2MASS J08152329-3859234

2MASS J08152329-3859234, also known as ASASSN-21qj, is a Sun-like main sequence star with a rotating disk of circumstellar dust and gas which are leftovers from its stellar formation around 300 million years ago. The star is located 1,850 light years (567.2 parsecs) from Earth in the constellation of Puppis.

Planetary collision event
In 2021 the All-Sky Automated Survey for Supernovae reported that this star was rapidly fading. The published Astronomer's Telegram asked for follow-up observations. On twitter the astronomers Dr. Matthew Kenworthy and Dr. Eric Mamajek speculated about this object and amateur astronomer Arttu Sainio made his own investigation and discovered a brightening in NEOWISE data. He then joined the discussion on social media. The star brightened 2.5 years before the dimming event. More contributions came from amateur and professional astronomers, such as spectroscopic follow-up by amateur astronomers Hamish Barker, Sean Curry and the amateur Southern Spectroscopic project Observatory Team (2SPOT) members Stéphane Charbonnel, Pascal Le Dû, Olivier Garde, Lionel Mulato and Thomas Petit. Dr. Franz-Josef Hambsch observed this object with his remote observatory ROAD and submitted his observations to AAVSO. Other observations from professional telescope include ATLAS, ALMA, LCOGT and TESS.

In 2023, a scientific paper reported observations consistent with two ice-giant type exoplanets of several to tens of Earth masses having undergone a planetary collision event. The collision occurred at a distance of 2-16 AU (astronomical units) from the star. The infrared brightening is thought to be the result of dust produced by the disruption being heated by the collision, reaching a temperature of 1000 K (727°C; 1340°F) and then the dust slowly cooled off and expanded in size. Together with the newly formed planet, the dust cloud orbited the star and 1000 days later the dust moved in front of the star, causing a dimming event. Because of the dust cloud had now reached a large size, the dimming event would last for 600 days. The newly formed planet did not cause a transit.

A few other planetary collisions were discovered in the past, such as around NGC 2354–ID8, HD 166191 and V844 Persei.