A composite image of the dust belt around Fomalhaut (the central star), the brightest star in the constellation Piscis Austrinus. The inset shows the dust cloud cs1 imaged in 2012 and the dust cloud cs2 imaged in 2023./Courtesy of NASA, ESA, Paul Kalas (UC Berkeley)

Scientists believe the early days of our solar system were, literally, a period of violent chaos. Asteroids, comets, or protoplanets collided from all directions, dust clouds formed and then clumped together, later evolving into planets. NASA's Hubble Space Telescope has, for the first time, observed bumper car-like collisions of small bodies around a star outside the solar system that repeatedly produce dust clouds.

A team led by Paul Kalas, a professor in the Department of Astronomy at the University of California, Berkeley (UC Berkeley), said, "With the Hubble Space Telescope, we observed the sudden appearance of a bright spot caused by a planetesimal collision around Fomalhaut, a star relatively close to Earth," in a paper released on the 19th in the journal Science.

Planetesimals are small bodies in the early stages of a stellar system that are thought to have grown into planets through collisions. Asteroids and comets that orbit the sun on long elliptical paths can also be seen as remnants of planetesimals. If so, the planetesimal collisions observed this time are as rare as what happened in the early solar system. In other words, it is like riding a time machine to see the dawn of our solar system.

An artist's rendering showing the sequence leading to the formation of the dust cloud cs2 around the star Fomalhaut. Fomalhaut appears in the upper left corner, and the two white dots in the lower right corner are two massive bodies orbiting Fomalhaut (1). The two bodies approach each other (2) and collide (3). The resulting dust cloud cs2 appears, and starlight pushes the dust particles outward (4)./Courtesy of NASA, ESA, STScI, Ralf Crawford (STScI)

◇ A time machine showing the early solar system

Fomalhaut is a yellowish star seen low in the southern autumn night sky and is the brightest in Piscis Austrinus. It is 25 light-years from Earth (one light-year is the distance light travels in a year, about 9.46 trillion km). In 2008, Kalas's team released a paper in Science announcing the discovery with Hubble of an exoplanet orbiting Fomalhaut. The paper analyzed observations from 2004 and 2005. The planet was named "Fomalhaut b."

But as observations continued, the planet became the subject of debate among scientists. Some said it was a planet larger than Jupiter, while others argued it could be a dust cloud formed by debris from collisions of protoplanets.

To settle the debate, the UC Berkeley team observed Fomalhaut again with Hubble. Kalas said, "It was the same instrument, but in 2023 we could not find Fomalhaut b," adding, "More surprising was the fact that a new Fomalhaut b exists."

Scientists searching for Fomalhaut b found another luminous object in a similar location. The team changed the previously released Fomalhaut b to a neutral name, "cs1 (circumstellar source 1)," and named the object newly found in 2023 cs2.

The team said, "The most compelling explanation for the two objects is that they are dust clouds created by a collision between two planetesimals about 60 km in diameter." David Kipping, a professor in the Department of Astronomy at Columbia University, also said, "These sources are noisy and irregular, so it is still too early to reach a firm conclusion," but added, "However, all the evidence so far neatly fits the broad explanation of collisions between protoplanets."

미 항공우주국(NASA)의 허블 우주망원경이 남쪽물고기자리의 가장 밝은 별인 포말하우트 주변에서 펼쳐지는 희귀하고 격렬한 충돌 현상을 포착했다. 이번 발견은 수십억 년 전 우리 태양계를 형성한 혼돈의 과정을 이해하는 데 도움을 줄 것으로 기대된다./NASA 고다드 우주비행센터

◇ The area around the star sparkles like a Christmas tree

However, capturing two collisions in a short time was unexpected. Kalas said, "According to existing theory, such collisions should occur once every 100,000 years or even more rarely, but we witnessed two in 20 years," adding, "It is astonishing that Fomalhaut is sparkling like a Christmas tree decorated with fairy lights." The team explained that if you fast-forward a movie covering the past 3,000 years, the region around Fomalhaut would have kept flashing with light from planetesimal collisions.

If these observations are correct, collisions between protoplanets around relatively young stars like Fomalhaut may occur more frequently than previously thought. The team said it will track Fomalhaut cs2 using Hubble and the more powerful James Webb Space Telescope (JWST) simultaneously.

Kalas said, "We no longer need to rely only on theory to understand the violent collisions of the early solar system; we can witness them directly," adding, "Further observations will contribute not only to understanding young planetary systems in general but also to identifying the early appearance and characteristics of our solar system."

References

Science (2025), DOI: https://doi.org/10.1126/science.adu6266

Science (2008), DOI: https://doi.org/10.1126/science.1166609

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