Planetary systems can be violent. In our own solar system, for example, the Moon probably was born from the debris left over when a planet as big as Mars rammed into Earth billions of years ago. And the giant planet Uranus probably was knocked over on its side by a similar collision about the same time.

A more recent example comes from ASASSN-21qj — a star system that’s about 1800 light-years away, in the constellation Puppis.

In December of 2021, astronomers noticed that a Sun-like star there suddenly got a lot fainter. They kept an eye on the system, and also looked through records of its appearance in the past. They found that about two and a half years before it faded, the system grew a lot brighter, especially at infrared wavelengths.

A team recently reported a possible cause for all the changes: a massive collision between two planets. Both worlds probably were several times the mass of Earth. The impact made the system look brighter. And it produced a super-heated cloud of debris — gas, dust, and big boulders. This cloud then passed in front of the star, making the system look fainter.

In time, much of this debris could come together — making a new giant planet.

ASASSN-21qj is quite low in the south as night falls. It’s too faint to see with the eye alone. But it’s close to the brightest star of Puppis, so at least you can spot the location of this violent star system.

Script by Damond Benningfield