SPHERE debris disk survey maps hidden asteroid and comet belts in young planetary systems

by Robert Schreiber
Berlin, Germany (SPX) Dec 03, 2025

Observations with the SPHERE instrument on ESO's Very Large Telescope have produced a large gallery of debris disks around nearby young stars, revealing where unseen asteroid- and comet-scale bodies are colliding and generating dust. Gael Chauvin, SPHERE project scientist and co-author, said: "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 the solar system, objects between about a kilometer and a few hundred kilometers across occupy distinct populations such as asteroids and comets, which are thought to be leftover planetesimals from the era of planet formation. These minor bodies preserve a record of the transition from dust grains to full-sized planets, because they represent building blocks that never grew into larger worlds. Astronomers distinguish comets as bodies that at least sometimes release gas and dust to form visible tails or comae, while asteroids do not show such activity.

More than 6000 exoplanets are now known, but directly imaging planets remains difficult and yields only unresolved points of light, and current indirect planet-detection methods do not provide direct information about small bodies in those systems. The new work instead traces much smaller dust grains that are generated when unseen planetesimals collide, offering an indirect way to study asteroid- and comet-like populations around other stars. When a kilometer-scale asteroid is broken down into micrometer-scale dust grains, the total volume stays the same but the combined surface area rises by about a factor of a billion, making reflected starlight from the dust detectable over large distances with sensitive instruments.

In young planetary systems, these ongoing collisions maintain bright debris disks that can be seen in reflected light. Over tens of millions of years, the collision rate decreases and the debris disk fades as dust is expelled by radiation pressure, accreted onto planets or planetesimals, or spirals into the central star. The solar system now shows the end state of this evolution, with two main planetesimal belts - the asteroid belt between Mars and Jupiter and the Kuiper Belt beyond the giant planets - plus a population of dust that produces the zodiacal light visible from Earth under dark skies just after sunset or before sunrise.

The study estimates that with current facilities, debris disks around nearby stars should be observable in scattered light for roughly the first 50 million years of their evolution, before collisional and dynamical processes remove most of the dust. Obtaining such images is technically demanding because a faint, extended cloud of dust must be detected right next to a very bright star, similar to trying to photograph a puff of cigarette smoke beside a stadium floodlight from several kilometers away.

SPHERE is designed to tackle this problem by combining a coronagraph with extreme adaptive optics on the VLT. The coronagraph inserts a small occulting disk into the optical path to block most of the starlight, like raising a hand to shield the Sun when trying to see a nearby object, while the adaptive optics system uses a deformable mirror and real-time wavefront sensing to correct for atmospheric turbulence and keep the stellar image stable. An additional observing mode uses polarimetric filtering to isolate polarized light that has been scattered by dust grains, which helps separate disk signal from residual starlight and increases sensitivity to faint structures.

For the new survey, the team processed SPHERE observations of 161 nearby young stars that show strong infrared excesses, a signature that indicates the presence of debris disks heated by the central star. Lead author Natalia Engler of ETH Zurich said: "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. 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."

The resulting gallery displays disks with a wide range of apparent sizes and orientations, from compact systems to extended structures that fill the SPHERE field of view. Some disks appear nearly edge-on, highlighting vertical thickness and midplane structure, while others are viewed close to face-on, emphasizing azimuthal features such as gaps, rings, and brightness asymmetries. The sample includes 51 resolved disks out of the 161 targets, confirming that a significant fraction of young, dusty systems can be mapped in scattered light with current high-contrast imaging techniques.

Comparing different systems in a uniform dataset allowed the team to identify global trends linking disk properties to stellar characteristics. The analysis shows that more massive young stars generally host more massive debris disks, consistent with models in which more massive protoplanetary disks around such stars leave behind larger planetesimal reservoirs. Disks where most of the material lies at larger stellocentric distances also tend to be more massive, suggesting that extended, cold belts can retain substantial populations of small bodies over tens of millions of years.

Within individual systems, many debris disks show ring-like or banded structures where dust is concentrated in relatively narrow belts at specific distances from the star. This morphology closely parallels the structure of the solar system's asteroid belt and Kuiper Belt, where planetesimals occupy preferred orbital zones rather than a continuous distribution. Sharp inner edges, offsets between disk center and stellar position, and one-sided brightness enhancements in several SPHERE images point to gravitational shaping by planets that clear or trap small bodies.

The belts in these systems appear to be associated with giant planets that have dynamically cleared their neighborhoods of smaller bodies, creating gaps and confining planetesimals into resonant rings. Some of these planets have already been detected in previous observations, providing direct links between known giant planets and the surrounding sculpted disks. In other systems, subtle disk asymmetries and sharply defined edges hint at additional, as-yet unseen planets whose properties can be constrained by modeling the observed dust structures.

The SPHERE debris disk gallery therefore not only maps current locations of small bodies but also defines priority targets for future planet searches. Facilities such as the James Webb Space Telescope and ESO's Extremely Large Telescope, now under construction, are expected to directly image some of the giant planets that shape these disks and to measure their atmospheres and orbits. Combining such data with the SPHERE disk morphologies will help refine models of how giant planets and planetesimal belts co-evolve and how debris production changes over time.