The rare cosmic spectacle left debris and dust that could form new celestial bodies.

Two Neptune-sized icy planets collided in 2021, a rare cosmic event that resulted in their mutual destruction and the formation of a scorching, torus-shaped debris cloud. It marks a historic moment for the community of astronomers, as it represents the very first observation of the immediate aftermath of such a planetary crash.

Matthew Kenworthy, an astronomer at Leiden University in the Netherlands, has described the event in a study published in the journal Nature as a co-lead author. The event sheds light on the intriguing phenomena of interplanetary collisions and their potential role in the genesis of new celestial bodies. 

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According to the study, the energy generated by the impact would transform the remaining debris into an object resembling a star. While fainter than the primary star in the system, this star-like remnant would be roughly seven times larger in size, casting its radiance throughout the entire stellar system.

Astronomers first became aware of this celestial event in 2021, when they noticed the recurring dimming of a star called ASASSN-21qj.

A deeper investigation revealed that this wasn't the first time the Sun-like star's brightness had fluctuated. 

Computer simulations of the intense heat and bright vapour as the two ice giant planets collide in a distant space system 1,800 light years away from Earth. Credit: Jingyao Dou/University of Bristol

In the three years leading up to the dimming, ASASSN-21qj had actually doubled in brightness in the infrared spectrum, raising a perplexing question: How could a star appear to brighten in one spectrum while fading in another?

The most plausible explanation, according to Kenworthy's theory, was a head-on collision between two ice giant planets, both approximately the size of Neptune and over 17 times the mass of Earth.

The outcome of this cataclysmic encounter was the mutual vaporization of the two worlds, giving rise to a torus-shaped cloud of intensely hot debris, with temperatures reaching around 1,300 degrees Fahrenheit. This searing heat accounted for the infrared brightening observed in ASASSN-21qj.

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Furthermore, as this freshly formed debris ring continued to orbit the star, it eventually passed between the star and Earth, obscuring the star's light from the perspective of astronomers.

Why did the planets collide?

Astronomers who spoke to New Scientist are still grappling with the enigma of what brought these two planets into a fateful collision course. Although a planetary collision appears to be the most likely explanation given the evidence, such events are considered rare, especially in a star system estimated to be around 300 million years old, which is typically considered too mature for such incidents. 

The authors of the study speculate that the debris cloud may eventually give birth to new planets and moons. Kenworthy offered his prediction that within the next five to ten years, they might begin to detect additional light emitted from the system due to the scattering of light by the dust cloud. However, if this doesn't materialize, it would suggest the presence of other, yet unknown, factors at play.

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