It was considered to be one of the first exoplanets that could be imaged directly – but researchers have reported that “Dagon” never really existed. According to their results, the alleged celestial body in the Fomalhaut star system was a cloud of dust that resulted from the collision of two large asteroids. Due to the continuous expansion, the collision cloud is no longer recognizable, astronomers report.
The eye is on a prominent star on the firmament: Fomalhaut is only 25 light years away from the earth and forms the brightest point of the constellation Southern Fish. In 2004 and 2006 this system was in the sights of the Hubble Space Telescope – planetary hunters were looking for celestial bodies in orbit around the star. Based on the data, astronomers then announced the discovery of exoplanet Fomalhaut b in 2008, which was later named “Dagon”. The special thing about the Jupiter-sized celestial body was its unusual visibility: Until then, exoplanets were usually only found using indirect detection methods. Dagon, on the other hand, shone so brightly in the visible wave range that Hubble could directly image the supposed exoplanet as a wandering point.
Puzzling from the start
But this brightness alone puzzled astronomers from the start. Because normally, exoplanets do not reflect enough light from their central star to be directly captured from Earth. In contrast to visible light, Fomalhaut b did not provide any recognizable infrared heat signature. This was also surprising since a young planet should be warm enough to emit infrared radiation. Another mystery was the apparently low gravitational effect of the suspected planet: its orbit crossed the debris ring around the star without causing characteristic disturbances.
As Andras Gaspar and George Rieke from the University of Arizona in Tucson report, these puzzles have now literally been solved. The object Fomalhaut b has now disappeared and the observation data show that it faded continuously. “Our analyzes, which include all available Hubble archive data on Fomalhaut, now suggest that the planet-sized object was never an exoplanet,” Gaspar sums up. The two astronomers come to the conclusion that it was an expanding cloud of very fine dust particles, which was created by the collision of two celestial bodies.
A cloud of debris from a cosmic crash
Gaspar and Rieke created model simulations for this scenario. They show that the collision between two celestial bodies with a diameter of around 200 kilometers could have caused the phenomenon. Corresponding simulations match the observed brightness profile, the orbital movement and the spatial extent of the structure in the sights of Hubble. The telescope probably just missed the collision: The results indicate that the two planetesimals crashed into each other in 2004 and created the cloud of dust, which then expanded and gradually became invisible. In the meantime, the cloud has probably reached a diameter of 320 million kilometers, the researchers report. “Such collisions are extremely rare, so it’s a stroke of luck that we actually saw evidence of such a collision,” says Gaspar.
Together with other astronomers, Gaspar and Rieke now want to keep their sights on the Fomalhaut system. They hope for an even sharper look from NASA’s new James Webb space telescope, which is scheduled to go into operation in 2021. The team plans to map the inner regions of the system directly in order to gain insight into the architecture of the asteroid belt of a distant system for the first time. “The Fomalhaut star system is the ultimate test laboratory for all our ideas about how exoplanets and star systems develop,” says Rieke. Perhaps the researchers will also discover a real planet in orbit around the bright star in our cosmic neighborhood.
Source: University of Arizona, technical article: PNAS, doi: 10.1073 / pnas.1912506117