What astronomers thought was a planet beyond our solar system has now seemingly vanished from sight. Though this happens in science fiction, such as Superman’s home planet Krypton exploding, astronomers are looking for a plausible explanation.
One interpretation is that, rather than being a full-sized planetary object, which was first photographed in 2004, it could instead be a vast, expanding cloud of dust produced in a collision between two large bodies orbiting the bright nearby star Fomalhaut (PNAS, “New HST data and modeling reveal a massive planetesimal collision around Fomalhaut”). Potential follow-up observations might confirm this extraordinary conclusion.
This diagram simulates what astronomers, studying Hubble Space Telescope observations, taken over several years, consider evidence for the first-ever detection of the aftermath of a titanic planetary collision in another star system. The color-tinted Hubble image on the left is of a vast ring of icy debris encircling the star Fomalhaut, located 25 light-years away. The star is so brilliant that a black occulting disk is used to block out its glare so that the dust ring can be photographed. In 2008, astronomers saw what they thought was the first direct image of a planet orbiting far from the star. However, by 2014, the planet candidate faded below Hubble’s detection. The best interpretation is that the object wasn’t ever a fully formed planet at all, but an expanding cloud of dust from a collision between two minor bodies, each about 125 miles across. The diagram at the right is based on a simulation of the expanding and fading cloud. The cloud, made of very fine dust particles, is currently estimated to be over 200 million miles across. Smashups like this are estimated to happen around Fomalhaut once every 200,000 years. Therefore, Hubble was looking at the right place at the right time to capture this transient event. (Image:: NASA, ESA, and A. Gspr and G. Rieke (University of Arizona) (click on image to enlarge)
“These collisions are exceedingly rare and so this is a big deal that we actually get to see one,” said Andrs Gspr of the University of Arizona, Tucson. “We believe that we were at the right place at the right time to have witnessed such an unlikely event with NASA’s Hubble Space Telescope.”
“The Fomalhaut system is the ultimate test lab for all of our ideas about how exoplanets and star systems evolve,” added George Rieke of the University of Arizona’s Steward Observatory. “We do have evidence of such collisions in other systems, but none of this magnitude has been observed in our solar system. This is a blueprint of how planets destroy each other.”
The object, called Fomalhaut b, was first announced in 2008, based on data taken in 2004 and 2006. It was clearly visible in several years of Hubble observations that revealed it was a moving dot. Until then, evidence for exoplanets had mostly been inferred through indirect detection methods, such as subtle back-and-forth stellar wobbles, and shadows from planets passing in front of their stars.
Unlike other directly imaged exoplanets, however, nagging puzzles arose with Fomalhaut b early on. The object was unusually bright in visible light, but did not have any detectable infrared heat signature. Astronomers conjectured that the added brightness came from a huge shell or ring of dust encircling the planet that may possibly have been collision-related. The orbit of Fomalhaut b also appeared unusual, possibly very eccentric.
“Our study, which analyzed all available archival Hubble data on Fomalhaut revealed several characteristics that together paint a picture that the planet-sized object may never have existed in the first place,” said Gspr.
The team emphasizes that the final nail in the coffin came when their data analysis of Hubble images taken in 2014 showed the object had vanished, to their disbelief. Adding to the mystery, earlier images showed the object to continuously fade over time, they say. “Clearly, Fomalhaut b was doing things a bona fide planet should not be doing,” said Gspr.
The interpretation is that Fomalhaut b is slowly expanding from the smashup that blasted a dissipating dust cloud into space. Taking into account all available data, Gspr and Rieke think the collision occurred not too long prior to the first observations taken in 2004. By now the debris cloud, consisting of dust particles around 1 micron (1/50th the diameter of a human hair), is below Hubble’s detection limit. The dust cloud is estimated to have expanded by now to a size larger than the orbit of Earth around our Sun.
Equally confounding is that the team reports that the object is more likely on an escape path, rather than on an elliptical orbit, as expected for planets. This is based on the researchers adding later observations to the trajectory plots from earlier data. “A recently created massive dust cloud, experiencing considerable radiative forces from the central star Fomalhaut, would be placed on such a trajectory,” said Gspr. “Our model is naturally able to explain all independent observable parameters of the system: its expansion rate, its fading, and its trajectory.”
This video simulates what astronomers, studying Hubble Space Telescope observations, consider evidence for the first-ever detection of the aftermath of a titanic planetary collision in another star system. The color-tinted Hubble image on the left is of a vast ring of icy debris encircling the star Fomalhaut, located 25 light-years away. The animated diagram on the right is a simulation of the expanding and fading cloud, based on Hubble observations taken over a period of several years.
Because Fomalhaut b is presently inside a vast ring of icy debris encircling the star, colliding bodies would likely be a mixture of ice and dust, like the comets that exist in the Kuiper belt on the outer fringe of our solar system. Gspr and Rieke estimate that each of these comet-like bodies measured about 125 miles (200 kilometers) across (roughly half the size of the asteroid Vesta).
According to the authors, their model explains all the observed characteristics of Fomalhaut b. Sophisticated dust dynamical modeling done on a cluster of computers at the University of Arizona shows that such a model is able to fit quantitatively all the observations. According to the author’s calculations, the Fomalhaut system, located about 25 light-years from Earth, may experience one of these events only every 200,000 years.
Gspr and Rieke — along with other members of an extended team — will also be observing the Fomalhaut system with NASA’s upcoming James Webb Space Telescope in its first year of science operations. The team will be directly imaging the inner warm regions of the system, spatially resolving for the first time the elusive asteroid-belt component of an extrasolar planetary system. The team will also search for bona fide planets orbiting Fomalhaut that might be gravitationally sculpting the outer disk. They will also analyze the chemical composition of the disk.
