New classification of planetary collisions

A team from the Space Science Institute in Boulder has identified two types of planetary collisions by analyzing the chemical composition of grains around 21 stars. Their findings, published in October 2026, reveal that the intensity of these impacts can be determined by the makeup of microscopic particles surrounding young stars, a detail previously unknown.

Extreme debris disks around young stars

Some young stars are surrounded by unusually dense and hot clouds called extreme debris disks. These rare structures are much brighter in infrared light than the stars themselves, with brightness levels 10 to several hundred times greater. They are found in only about 1% of systems of the same age.

Observing debris disks with advanced telescopes

Kate Su's team used the James Webb Space Telescope and data from the Spitzer Telescope to study these 21 systems in the mid-infrared. The study, published in The Astrophysical Journal, found that the grains in these disks are much smaller than those in classical disks, measuring less than a micrometer. These tiny grains are quickly destroyed by stellar radiation and can only survive a few thousand years.

Silica-rich systems indicate large planetary collisions

Eight of the 21 systems studied contain high levels of silica, a component found in Earth's volcanic glass like obsidian. Silica forms at temperatures between 1,200 and 1,800 kelvins, indicating powerful collisions between planetary embryos the size of Mars, strong enough to melt rock.

Forsterite points to less intense impacts

In the other 13 systems, the presence of forsterite, a crystalline magnesium silicate found in some Hawaiian green sands, indicates less intense impacts. This mineral forms at temperatures between 1,000 and 1,200 kelvins, suggesting collisions involving bodies the size of the Moon.

Challenges in observing planetary embryos

Ágnes Kóspál from the Konkoly Observatory in Budapest notes that directly observing planetary embryos at such distances is currently impossible. Instead, scientists rely on the infrared light from debris disks to detect them. The James Webb Space Telescope has significantly expanded the known sample of such systems.

Age distribution matches formation simulations

The silica-rich systems in the sample are all less than 300 million years old, while only three of them remain after that age. This pattern aligns with simulations of planetary formation, where giant collisions between rocky embryos would occur in the first few hundred million years.

Link to ancient solar system events

About 4.5 billion years ago, a collision between the early Earth and a Mars-sized body called Theia is believed to have created the Moon. Later, the migration of the giant planets likely caused new waves of collisions. The three oldest systems in the sample, aged between 600 and 5,500 million years, may be evidence of this late orbital instability. The oldest, J1044, orbits a K-type star that is 5.5 billion years old.