A young, Sun-like star in the southern constellation “Aft Deck of the Ship” suddenly lost its brightness in December 2021. A research team led by Matthew Kenworthy of Leiden University compiled observations from several years ago and published the results in the journal “nature”. In it, they concluded that a collision a few years ago between two super-Earths – large exoplanets made of rock and ice – in the star’s orbit may have been responsible for the changes in brightness.
The dimming of the star, which is only 300 million years old and is about 1,800 light-years from Earth, was recorded in an automatic scan of the sky by the Automated All-Sky Supernova Survey (ASAS-SN). Such changes in brightness are rare, but do not necessarily indicate unusual events. The most famous is probably the giant star Betelgeuse, which turned out to be a cloud of gas and dust ejected by the red giant. However, the star identified by ASAS-SN, dubbed ASASSN-21qj, was intended to surprise the research team.
According to Keorthy, after the star’s dimming became public, an astronomer pointed out on social media that ASASSN-21qj had been noticeable before, but not in visible light. About 2.5 years ago, it suddenly became so bright at infrared wavelengths that the radiation could no longer be explained by the star alone. Instead, the data suggested another source with a temperature of about 1,000 K and about 4% of the star’s luminosity. Both events – the sudden flash of infrared light and the dimming in visible wavelengths after a few years – are rare on their own, but together and at such a short distance between them, they are more than unusual.
The researchers explored several possible explanations, some of which took a dust cloud approach, as in the case of Betelgeuse. The data, obtained from archives of various telescopes and supplemented by new observations in several wavelength ranges, spans several years. Ultimately, this can only be explained by one of the scenarios examined: the collision of two super-Earths in the still-young disk around the star.
According to the team, the glow at infrared wavelengths is nothing more than the afterglow of the collision that occurred at a distance of 300 million to 2.4 billion kilometers from the star. This roughly corresponds to the region between the asteroid belt and the planet Uranus in our solar system. It left behind a hot object that slowly cooled and was surrounded by a cloud of debris. This will become increasingly distributed throughout the orbit in the coming years and will dim the star. Parts of the debris cloud could coalesce to form moons orbiting the newly formed planet. This is similar to the story of the formation of our Earth’s moon, and underscores how important monitoring such events is for research.
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