The James Webb telescope has just observed the traces of collisions so violent that they can reduce entire rocky worlds to dust.
Around some young stars, astronomers have detected far more hot dust than expected. This material forms debris disks close to the star, in a region comparable to that where the rocky planets of our Solar System are found. Their origin had remained difficult to determine precisely.
A team used the James Webb Space Telescope to study 21 of these systems, described as extreme. Its MIRI instrument observes in the mid-infrared, a range of wavelengths particularly suited to analyzing hot dust. The resulting spectrum reveals which minerals are present in the debris.
Several disks contain silica and other materials transformed by extremely high temperatures. Such signatures can appear when a collision between large rocky bodies releases enough energy to melt or vaporize part of their material. The fragments then cool into dust and tiny grains.
These impacts are difficult to imagine in their violence. They occur during the final stages of a planetary system's formation, when planetary embryos still share unstable orbits. Some eventually collide at several kilometers per second.
Our own Solar System likely experienced this kind of event. The leading scenario for the birth of our Moon involves a collision between the young Earth and another large body about 4.5 billion years ago. The systems observed by Webb therefore provide access to phenomena that probably shaped the rocky planets around the Sun.
However, not all disks have the same composition. This diversity indicates that collisions can involve bodies formed from different materials or occur with different energies. Some dust may come from rocky surfaces, while other dust may originate from deeper layers exposed by the impact.
Astronomers can also follow these systems over time. The amount of dust changes after a collision: the grains disperse, are destroyed, or fall back onto other bodies. Repeated observations will therefore make it possible to estimate the frequency of impacts and how long their traces remain visible.
The researchers now want to link these mineral signatures to the ages of the stars and the architecture of their planetary systems. This will allow them to determine when giant collisions are most frequent and better reconstruct the chaotic period during which Earth-like planets complete their formation.