Hubble captures collision of two celestial bodies in Fomalhaut system

The Hubble space telescope has presumably detected the collision of two celestial bodies in the system of the star Fomalhaut – and for the second time in two decades. Scientists previously believed that such events occur much less frequently.

Fomalhaut is a star in the constellation Piscis Austrinus, or Southern Fish, and one of the brightest stars in the sky. At only about 25 light-years away, the star is still relatively young at 440 million years old; the sun is about 4.57 billion years old. Fomalhaut belongs to a triple system with two dwarf stars, surrounded by dust rings in which planets form.

In 2004 and 2006, astronomers discovered an object in one of these belts that they believed to be an exoplanet and named Fomalhaut B. In 2023, they wanted to observe the planet again with the Hubble telescope but found that it was not where they expected it to be.

However, they found a point of light in a different location, near the first. Upon comparing the images, it became clear that the two points of light could not have originated from the same source, said Jason Wang of Northwestern University. They renamed Fomalhaut b to Fomalhaut Circumstellar Source 1 (CS1), and the second point of light was designated Fomalhaut CS2.

Collision of Planetesimals

Researchers explain the appearance and disappearance of the points of light as debris clouds formed by the collision of planetesimals, which are building blocks of planets. From the brightness of the points of light CS1 and CS2, they concluded that the colliding objects themselves must have been around 60 kilometers in size – and thus too small to be visible even in the images from the space telescope. The spreading debris clouds, on the other hand, are illuminated by the central star.

"Discovering a new light source in the dust belt around a star surprised us. We didn't expect that," said Wang. "Our hypothesis is that we have observed two collisions of planetesimals – small rocky objects, similar to asteroids – within two decades. Collisions of planetesimals are very rare events, and this is the first time we have seen one outside our solar system."

Researchers were also astonished to have observed two such collisions in approximately 20 years: "According to theory, such a collision should occur once every 100,000 years or even less frequently," said Paul Kalas, an astronomer at the University of California, Berkeley, and lead author of the study published in the journal Science. He believes the two sightings in 20 years could have been a coincidence, or the theoretical models would need to be adjusted.

Researchers plan to observe the Fomalhaut system with the James Webb Space Telescope in the future, hoping for new insights into CS 2 and the nature of the collided planetesimals – and possibly to observe new collisions.

(wpl)