This illustration shows what a debris cloud around the young star called HD 166191 might look like up close. Astronomers using NASA's now-retired Spitzer Space Telescope saw a debris cloud briefly block the light from that star. The cloud was likely created by a smashup between two large asteroid-sized objects that might be the seeds of future planets around the star.

HD 166191 is about 10 million years old. Around this time in a star's life, dust left over from its formation has clumped together to form rocky bodies called planetesimals. Asteroids are leftover planetesimals from the formation of our own solar system. Around other stars, these objects can be the seeds of future planets. Catastrophic collisions between them become common once the gas that previously filled the space between the objects disperses.

Spitzer has previously found evidence of these collisions around young stars where rocky planets are forming. Kate Su of the University of Arizona led the team that saw the debris cloud transit around HD 166191 – the first transit of its kind ever found.

Anticipating they might see evidence of one of these collisions around the star, the team ultimately used Spitzer to conduct more than 100 observations of the system between 2015 and 2019. While the objects are too small and distant to resolve by telescope, their smashups produce large amounts of dust that is detectable. Infrared light, which Spitzer detected, is an ideal range for detecting dust, including the debris created by protoplanet collisions.

In mid-2018, the space telescope saw the HD 166191 system become significantly brighter, suggesting an increase in debris production. During that time, Spitzer also detected a debris cloud blocking the star. Combining Spitzer's observation of the transit with observations by telescopes on the ground, the team could deduce the size and shape of the debris cloud.

Their work suggests the cloud was highly elongated, with a minimum estimated area three times that of the star. However, the amount of infrared brightening Spitzer saw suggests only a small portion of the cloud passed in front of the star, and that the debris from this event covered an area hundreds of times that of the star.

To produce a cloud that big, the objects in the main collision must have been the size of dwarf planets, like Vesta in our solar system – an asteroid 330 miles (530 kilometers) wide in the main asteroid belt between Mars and Jupiter. By 2019, the cloud that passed in front of HD 166191 was no longer visible, but the system contained twice as much dust as it had before Spitzer spotted the cloud.

NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology, also in Pasadena. Caltech manages JPL for NASA.