December 4, 2023

On the path to a planetary collision

There could be violent collisions in planetary systems: Astronomers have reported direct signs of a massive planetary collision in a distant star system, study shows. This event is reflected in a flash in the infrared range, which may have been caused by the hot collision between two ice giants. Then the glow was followed by a distinct blackening of starlight. As astronomers explain, this effect is due to the resulting debris cloud, which is now orbiting the star along with the fusion product.

Collision processes have shaped the formation stories of planetary systems from the beginning: they begin with the increasing clumping of matter into disks of dust around young stars. The union of the larger pieces eventually gives rise to celestial bodies of planetary proportions. But things could also go on after that: collisions of large celestial bodies supposedly played an important role in the formation of the planets and their star clusters. This also applies to our solar system and our home planet: proto-Earth is thought to have once collided with a Mars-sized body, creating this situation.

Exciting radiation fluctuations on the horizon

Evidence of such events has also been found in some distant planetary systems. In the current case, it is an observation of direct signs of such a planetary collision. The focus is on the Sun-like star system ASASSN-21qj, which is estimated to be about 300 million years old. This was observed for the first time in 2021: telescope observations as part of the ASASSN project recorded the fading of the star in the visible wavelength range. In order to investigate the unusual effect in more detail, an international team of astronomers has now evaluated more observational data for the star, which also includes the time before darkness.

As the team reported, there was already an unusual fluctuation in radiation in the ASASSN-21qj system three years before the star faded in visible light: telescopic images showed a doubling of brightness in the infrared wavelength range. This “glow” was clearly noticeable for about 1,000 days. The combination of the two events suggests that they were caused by a collision between two celestial bodies in orbit around ASASSN-21qj: the infrared radiation was due to the heat generated by the collision and the subsequent dimming of starlight was due to the passage of the resulting debris cloud. Since the dimming of visible light began about three years after the infrared glow, scientists say the orbital period of this material can be assumed to be at least that long.

Collision between two ice giants

To further explain the event, they then developed simulation models based on the collected data. As the team reported, the results showed that it was a collision of two planets with masses ranging from several to tens of Earth’s masses. “The results are particularly consistent with the collision of two icy giant exoplanets,” says co-first author Simon Locke from the University of Bristol. They collided with each other at a distance from the central star equivalent to two to 16 times the distance between the Earth and the Sun. The violent collision created a hot fusion cluster whose luminosity matches infrared observations. Astronomers explained that the orbital motion turned the resulting debris into a long cloud.

They now want to continue monitoring the exciting system using ground-based telescopes and NASA’s James Webb Space Telescope to investigate further developments. Because it could be interesting, confirms co-author Zoe Lenhardt from the University of Bristol: “Eventually, the mass of material could condense around the remains and form a chain of moons orbiting this new planet,” the astronomer said.

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Source: University of Bristol Specialized article: Nature, doi: 10.1038/s41586-023-06573-9