Exploring Giant Planet Collisions: Witnessing ASASSN-21qj’s Collision Event and Beyond

2023-10-21 11:54:09
It is thought that events in which another giant celestial body collides with a planet occur frequently immediately after the planet’s birth. However, there has never been any direct observation of this. A research team led by Matthew Kenworthy of Leiden University observed long-term changes in the brightness of the star ASASSN-21qj and claimed that a collision between planets occurred around ASASSN-21qj. If this claim is correct, it is thought that observations have captured the collision of two planets that are several times to several tens of times heavier than Earth.[▲ Figure 1: Artist’s impression of ASASSN-21qj, where two giant ice planets collided (Credit: Mark Garlick)]■Do planets frequently have giant collisions? At the site where a new star is born, a “protoplanetary disk” made of dust and gas exists around the star. It is believed that planets are formed by dust gathering inside protoplanetary disks. It is thought that the dust disk will eventually disappear little by little due to radiation from the star. At this stage, the orbits of the planets change, and it is thought that they may sometimes collide with each other. For example, in the case of Earth, it is thought that a celestial body about the size of Mars collided with it shortly after its birth. It is thought that the debris created at this time eventually came together to form the moon, and this is called the “giant impact hypothesis.” Similar large-scale collisions are not uncommon, and are thought to have occurred on other celestial bodies in the solar system, such as the formation of Pluto’s moon Charon and the overturning of Uranus. However, due to the large collisions occurring in ancient times, little evidence remains, and so far they remain only hypotheses. When we observe stars other than the Sun, we sometimes find planetary systems that have just formed. Observing other planetary systems can be said to be similar to using a time machine to see the solar system of the past, so the state of young planetary systems is often an interesting subject for observation. If there is a violent phenomenon such as a collision between planets, it would be possible to observe changes in the dust caused by the collision using an infrared telescope. For example, NASA’s (National Aeronautics and Space Administration) infrared telescope “Spitzer” has observed significant dust changes at NGC 2354–ID8, HD 166191, and V488 Perseus. However, it was not conclusive whether these observations were due to collisions between planets. ■Capturing collisions between giant planets with “ASASSN-21qj”! Kenworthy and his team focused on the star 2MASS J08152329-3859234, which is located approximately 1,800 light-years from Earth in the direction of the constellation Pupil, after seeing a post on social media about this star. The brightness of this star has dimmed for about 500 days since December 2021, but for about 1000 days before that it was twice as bright in infrared light. This change in brightness was detected by the All Sky Automated Survey for SuperNovae (ASASSN), which searches for supernovae, so 2MASS J08152329-3859234 was renamed “ASASSN-21qj” and published in the paper. However, this name has been adopted. When we analyzed changes in the brightness of ASASSN-21qj using past observation data and additional observation data conducted after it went dark, we discovered something unexpected. First, the reason why it remained dark for about 500 days is thought to be caused by a huge dust cloud blocking the star’s light. However, the reason for the approximately 1000-day bright period before darkening was not immediately clear. However, analysis of observational data shows that the temperature is around 1000K (about 700℃) and the amount of light accounts for a considerable proportion (about 4%) of the total radiation of the star, so it is a fairly high-energy phenomenon. It has gradually become clear that.[▲ Figure 2: This research has determined that ASASSN-21qj is likely to have experienced a scenario in which two giant ice planets collided, generating a large amount of heat and dust clouds (Credit: Kenworthy , et al.) Kenworthy and his colleagues conducted an analysis that also included simulation results, and concluded that the approximately 1,000 days of infrared radiation was derived from the enormous amount of heat generated by the collision between the planets. The observational results are best explained by the existence of two ice giant planets (planet like Uranus and Neptune) that are several to tens of times the size of Earth and are 2 to 16 astronomical units (300 million to 2.4 billion km) from the star. This is a scenario in which there is a collision. Not only do collisions generate enormous amounts of heat, but the resulting dust clouds are stretched out by their orbital motion, making them well suited for both the approximately 1,000 days of brightness and the approximately 500 days of darkness that follow. I can explain. Especially during the dark period of about 500 days, the brightness changes are complex, which can be explained as a result of the fragmentation of the dust cloud by the orbital motion. ■Observations on other stars also on the lookout If the scenario concluded by Kenworthy and his colleagues is correct, ASASSN-21qj would mean that two giant icy planets collided 300 million years after their birth. This is consistent with previous predictions that the dust in the protoplanetary disk disappears due to radiation from the star, making it easier for planets to collide with each other. ASASSN-21qj may be the first detailed observation of an event predicted by planet formation theory, in which planets collide. The fate of the planetary debris from the ASASSN-21qj collision is not well understood, but perhaps some of the dust will reassemble into a small planet or a satellite orbiting it. Progress at this stage is quite slow, so it seems impossible to continue observing it forever. However, if we observe similar phenomena in other stars, we may be able to see a snapshot of another stage. Source text/Riri Aya
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