Like a game of cosmic bumper cars, scientists believe that the early days of our solar system were a time of violent upheaval, when planetesimals, asteroids and comets collided with each other and showered the Earth, Moon and other inner planets with debris. Now, in a historic milestone, NASA Hubble Space Telescope directly imaged similar catastrophic collisions in a nearby planetary system around another star, Fomalhaut.
“This is certainly the first time I've seen a point of light appear out of nowhere in an exoplanet system,” said principal investigator Paul Kalas of the University of California, Berkeley. “It's not in all of our previous Hubble images, meaning we've just witnessed a violent collision between two massive objects and a huge cloud of debris unlike anything in our solar system today. Marvelous!”
Just 25 light-years from Earth lies Fomalhaut, one of the brightest stars in the night sky. Located in the constellation Pisces Austrian, also known as Pisces Austrine, it is more massive and brighter than the Sun and is surrounded by several belts of dusty debris.
This composite image from the Hubble Space Telescope shows the ring of debris and dust clouds cs1 and cs2 around the star Fomalhaut. Fomalhaut itself is camouflaged to allow fainter details to be seen. Its location is marked with a white star.
Image: NASA, ESA, Paul Kalas (UC Berkeley); Image Processing: Joseph DePasquale (STScI)
In 2008, scientists used Hubble to detect a planet candidate around Fomalhaut, making it the first star system to have a possible planet detected using visible light. This object, named Fomalhaut b, is now looks like a cloud of dust masquerading as a planet – the result of a collision of planetesimals. While searching for Fomalhaut b in recent Hubble observations, scientists were surprised to find a second point of light in a similar location around the star. They call this object “circumstellar source 2” or “cs2”, and the first object is now known as “cs1”.
Why astronomers see both of these debris clouds so physically close to each other remains a mystery. If asteroid and planetesimal collisions were random, CS1 and CS2 should randomly appear in unrelated locations. However, they are curiously located next to each other in the inner part of the outer disk of Fomalhaut debris.
Another mystery is why scientists witnessed these two events in such a short period of time. “Previous theory suggested that there should be one collision every 100,000 years or longer. Here, in 20 years, we saw two,” Kalas explained. “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 would see during that time. The planetary system of Fomalhaut would sparkle with these collisions.”
Collisions are fundamental to the evolution of planetary systems, but they are rare and difficult to study.
This artist's concept shows the sequence of events that led to the creation of the cs2 dust cloud around the star Fomalhaut. In panel 1, the star Fomalhaut appears in the upper left corner. The two white dots located in the lower right corner represent two massive objects orbiting Fomalhaut. In panel 2, objects move closer to each other. Panel 3 shows the violent collision of these two objects. In panel 4, the resulting dust cloud cs2 becomes visible, and the light from the star pushes the dust particles away from the star.
Works: NASA, ESA, STScI, Ralph Crawford (STScI)
“The exciting aspect of this observation is that it allows researchers to estimate both the size of the colliding bodies and the number of them in the disk—information that would be virtually impossible to obtain any other way,” said co-author Mark Wyatt of the University of Cambridge in England. “We estimate that the planetesimals that were destroyed to create CS1 and CS2 are only 37 miles, or 60 kilometers, in diameter, and we conclude that there are 300 million such objects orbiting the Fomalhaut system.”
“This system provides a natural laboratory for studying how planetesimals behave in collisions, which in turn tells us what they are made of and how they formed,” Wyatt explained.
The transient nature of Fomalhaut cs1 and cs2 poses challenges for future space missions aimed at directly imaging exoplanets. Such telescopes may mistake dust clouds like CS1 and CS2 for real planets.
“Fomalhaut CS2 looks exactly like an extrasolar planet reflecting starlight,” Kalas said. “By studying CS1, we learned that a large dust cloud can masquerade as a planet for many years. This is a warning for future missions that aim to detect extrasolar planets in reflected light.”
Kalas and his team were given Hubble time to monitor CS2 for the next three years. They want to see how it develops: does it dim or brighten? Being closer to the dust belt than CS1, the expanding CS2 cloud is more likely to start colliding with other material in the belt. This can cause dust to suddenly be released into the system, which can cause the entire surrounding area to become brighter.
“We will monitor CS2 for any changes in its shape, brightness and orbit over time,” Kalas said. “It is possible that CS2 will begin to become more oval or comet-like in shape as dust grains are pushed outward by the pressure of starlight.”
The team will also use the NIRCam (near-infrared camera) instrument on the NASA satellite. James Webb Space Telescope watch cs2. Webb's NIRCam camera is capable of providing color information that can reveal the size of dust grains in a cloud and their composition. It can even detect whether a cloud contains water ice.
Hubble and Webb are the only observatories capable of taking such images. While Hubble sees primarily in visible wavelengths, Webb could see CS2 in infrared wavelengths. These different, complementary wavelengths are needed to provide a broad multispectral survey and a more complete picture of the enigmatic Fomalhaut system and its rapid evolution.
This research appears in the December 18 issue of Science.
The Hubble Space Telescope has been in operation for more than three decades and continues to make groundbreaking discoveries that shape our fundamental understanding of the universe. Hubble is an international cooperation project between NASA and ESA (European Space Agency). NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope and mission operations. Lockheed Martin Space, based in Denver, also supports mission operations at Goddard. The Space Telescope Science Institute in Baltimore, managed by the Association of Universities for Astronomical Research, conducts Hubble science operations for NASA.
