Did A Star Crash Into The Early Solar System?

In 1936, a star in Orion changed. Over the course of a year, the star FU Orionis (also known as FU Ori) progressively became 250 times brighter. The star has remained mysteriously brighter ever since. 

However, an international research team led by Monash University may finally have an answer as to why FU Orionis, and some other young stars, can appear to become brighter over a period of 1 to 2 years.  

FU Orion is only one of several low-mass, presequence stars which have similarly become and stayed brighter than when they were first observed. This class of event is referred to as an FU Orionis event. The main questions surrounding these events are: why do these particular stars become brighter, and why do they remain that way? 

“FU Ori has remained bright ever since,” said Elisabeth Borchert, PhD candidate from Monash School of Physics and Astronomy, and the lead author of the study. “Many explanations have been proposed, but none completely explain the phenomenon.”

Explanations so far have ranged from interactions within the disc of debris and proto-planetary materials surrounding the young star, binary star system interactions, and a mixture of these two theories. The research team chose to explore the proto-planetary disc of stars as a source of FU Orionis events.  

Once a star forms, it is surrounded by a disc of debris. This disc may one day turn into planets, but at the beginning of a star’s life, it is simply a disc of gas and dust. The team found that when another star crashes into the planetary forming disc, also known as a stellar flyby, the star grows brighter.

“We show that another star crashing into the surrounding disc of gas and dust results in 250 times change in brightness in one to two years,” said co-author Associate Professor Christophe Pinte, also from the Monash School of Physics and Astronomy.

The team’s 3D hydrodynamics computer simulations showed that as the disturbing star moves through the disc, it accretes materials and becomes brighter. 

“The surprise of the study was that the small star is the one that becomes bright, which is nice as in FU Ori the low-mass star is the bright one of the pair,” Elisabeth said.

Another interesting find from this study is that this type of interaction would cause rapid heating of the disc and melting of dust, an effect seen in our own solar system.

“A mystery in our solar system is that a lot of the dust found in meteorites seems to have been melted rapidly, which could be explained by a similar disturbance to our solar system during its formation,” said study co-author Professor Daniel Price, also from Monash School of Physics and Astronomy.