Astronomers spot collision between two exoplanets, both feared vaporized

Dust blocks light from Sun-like star as far-off worlds meet

Evidence of a collision between planets outside our solar system was published for the first time today.

Data on infrared radiation and the dimming of the host star's light led researchers to conclude they may have witnessed planets between the size of Earth and Neptune smashing into each other while circling the distant stellar orb.

In 2021, researchers spotted distant star ASASSN-21qj dimming. A subsequent investigation of infrared radiation from the region added to the intrigue. A research team led by Matthew Kenworthy, associate professor at the Netherland's Leiden Observatory, concluded that the best explanation was a collision that generated enough heat for the infrared radiation while also throwing up enough dust to dim the star's light.

"These observations are consistent with a collision between two exoplanets of several to tens of Earth masses… Such an impact produces a hot, highly extended post-impact remnant with sufficient luminosity to explain the infrared observations. Transit of the impact debris, sheared by orbital motion into a long cloud, causes the subsequent complex eclipse of the host star," says the paper published in Nature today.

The researchers suggest the power of the impact would be enough to destroy both exoplanets. "Giant impacts are one of the most energetic events that planets experience. For example, the kinetic energy of impacts between two half-Neptune-mass bodies range from 1033 to 1034 Joules, enough to vaporize the colliding bodies several times over," the paper says.

The early stages of our solar system were marked by planetary impacts. On Earth, this led to the creation of the Moon and the arrival of water. These events were driven by instability in the orbits of Jupiter and Saturn.

In an accompanying article by Carl Melis, the associate researcher at the Center for Astrophysics and Space Sciences, University of California praises the research. "The observation is unprecedented, and brings fresh insight into the early evolution of planetary systems and worlds like our own," he writes.

He also tries to explain the findings in the context of the evolution of the solar system.

"If the system were around 100 million years old, it would be comfortably within the realm of the final stages of rocky-planet assembly. However, the authors' estimates put its age at 300 million years, conflicting with such an interpretation and placing it in the ranks of other older Sun-like stars that have undergone major collisional events."

Researchers could also rule out comparisons with our solar system's late heavy bombardment phase, in which the inner planetary system was subjected to a spate of asteroid impacts.

"Unlike in that phase… the star observed by the authors is experiencing true chaos: the collision of mature planets in its outer planetary system," Melis says.

"Combining simulation results with Kenworthy and co-workers' findings, it seems plausible that collisions between mature planets are much more common than was previously thought. Large-scale violent planetary collisions could well be the norm, and have probably had a role in shaping planetary systems throughout the Universe." ®

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