The glow resulting from a massive collision between two giant planets may have been detected for the first time.
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The debris from the collision could eventually cool and form an entirely new planet. If this observation is confirmed, it will be a wonderful opportunity to observe the birth of a new world in real time and open a window into how planets form.
In December 2021, astronomers observing a natural sun-like star saw it begin to twinkle. For a few months, visible light (the light we can see with our eyes) continued to change. Sometimes it almost disappeared before returning to its previous brightness.
The star, which is about 1,800 light-years from Earth, was given the identification ASASSN-21qj, after the ASASN-SN astronomical survey first observed the dimming of the star.
Seeing dim stars like this is not uncommon. This is generally due to the passage of material between the star and Earth. ASASSN-21qj would have been added to a growing list of similar observations if not for amateur astronomer, Arttu Sainio.
Sainyu explained on social media that two and a half years before the star’s light disappeared, the emission of infrared light from its location increased by about 4%.
Infrared light is most strongly emitted by objects at relatively high temperatures of up to a few hundred degrees Celsius. This raised questions: Were these two observations related, and if so, what the hell was going on around ASASSN-21qj?
Planetary disaster
By publishing our findings in Nature, we suggest that both sets of observations can be explained by a catastrophic collision between two planets.
Giant impacts, as these collisions are known, are thought to be common in the final stages of planet formation. They determine the final sizes, compositions, and thermal states of planets, and shape the orbits of objects in these planetary systems.
In our solar system, giant impacts are thought to be responsible for the strange tilt of Uranus, the high density of Mercury, and the presence of Earth’s moon. However, until now, we have had little direct evidence of ongoing giant impacts in the galaxy.
To explain the observations, the collision must release more energy in the first few hours after the collision than the star would. Materials from the colliding objects could have been superheated and melted, evaporated, or both.
The collision would have created a hot, glowing mass of material hundreds of times larger than the original planets. The infrared glow of ASASSN-21qj was observed by NASA’s WISE space telescope. WISE only looks at the star every 300 days or so and likely misses the initial flash of light from the impact.
However, the expanding planetary body resulting from the collision would take a long time, perhaps millions of years, to cool and contract into something we would recognize as a new planet.
Initially, when this “post-impact object” was at its greatest extent, the light it emitted could have been several percent of the star’s emission. Such an object could produce the infrared glow we saw.
The collision would also have ejected large clouds of debris into a series of different orbits around the star. Some of this debris likely vaporized due to the shock of the impact, later condensing to form clouds of small ice and rock crystals.
Over time, part of this clumped cloud of material passed between ASASSN-21qj and Earth, blocking part of the star’s visible light and producing an irregular dimming.
If our interpretation of events is correct, studying this star system could help us understand the main mechanism of planetary formation. Even with the limited set of observations we have so far, we’ve learned some very interesting things.
First, to release the observed amount of energy, the object’s post-impact size would have to be hundreds of times the size of Earth. To create such a massive body, the mass of each of the colliding planets would have to be several times the mass of Earth — perhaps as large as the “ice giants” Uranus and Neptune.
Second, we estimate the post-impact body temperature to be around 700°C. For the temperature to be so low, the colliding objects could not have been made entirely of rock and metal.
Ice giants
The outer regions of at least one of the planets must contain elements with low boiling temperatures, such as water. So we think we’ve seen a collision between two ice-rich, Neptune-like worlds.
The observed delay between the infrared emission and the detection of debris passing through the star indicates that the collision occurred far from the star, farther than Earth’s distance from the Sun.
Such a system, in which ice giants exist far from the star, is more similar to our solar system than many of the compact planetary systems that astronomers often observe around other stars.
The most exciting aspect of this is that we can continue to monitor the evolution of the system over many decades and test our conclusions. Future observations, using telescopes like NASA’s James Webb Space Telescope, will determine the sizes and compositions of particles in the debris cloud, determine the chemistry of the upper layers of the object after the impact, and track how this hot mass of debris cooled. We may even see new moons appear.
These observations can inform our theories, helping us understand how massive impacts shape planetary systems. So far, the only examples we have are echoes from collisions in our solar system. Now we will be able to watch the birth of a new planet in real time.
Translated by Matthews Lineker from Science Alert
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