The afterglow of an explosive collision between giant planets may have been detected in a far-off star system

Subject: Science and Tech

Section: Space technology

Context:

The afterglow of a massive collision between two giant planets may have been detected for the first time.

Details:

The wreckage of the collision could eventually cool and form an entirely new planet.
If the observation is confirmed, it provides an amazing opportunity to watch the birth of a new world in real time and open a window into how planets form.
Observations:
- To emit the amount of energy observed, the post-impact body must have been many hundreds of times the size of Earth as large as the “ice giant” planets Uranus and Neptune.
- The temperature of the post-impact body is around 700°C. For the temperature to be that low, the colliding bodies could not have been entirely made of rock and metal.

About the star- Aaan-21qj:

In December 2021, astronomers observed a star and at times it would almost disappear, before returning to its previous brightness.
The star, which sits roughly 1,800 light years from Earth, was given the identifier ASASSN-21qj, after the ASASN-SN astronomy survey that first observed the star’s dimming.
- The dimming of stars is generally attributed to material passing between the star and Earth.
Infrared light is most strongly emitted by objects at relatively high temperatures of a few hundred degrees Celsius.
- The infrared brightening of ASASSN-21qj was observed by NASA’s WISE space telescope. WISE only looks at the star every 300 days or so and probably missed the initial flash of light from the impact.

Planetary cataclysm:

Both sets of observations could be explained by a cataclysmic collision between two planets.
Giant impacts and collisions are common in the final stages of the formation of planets. They dictate the final sizes, compositions and thermal states of planets and mould the orbits of objects in those planetary systems.
In our solar system, giant impacts are responsible for the odd tilt of Uranus, the high density of Mercury and the existence of Earth’s Moon.
But, until now, we had little direct evidence of giant impacts ongoing in the galaxy.
A collision would have needed to release more energy in the first few hours after impact than would be emitted from the star. Material from the colliding bodies would have been superheated and melted, vaporized or both.

New planet or the post-impact body:

The impact would have formed a hot, glowing mass of material hundreds of times larger than the original planets.
However, the expanded planetary body produced by the impact will take a long time, perhaps millions of years, to cool and shrink to something we might recognise as a new planet.
Initially, when this “post-impact body” was at its greatest extent, it could have produced the infrared brightening.
The impact would also have ejected great plumes of debris into a range of different orbits around the star. A fraction of this debris would have been vaporized by the shock of the impact, later condensing to form clouds of tiny ice and rock crystals.
Over time, some of this clumpy cloud of material passed between ASASSN-21qj and Earth, blocking out a fraction of the visible light from the star and producing the erratic dimming.

Ice giants:

The outer regions of at least one of the planets must have contained elements with low boiling temperatures, such as in water. It is like a collision between two Neptune-like worlds that are rich in ice.
The delay between the emission of infrared light and the observation of debris crossing the star suggests that the collision took place quite far away from the star.

WISE telescope:

Wide-field Infrared Survey Explorer (WISE) is a NASA infrared astronomy space telescope in the Explorers Program. It was launched in December 2009 and placed in hibernation mode in February 2011, before being re-activated in 2013 and renamed the Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE).
WISE discovered thousands of minor planets and numerous star clusters. Its observations also supported the discovery of the first Y-type brown dwarf and Earth trojan asteroid.
After its solid hydrogen coolant depleted, a four-month mission extension called NEOWISE was conducted to search for near-Earth objects (NEO) such as comets and asteroids using its remaining capability.

Source: DownToEarth