Hubble sees asteroids colliding at nearby star for first time
In a historical milestone, catastrophic collisions in a nearby planetary system were witnessed for the first time by astronomers using the NASA/ESA Hubble Space Telescope. As they observed the bright star Fomalhaut, the scientists saw the impact of massive objects around the star. The Fomalhaut system appears to be in a dynamical upheaval, similar to what our solar system experienced in its first few hundred million years after formation.
“This is certainly the first time I’ve ever seen a point of light appear out of nowhere in an exoplanetary system,” said principal investigator Paul Kalas of the University of California, Berkeley. “It’s absent in all of our previous Hubble images, which means that we just witnessed a violent collision between two massive objects and a huge debris cloud unlike anything in our own solar system today. Amazing!"
Just 25 light-years from Earth, Fomalhaut is one of the brightest stars in the night sky. Located in the constellation Piscis Austrinus, also known as the Southern Fish, it is more massive and brighter than the Sun and is encircled by several belts of dusty debris.
In 2008, scientists used Hubble to discover a candidate planet around Fomalhaut, making it the first stellar system with a possible planet found using visible light. That object, called Fomalhaut b, now appears to be a dust cloud masquerading as a planet – the result of colliding planetesimals. While searching for Fomalhaut b in recent Hubble observations, scientists were surprised to find a second point of light at a similar location around the star. They call this object “circumstellar source 2” or “cs2” while the first object is now known as “cs1.”
Tackling mysteries of colliding planetesimals
Why astronomers are seeing both of these debris clouds so physically close to each other is a mystery. If the collisions between asteroids and planetesimals were random, cs1 and cs2 should appear by chance at unrelated locations. Yet, they are positioned intriguingly near each other along the inner portion of Fomalhaut’s outer debris disk.
Another mystery is why scientists have witnessed these two events within such a short timeframe. “Previous theory suggested that there should be one collision every 100,000 years, or longer. Here, in 20 years, we've seen two,” explained Kalas. “If you had a movie of the last 3,000 years, and it was sped up so that every year was a fraction of a second, imagine how many flashes you'd see over that time. Fomalhaut’s planetary system would be sparkling with these collisions.”
Collisions are fundamental to the evolution of planetary systems, but they are rare and difficult to study.
“The exciting aspect of this observation is that it allows researchers to estimate both the size of the colliding bodies and how many of them there are in the disk, information which is almost impossible to get by any other means,” said co-author Mark Wyatt at the University of Cambridge in England. “Our estimates put the planetesimals that were destroyed to create cs1 and cs2 at just 30 kilometres in size, and we infer that there are 300 million such objects orbiting in the Fomalhaut system.”
“The system is a natural laboratory to probe how planetesimals behave when undergoing collisions, which in turn tells us about what they are made of and how they formed,” explained Wyatt.
Cautionary tale
The transient nature of Fomalhaut cs1 and cs2 poses challenges for future space missions aiming to directly image exoplanets. Such telescopes may mistake dust clouds like cs1 and cs2 for actual planets.
“Fomalhaut cs2 looks exactly like an extrasolar planet reflecting starlight,” said Kalas. “What we learned from studying cs1 is that a large dust cloud can masquerade as a planet for many years. This is a cautionary note for future missions that aim to detect extrasolar planets in reflected light."
Looking to the future
Kalas and his team have been granted Hubble time to monitor cs2 over the next three years. They want to see how it evolves -- does it fade, or does it get brighter? Being closer to the dust belt than cs1, the expanding cs2 cloud is more likely to start encountering other material in the belt. This could lead to a sudden avalanche of more dust in the system, which could cause the whole surrounding area to get brighter.
“We will be tracing cs2 for any changes in its shape, brightness, and orbit over time,” said Kalas, “It’s possible that cs2 will start becoming more oval or cometary in shape as the dust grains are pushed outward by the pressure of starlight.”
The team also will use the NIRCam (Near-Infrared Camera) instrument on the NASA/ESA/CSA James Webb Space Telescope to observe cs2. Webb’s NIRCam has the ability to provide color information that can reveal the size of the cloud’s dust grains and their composition. It can even determine if the cloud contains water ice.
Hubble and Webb are the only observatories capable of this kind of imaging. While Hubble primarily sees in visible wavelengths, Webb could view cs2 in the infrared. These different, complementary wavelengths are needed to provide a broad multi-spectral investigation and a more complete picture of the mysterious Fomalhaut system and its rapid evolution.
This research appears today in the December 18 issue of Science.
Fomalhaut cs1 and cs2 (annotated)
Fomalhaut cs1 and cs2 (clean image)
Fomalhaut cs2 (artist’s concept)
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