(ORDO NEWS) — Major collisions between rocky bodies shaped our solar system. Observations of such an accident give an idea of how often these events occur around other stars.
Most of the rocky planets and moons in our solar system, including the Earth and the Moon, were formed by massive collisions early in the solar system’s history. By colliding together, rocky bodies can accumulate more material, growing in size, or they can break up into several smaller bodies.
Astronomers using NASA’s now retired Spitzer Space Telescope have in the past found evidence of these types of collisions around young stars where rocky planets form. But these observations did not provide many details about the collisions, such as the size of the objects involved.
In a new study in the Astrophysical Journal, a team of astronomers led by Keith Su of the University of Arizona reports the first sightings of a debris cloud from one of these collisions as it passed in front of its star and briefly blocked the light. Astronomers call this a transit.
Combined with knowledge of the size and brightness of the star, the observations allowed the researchers to directly determine the size of the cloud shortly after the impact, estimate the size of the objects that collided, and observe the rate at which the cloud dissipated.
“There is no substitute for being an eyewitness to an event,” said George Ricke, also of the University of Arizona and co-author of the new study. “All cases previously reported from Spitzer were unresolved, and only theoretical hypotheses about what the actual event and debris cloud might have looked like.”
Beginning in 2015, Su’s team began making regular observations of the 10-million-year-old star HD 166191. Around this early time in a star’s life, the dust left over from its formation clung together to form rocky bodies called planetesimals, the seeds of future planets. Once the gas that previously filled the space between these objects has dissipated, catastrophic collisions between them become commonplace.
Expecting they might see evidence of one of these collisions around HD 166191, the team used Spitzer to make more than 100 observations of the system between 2015 and 2019.
While planetesimals are too small and far away to be seen with a telescope, their collisions produce large amounts of dust. Spitzer discovered infrared light. Infrared is ideal for detecting dust, including debris created by protoplanetary collisions.
In mid-2018, the space telescope saw that the HD 166191 system became significantly brighter, indicating an increase in space debris production. During this time, Spitzer also detected a cloud of debris blocking the star. By combining Spitzer’s observations of the transit with observations from telescopes on the ground, the team was able to determine the size and shape of the debris cloud.
Their work suggests that the cloud was highly elongated, with a minimum estimated area of three times that of the star. However, the amount of infrared light that Spitzer saw suggests that only a small portion of the cloud passed in front of the star, and that debris from this event covered an area hundreds of times larger than that of the star.
To create such a large cloud, the objects in the main collision would have to be the size of dwarf planets, such as Vesta in our solar system, a 530-kilometer-wide object located in the main asteroid belt between Mars and Jupiter.
The initial collision generated enough energy and heat to vaporize some of the material. It also set off a chain reaction of impacts between fragments of the first impact and other small bodies in the system, which likely created the significant amount of dust that Spitzer saw.
Over the next few months, the large dust cloud increased in size and became more transparent, indicating that dust and other debris is rapidly dispersing throughout the young star system.
By 2019, the cloud that passed in front of the star was no longer visible, but the system contained twice as much dust as before Spitzer noticed the cloud. This information, according to the authors of the article, can help scientists test theories about how terrestrial planets form and grow.
“By looking at the dusty disks of debris around young stars, we can essentially look back in time and see the processes that may have shaped our own solar system,” Su said. “By learning about the results of collisions in these systems, we can also get a better idea of how often rocky planets form around us and other stars.”
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