical comparison. After demanding data reduction and processing, the researchers managed to extract clear images of disks around 51 stars. Four of these disks had never been directly imaged before, so the new gallery brings completely new objects along with detailed improved displays of already known systems.

Diversity of disk architectures

The resulting images reveal a stunning diversity of geometries and structures. Some disks are observed almost edge-on, like thin, bright lines that cut through the darkness of the frame and barely reveal their true three-dimensional nature. Others are imaged almost face-on towards us and resemble rings or wide belts of dust framing the central mask obscuring the star. Certain systems show narrow, very well-defined rings, while in others, disks are more diffuse and extended to greater distances. Asymmetries are also visible – one part of the disk may be brighter or deformed relative to the opposite side – indicating gravitational influences of invisible massive bodies.

Global trends: star mass and disk mass

When such a rich collection of images is observed as a whole, systematic connections between the properties of stars and their disks also come to light. The analysis shows that more massive young stars generally possess more massive debris disks. This is consistent with the expectation that more massive stellar systems begin with a larger amount of dust and gas in the original protoplanetary disk, so after the planet formation phase, more material remains in the form of planetesimals. A correlation was also observed between the distance at which the disk is located and its total mass: disks whose main dust belt extends further from the star are often richer in material than more compact, inner belts.

Debris belts as traces of planets

The most intriguing aspect of the new gallery, however, is the internal structures within the disks themselves. In many systems, dust is not distributed smoothly but is concentrated in one or more ring-like belts. Such a ring structure strongly resembles the distribution of small bodies in the Solar System, where we have the asteroid belt between Mars and Jupiter and the Kuiper belt beyond the orbit of Neptune. In both cases, these are regions where the gravitational influences of large planets have cleared and "carved" the disk, leaving belts of increased density at certain distances.

A similar mechanism is likely at work in the observed exoplanetary systems. Giant planets, especially those on wider orbits, act like cosmic architects: while orbiting their star, through resonances and gravitational interactions, they eject part of the planetesimals from the system, send part into inner, unstable orbits, and trap part in shared resonant orbits. The result is a debris belt with a sharply defined inner edge or an asymmetric distribution of dust. In some of SPHERE's images, precisely these sharply cut edges and prominent bright spots in the rings serve as "traces" of possible, as yet only indirectly detected planets.

Disks as a map for future observations

It is important to emphasize that in individual systems, giant planets have already been directly imaged by other programs or instruments, and their presence fits nicely into the patterns seen in debris disks. In other cases, disks serve as a map for future research: numerical simulations can show what kind of planet – with what mass and on what orbit – could create the observed structure. Such models then guide new imaging campaigns, whether with the SPHERE instrument or the James Webb Space Telescope, which can search for the thermal signature of these hidden giants in the infrared range.

What future telescopes bring

The new collection of disks is therefore also a kind of catalog of priority targets for future observatories. A special role in this regard will be played by the European Extremely Large Telescope (ELT), which according to current plans should achieve first light at the end of this decade. With a primary mirror of 39 meters in diameter, the ELT will offer significantly higher resolution and light-gathering power than today's telescopes, so it will be able to map faint disks in more detail and directly image the planets that shape them. In combination with high-contrast techniques developed on SPHERE, future instruments on the ELT should enable the systematic study of phenomena like asteroid and Kuiper belts in various stellar environments.

How special is our system?

The results of the study also fit into the broader story of the diversity of planetary systems. Although the abundance of "hot Jupiters", mini-Neptunes, and other exotic worlds teaches us that the Solar System is not typical in every respect, the fact that many debris disks show belts similar to our asteroid and Kuiper belts suggests that certain architectural elements are nevertheless common. In such belts, collisions constantly create fresh dust, but also throw icy comets towards the inner parts of the system, where they can bring water and volatiles to young rocky planets. In this sense, images of debris disks are not only aesthetically impressive pictures but also a key to understanding the long-term conditions that may, much later, decide on the appearance of oceans, atmospheres, and potentially habitable worlds.

For astronomers studying planet formation, SPHERE's gallery represents an important link between early, gaseous protoplanetary disks and mature systems like the one we live in today. Debris disks show how the first generations of planets carved into the original disk, where supplies of small bodies remained, and how the overall structure changes as the system ages. By comparing different stars, disks, and eventual planets, it will be possible to build an increasingly detailed chronology of the development of planetary systems and answer the question of how common the combination is that in our case led to stable orbits, moderate conditions, and a diverse menu of potential "Earths" around other stars.