Twinkle, twinkle little star, the reason this star is dimming is →

Stars that brighten and dim

In 2021, Matthew Kenworthy, an astronomer at Leiden University in the Netherlands, was searching for rings around exoplanets. He searched for stars that flickered or dimmed in unusual ways in data collected by numerous astronomical telescopes. In data from the ASASN-SN survey mission, which monitors exploding stars in the sky, he captured a Sun-like star that flickered repeatedly in visible light.

The star, named ASASSN-21qj, about 1,800 light-years from Earth, dimmed and then returned to its original brightness over a period of several months. Such dimming is not uncommon and is usually attributed to material passing between the star and Earth.

But after Kenworthy posted about ASASSN-21qj on a social networking site, it caught the attention of amateur astronomer Arttu Sainio, who noted that about 900 days before the observed dimming of ASASSN-21qj's visible light, the star's infrared emission had increased by about 4%.

This information completely changed the story. They asked: Are the two observations related? If so, what exactly happened to ASASSN-21qj?

Now, in a new study published in the journal Nature, Kenworthy and his colleagues have a possible answer: They suggest that both sets of observations could be explained by a giant collision between two planets.

This image shows a giant, glowing planetary body created by a planetary collision. After the collision, some of the icy and rocky debris was ejected and then passed between Earth and the host star in the background of the image. (Image: Mark Garlick)

After the huge collision

Giant collisions are thought to be common in the final stages of planet formation, determining the final size, composition and thermal state of the planets, and shaping the orbits of objects in these planetary systems. In our own solar system, the strange tilt of Uranus, the high density of Mercury, and the existence of the Moon are all thought to have been caused by giant collisions. However, until now, astronomers have had little direct evidence that these giant collisions are occurring in the Milky Way.

For a giant collision to explain the observations of ASASSN-21qj, the collision would have released more energy than the star itself in the first few hours after the collision. The material created by the collision would have been superheated, either melting, vaporizing, or both.

In addition, such a collision could also create hot, glowing material hundreds of times more massive than the original planet. Although the Wide-field Infrared Survey Explorer (WISE), which captured the infrared brightening of ASASSN-21qj, did not observe this event, it may be because WISE only observes ASASSN-21qj every 300 days or so and therefore missed the initial flashes of light produced by the collision.

However, it takes a long time for the expanded planetary body created by such a collision to cool and shrink to a size that is recognizable as a new planet. Initially, when such a "post-collision object" reaches its maximum size, its light may still be a few percent of the light emitted by the star. Such an object may produce the infrared brightening phenomenon (as observed).

At the same time, such a collision will also eject a large amount of debris into a series of different orbits around the star. A small part of this debris will evaporate under the impact of the collision and then condense to form a cloud composed of tiny ice and rock crystals. Over time, some of the material in these clumpy clouds will pass between ASASSN-21qj and the Earth, blocking a small part of the visible light from the star, causing the star to dim in observations.

Ice giant planets

If the above explanation proves correct, then studying this star system could provide an excellent opportunity to observe the birth of new worlds in real time and open a new window into a key mechanism of planet formation.

Even from the limited observations available so far, astronomers have learned some very interesting information. First, the researchers believe that to release the observed energy, the "post-collision body" would have to be several hundred times the size of Earth. To form such a large body, each of the colliding planets would have to be several times the mass of Earth, like the "ice giants" Uranus and Neptune.

These two planets may be similar in size to Neptune. (Photo/NASA/JPL)

Second, the researchers estimate the temperature of the "post-collision object" to be about 700°C. For such a temperature, the colliding objects could not have been made entirely of rock and metal; at least the outer region of one of the planets must have contained elements with lower boiling points, such as those in water.

Combining these two points, astronomers believe that this should be a collision between two ice-rich, Neptune-like planets.

The time delay between the brightening of infrared light and the dimming of visible light means that the collision occurred far away from the star. Such a system with icy giant planets far away from the star is more similar to our own solar system than the many tightly packed planetary systems astronomers often observe around other stars.

What's most exciting is that astronomers can continue to observe the evolution of this system over the next few decades, watch the birth of a new planet in real time, and test their conclusions. In the future, they will use powerful instruments such as the Webb Space Telescope (JWST) to determine the size and composition of particles in the debris cloud, the chemical composition of the upper layers of the "post-collision body", and track how these hot debris will cool down. In the process, it may even be possible to observe the emergence of new satellites. These observations can provide astronomers with theoretical information to help them understand how giant collisions shape planetary systems.

#Creative Team:

Compiled by: Fujihokuto

Layout: Wenwen

#Reference source:

https://theconversation.com/the-afterglow-of-an-explosive-collision-between-giant-planets-may-have-been-detected-in-a-far-off-star-system-215466

https://www.bristol.ac.uk/news/2023/october/exoplanet-collision.html

https://www.sciencenews.org/article/exoplanet-collision-first-afterglow-infrared-light

#Image source:

Cover photo & first photo: Mark Garlick