Key Takeaways:

  • Our Solar System was born in chaos, and the Earth and the other planets were formed and shaped by collisions.
  • By studying the aftermath of collisions in young solar systems, astronomers can gain insight into how our own Solar System formed.
  • A collision of dwarf planet-sized objects was observed in a young solar system called HD 166191. This is the first time that such a collision has been observed.
  • The collision produced a large amount of dust, which was observed by the Spitzer Space Telescope.
  • These observations of the collision in HD 166191 help us to understand how planets form in our solar system and other solar systems.

Our Solar System was born in chaos. The Earth and the other planets were formed and shaped by collisions, which also provided the building blocks for life. We might not be here if things hadn’t crashed into each other.

Thankfully, most of the collisions are in the past, and now our Solar System is a relatively calm place. However, in other younger solar systems, collisions still happen often, and astronomers can observe their consequences.

One can gain insight into planet formation by observing the aftermath of a collision. Astronomers use their telescopes to focus on distant objects in collision to determine the nature of the impact. Utilizing the Spitzer Space Telescope and additional resources, astronomers monitored the aftermath of a collision near the young star HD 166191.

“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.” Kate Su, study lead author, University of Arizona.

Ten million years old, HD 166191 is located roughly 330 light-years away from the Sun. Since it is a protoplanetary disk changing into a debris disk, astronomers are interested in it. Scientists were able to gain a new perspective on collisions and their aftermath when years of observations revealed a cloud of post-collision debris passing in front of the star.

A Star-sized Impact-produced Dust Clump in the Terrestrial Zone of the HD 166191 System” is the title of the study. Astronomer Kate Su of the University of Arizona’s Steward Observatory is the primary author. The Astrophysical Journal has the paper available online.

According to a press release from Su, “we can essentially look back in time and see the processes that may have shaped our solar system by looking at dusty debris disks around young stars.”

The group of astronomers first observed HD 166191 in 2015. They observed the young solar system more than 100 times between 2015 and 2019. The system is too young to have fully-grown planets, but planetesimals and maybe even dwarf planets are bound to be orbiting the star. Regretfully, telescopes cannot see them because they are too small and far away.

However, planetesimals are the basic units of a planet. And those blocks don’t accrete in an orderly fashion. Rather, they collide with one another, dividing into smaller pieces occasionally and merging to form larger rocky planets in the end.

Infrared dust clouds are produced by those collisions. When the star’s energy hits the dust, infrared light is released. And that’s what the group observed when using Spitzer. In 2018, the team observed a marked increase in brightness at HD 166191. The increase implies a significant increase in the amount of dust surrounding the star. “The large-scale infrared brightening obviously points to a huge increase in the debris emission…” the authors write.”

Over time, the scientists observed as a cloud of debris from the collision passed in front of the star. They computed the impact speed, the moment the collision happened, and the sizes of the colliding objects. They also watched how quickly the debris cloud dispersed.

It was obvious from their observations that the debris cloud was longer. Additionally, they demonstrated that the cloud covered roughly three times the area of the star. However, the 2018 increase in infrared light pointed to a different possibility. It meant that the dust cloud had to be much larger because only a small fraction of the debris cloud passed in front of the star directly. Their research indicated that it needed to encompass a region hundreds of times bigger than the star. All of that infrared light can only have one explanation.

The objects in the collisions had to be as big as dwarf planets in order to produce that much dust. The most famous dwarf planet in our solar system is Vesta, which was visited in 2011 by NASA’s Dawn mission. According to the astronomers who conducted this study, the objects that collided around HD 166191 had a diameter of roughly 530 km (330 miles), which is comparable to Vesta. “The amount and rapidity of the increase in the infrared flux requires a catastrophic event, such as a collision between two large bodies (?500 km in diameter) occurring in the terrestrial zone,” the authors write in their paper.

Artist's concept of the Dawn spacecraft arriving at Vesta. Image credit: NASA/JPL-Caltech
Artist’s concept of the Dawn spacecraft arriving at Vesta. Image credit: NASA/JPL-Caltech

A significant amount of heat was produced by the collision, and some of the material disappeared. Similar to balls on a billiard table, the impact set off a series of smaller collisions between debris and other rocky objects in orbit. The large amount of dust and the increase in infrared energy in mid-2018 can be explained by a cascade of multiple collisions. “Simulations indicate a substantial rate of fragment collisions rapidly following the impact disruption of asteroid-sized bodies, which would expedite the initiation of an intense collisional cascade,” the paper states. “This intense activity is further highlighted by detecting a star-size dust clump, passing in front of the star, in the midst of its infrared brightening.”

Throughout the next few months, the dust cloud grew larger and more transparent. The dust and the rocky debris from the collision were dispersing around the solar system. There was twice as much dust in the system by 2019, although observations indicated that the dust cloud that passed in front of the star had vanished.

Researchers who work on planets and solar systems will find value in these observations. The formation of planets is mostly obscured. Initially, young solar systems are dusty environments, and only specific observatories have the ability to see through the dust. Thanks to observatories like ALMA, we can see the lanes carved in the dust around young solar systems as planets form and sweep up the dust.

Image of the HL Tau planet-forming disk (not part of this study) taken with the Atacama Large Millimeter Array. Astronomers think that the dark lanes are where planets are forming. Credit: ALMA (ESO/NAOJ/NRAO)
Image of the HL Tau planet-forming disk (not part of this study) taken with the Atacama Large Millimeter Array. Astronomers think that the dark lanes are where planets are forming. Credit: ALMA (ESO/NAOJ/NRAO)

Lead study author Kate Su said, “We may also get a better idea of how frequently rocky planets form around other stars by learning about the outcome of collisions in these systems.”

Scientists will continue tracking HD 166191 to track the system’s development. It is believed by astronomers to be changing from a protoplanetary disk to a debris disk. The star itself isn’t accreting any more material. The system appears to be dusty despite the star having mostly evaporated the gas leftover from star formation through photoevaporation, based on the excessive infrared emissions.

“Theoretical simulations indicate that the very existence of terrestrial planets depends on the collisional merging of planetary embryos and oligarchs,” the paper says. This collisional merging generally happens during the first 200 million years after the protoplanetary disks have cleared out much of the remnant gas from the star formation. “Large-scale collisions among planetesimals and planetary embryos in the terrestrial-planet region are expected to be common after the dissipation of the gas in a protoplanetary disk, a stage we think HD 166191 is in,” the authors write.

In our Solar System there were probably dozens of Moon-sized to Mars-sized rocky bodies in the inner Solar System. Scientists believe that Theia, a protoplanet, collided with Earth to form the Moon. The inner Solar System’s architecture is the product of many more collisions that most likely occurred.

Still, astronomers are forced to investigate our Solar System like forensics. If they want to see any of this happening, they will need to study distant systems like HD 166191.

The paper states that future observations of the system will be extremely valuable in learning about the process of terrestrial-planet formation and planetary architecture, given its evolutionary state—that is, right after the dispersal of its gas-rich disk.

What happens to HD 166191 next? “We might be witnessing the early giant impact phase in assembling terrestrial planets in the inner zone,” the authors write in the conclusion of their paper. Or the activity they’re seeing in the young system “… might be triggered by perturbation from nearby, unseen planet-mass objects, creating orbit-crossing collisions in the existing asteroid population.” Alternatively, there could be a giant planet migrating.

Young dusty systems are dynamic environments that will only be fully understood through additional observations. Extremely dusty systems are temporary, which emphasizes the significance of ongoing observation. Future observations of this unique system would further shed light into our understanding of terrestrial-planet formation and overall assembling planetary architecture,” they conclude.

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