The directly-imaged massive exoplanet Fomalhaut b is actually a dispersing cloud of dust, produced by a catastrophic collision between two large planetesimals in an extrasolar planetary system, according to a duo of astronomers from the Steward Observatory at the University of Arizona.
Hubble data revealed an expanding cloud of dust produced in a collision between two large bodies orbiting Fomalhaut; this is the first time such a catastrophic event around another star has been imaged. Image credit: NASA / ESA / Hubble / M. Kornmesser.
Fomalhaut is an A-type star located just 25 light-years away in the constellation of Piscis Austrinus.
The star is twice as massive as the Sun and 20 times brighter and is surrounded by a ring of dust and debris.
In 2008, Hubble astronomers announced the discovery of a planet called Fomalhaut b, as the first extrasolar planet to be directly detected in visible light.
However, subsequent observations of the planet raised questions about its status: unlike other exoplanets, it is bright in the optical and non-detected in the infrared.
Astronomers proposed that the added brightness came from a huge shell or ring of dust encircling the object that may have been collision-related. Also, the Hubble observations suggested the object might not be following an elliptical orbit, as planets usually do.
“The Fomalhaut system is the ultimate test lab for all of our ideas about how exoplanets and star systems evolve,” said Dr. George Rieke, co-author of the study.
“We do have evidence of such collisions in other systems, but none of this magnitude has ever been observed. This is a blueprint for how planets destroy each other.”
“These collisions are exceedingly rare and so this is a big deal that we actually get to see one. We believe that we were at the right place at the right time to have witnessed such an unlikely event with Hubble,” added Dr. András Gáspár, corresponding author of the study.
“Our study, which analyzed all available archival Hubble data on Fomalhaut b, including the most recent images taken by Hubble, revealed several characteristics that together paint a picture that the planet-sized object may never have existed in the first place.”
Hubble observations of Fomalhaut b’s expanding dust cloud from 2004 to 2013; the cloud was produced in a collision between two large bodies orbiting Fomalhaut. Image credit: NASA / ESA / Hubble / A. Gáspár & G. Rieke, University of Arizona.
Hubble images from 2014 showed Fomalhaut b had vanished. Adding to the mystery, earlier images showed the object to continuously fade over time.
“Clearly, Fomalhaut b was doing things a bona fide planet should not be doing,” Dr. Gáspár said.
The resulting interpretation is that Fomalhaut b is not a planet, but a slowly expanding cloud blasted into space as a result of a collision between two lcomet-like bodies.
The researchers estimate that each of these bodies measured about 200 km (124 miles) across. They suggest that the Fomalhaut system may experience one of these collision events only every 200,000 years.
They also believe the collision occurred not too long prior to the first observations of Fomalhaut b.
By now the debris cloud, consisting of dust particles around 1 micron, 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 object is not on an elliptical orbit, as expected for planets, but on an escape trajectory, or hyperbolic path.
“A recently created massive dust cloud, experiencing considerable radiative forces from the central star Fomalhaut, would be placed on such a trajectory,” Dr. Gáspár said.
“Our model is naturally able to explain all independant observable paramters of the system: its expansion rate, its fading and its trajectory.”
The study was published in the Proceedings of the National Academy of Sciences.
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András Gáspár & George H. Rieke. New HST data and modeling reveal a massive planetesimal collision around Fomalhaut. PNAS, published online April 20, 2020; doi: 10.1073/pnas.1912506117
