Star-sized debris cloud from distant collision spotted by NASA telescope
Major smashups between rocky bodies shaped our solar system. Observations of a similar crash give clues about how frequently these events occur around other stars.
Most of the rocky bodies in our solar system, including Earth and the moon, were formed or shaped by massive collisions early in the solar system's history. By smashing together, the bodies might accumulate more material, increasing in size and potentially forming planets. The bodies might also lose material, but in the process, satellites like the moon could form.
Astronomers using NASA's now-retired Spitzer Space Telescope have found evidence of these types of collisions in the past around young stars where rocky planets are forming. But those observations provided few details about the smashups, such as the direct estimate of the size of the debris cloud generated in such a giant collision.
In a new study published in the Astrophysical Journal, a group of astronomers led by Kate Su, a research professor in the University of Arizona Steward Observatory, report the first observations of a debris cloud from one of these collisions as it passed in front of its star and briefly blocked the starlight. Astronomers call this a transit. Coupled with knowledge about the size and brightness of the star, this allowed researchers to directly determine the size of the cloud shortly after impact, estimate the size of the objects that collided, and watch the speed with which the cloud dispersed.
Beginning in 2015, Su's team started making routine observations of a 10-million-year-old star called HD 166191. Around this time in a star's life, dust leftover from its formation has clumped together to form rocky bodies called planetesimals. Asteroids are leftover planetesimals from the formation of our own solar system, and around other stars they are the seeds of future planets. Once the gas that previously filled the space between those objects has dispersed, catastrophic collisions between them become common.
Anticipating they might see evidence of one of these collisions around HD 166191, the team conducted more than 100 observations of the system between 2015 and 2019 with Spitzer. While the objects would be too small and distant to resolve by telescope, their smashups produce large amounts of dust. Spitzer detected infrared light – wavelengths slightly longer than what human eyes can see; it’s an ideal range for detecting dust, including the debris created by protoplanet collisions.
In mid-2018, Spitzer showed the system became significantly brighter, suggesting an increase in debris production. During that time, Spitzer also detected a debris cloud blocking the star – a transit at both 3.6-micron and 4.5-micron bands. The same transit was also detected by a ground-based, all-sky survey telescope in the visible wavelength at the same time. In addition to the transit detected by Spitzer, ground-based optical data also revealed a similar transit 142 days earlier during a gap in the Spitzer observations.
The team found an amazing "smoking gun" of a planetary collision – a newly formed dust cloud.
"For the first time, we captured both the infrared glow of the dust and the haziness that dust introduces when the cloud passes in front of the star," said Everett Schlawin, a co-author of the paper and assistant research professor at Steward Observatory.
Multi-wavelength transit data confirm that the transits were caused by a dust cloud passing in front of the star and evolving at a fast pace – as it grew wider and more opaque during the first two transits but showed very little hint of the cloud afterward. These data allowed the team to determine the location, shape and size of the debris cloud. The team's work suggests the cloud was highly elongated, with a minimum estimated area of 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 total amount of debris created by this violent event covered an area hundreds of times that of the star.
"There is no substitute for being an eyewitness to an event," said George Rieke, study co-author and a Regents Professor of Astronomy at UArizona. "All the cases reported to date from Spitzer have been unresolved, with only theoretical hypotheses about what the actual event and debris cloud might have looked like."
To produce a cloud that big, the objects in the main collision must have been the size of dwarf planets, like Vesta – an object 330 miles wide located in the main asteroid belt between Mars and Jupiter. The initial clash generated enough energy and heat to vaporize some of the material. It also set off a chain reaction of impacts among fragments from the first collision, as well as with other small bodies in the system, which likely created a significant amount of the dust Spitzer saw.
Over the next few months, the large dust cloud grew larger and became more translucent, indicating that the dust and other debris were quickly dispersing throughout the young star system. Although the cloud that passed in front of the star was no longer visible, the system contained twice as much dust as it had before Spitzer spotted the cloud. This information, according to the paper's authors, can help scientists test theories about how terrestrial planets form and grow.
"By looking at dusty debris disks around young stars, we can essentially look back in time and see the processes that may have shaped our own solar system," Su said. "Learning about the outcome of collisions in these systems, we may also get a better idea of how frequently rocky planets form around other stars."
The team is continuing to keep an eye on the star with other NASA infrared facilities including the Stratospheric Observatory for Infrared Astronomy and the Infrared Telescope Facility. Such observations not only keep a record of overall infrared flux evolution, but also provide additional information on the dust composition of the freshly generated fine dust with spectroscopy. Particularly, future observations with the Mid-InfraRed Instrument onboard NASA's James Webb Space Telescope will provide further insights into the physical condition of these large collisions by studying the dust mineralogy.
Other co-authors of the paper are Grant Kennedy from the University of Warwick and Alan Jackson from Arizona State University.
The entire body of scientific data collected by Spitzer during its lifetime is available to the public via the Spitzer data archive, housed at the Infrared Science Archive at the Infrared Processing and Analysis Center, or IPAC, at Caltech in Pasadena, California. The NASA Jet Propulsion Laboratory, a division of Caltech, managed Spitzer mission operations for NASA's Science Mission Directorate in Washington, D.C. Science operations were conducted at the Spitzer Science Center at IPAC at Caltech. Spacecraft operations were based at Lockheed Martin Space in Littleton, Colorado.
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