Astronomers have solved the riddle of a disappeared exoplanet— finding in its place a massive dust cloud from a cosmic collision.
A team of astronomers from the University of Arizona have discovered that an exoplanet spotted in a distant star system by the Hubble Space Telescope, actually never existed. Fomalhaut b —located in the Fomalhaut system 25 light-years from Earth — is actually an expanding dust cloud leftover from a massive collision between two planetesimals — icy bodies that often act as the seed for planets.
The team made their discovery whilst reexamining data collected by the Hubble Space Telescope (HST) between 2004–2012. In addition to these four data sets which had been thoroughly investigated in the past, the team had access to two data sets from 2013 and 2014 which had not previously been published.
It was in these two data sets that the astronomers found something fishy about Fomalhaut b which was announced as an exoplanet in 2008, based on data collected in 2004 and 2006. The planet had seemingly disappeared.
The astronomer’s results are published in the journal Proceedings of the National Academy of Sciences.
“One of the programs, that I am leading, will be observing the massive debris disk system around Fomalhaut using the Mid-Infrared Instrument (MIRI),” Explains Andras Gáspár, a member of James Webb Space Telescope NIRCam and MIRI Science Teams at the University of Arizona, and first author on the paper. “ While doing my preparations, I downloaded all archival HST data on the system and reduced them to see if there is anything interesting that someone may not have noticed before.
“I decided to check on Fomalhaut b, and to my surprise, it was not present on the latest images. So, I went through all the data and started to analyze it and noticed a pattern: it was fading.”
This stroke of pure luck inspired Gáspár and his colleague George Rieke, a Regents Professor of Astronomy at Steward Observatory, to look at the object in greater detail, an opportunity that the researcher could not pass up given his work primarily concerns modelling collisions and dust production.
The astronomer explains that the team found three unique observable aspects of the Fomalhaut b object in the data acquired between 2004 and 2014. Firstly, the object is fading, whilst simultaneously becoming large enough for the HST to spatially resolve it. Finally, its trajectory was an escape path more consistent with an object being ejected from a system than a planet on an orbit.
“All of these observables point towards a singular model, which is that it is a dust cloud produced in a massive collision between two large bodies, expanding and being removed from the system due to stellar radiative forces,” Gáspár continues. “I must stress here that we were by no means the first ones to suggest that the object is, in fact, a large dust cloud. But, we are the first ones to show this to be a viable model.”
“The Fomalhaut star system is the ultimate test lab for all of our ideas about how exoplanets and star systems evolve,” adds Rieke. “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.”
An Unusual Exoplanet
Even before the team developed its successful expanding dust cloud model astronomers were aware that Fomalhaut b displayed some unusual traits for an exoplanet. The planet was bright in optical wavelengths but invisible in infrared. Initial speculation suggested that this could be because the exoplanet was surrounded by a ring of dust or dust, thus explaining how such a small planet could be shown to be reflecting visible light.
“That was a secure detection of something — there was no doubt. Planets don’t shine on their own at these wavelengths,” Gáspár says, adding that the real thorn in the side of the theory that Fomalhaut b is an exoplanet is the fact it was not detected at infrared wavelengths. “A planet of this size and relatively young at 400 million years old should be emitting thermally on its own, due to its internal heat,” says Gáspár. “So the worrying part had to do with whether it was a real planet, not with its existence. It’s certainly not an exoplanet.”
Another smoking gun that led to reidentifying Fomalhaut b as a slowly expanding cloud of dust is the fact that the team discovered that it had disappeared in the 2014 data set after a period of slowly fading.
It may seem odd that an expanding cloud of dust could be mistaken for a planet, but Gáspár believes that the misidentification could have happened quite easily as a result of bad timing.
“The collision happened not much before the first set of data were taken, so the object was still more or less point-like. The fragments of the collision continue on a trajectory that is consistent with an orbital path,” the astronomer explains. “In the beginning, even the smallest particles approach this trajectory.”
Over time, these small particles are ejected — thus diverging from this orbit-like trajectory — and this is what the team spotted in the 2013/14 datasets. All this means that in the beginning, the object did look like a planet detected at optical wavelengths. “The announcement in 2008 was not a mistake, but the best available analysis given the available data,” Gáspár is keen to point out. “Science does change its view when new evidence is presented, as in this case.”
Categorising Fomalhaut b as an expanding dust cloud rather than an exoplanet is far from the end of the story for this object, however. There are still many questions to be asked about the objects and the collision that proceeded it, and for Gáspár and his team, this is where the excitement begins.
The Importance of Dust Clouds
The team estimate that the observed dust cloud is the result of a collision between two bodies each around 100 km in radius. Such collisions produce fragments of all sizes, and whilst most of the mass still resides in the few largest fragments, the number of dust particles produced will be many orders of magnitude higher than that of the larger fragments. Thus, Gáspár explains, whilst the total mass in the dust is negligible compared to that in the larger fragments, they do provide a much larger total surface area.
“Imagine someone smoking a cigarette in a room with a light beam going across it. You will see the smoke easily in the light beam, as it scatters the Sun’s light, even though the mass of the smoke is rather small,” says Gáspár. “The question is, how can we extrapolate the ‘smoke’ to the sizes of the colliding bodies?”
This is where the team’s collisional modelling understanding of outcomes of collisions, which are based mostly on laboratory measurements, comes into play.
As Fomalhaut b is currently sitting in a vast ring of icy debris surrounding its parent star — Fomalhaut — the bodies that collided to create it were very likely comprised of ice and dust. This means they are similar to the bodies found in the Kupier Belt at the outer edge of our own solar system.
The team’s findings have wider implications both for the Fomalhaut system and for the evolution of planetary systems in general. “For us to observe this low likelihood event in the Fomalhaut system means that there may be other dynamical events at play in the system, such as a giant planet migrating, that would place planetesimals on orbits with higher chances of collisions,” Gaspar explains.
For Gáspár, the next steps are to examine the system with the James Webb Space Telescope (JWST) after it begins operations in 2021. “The Fomalhaut system already has a massive debris disk that we know of and we will be observing it with the JWST in Cycle 1 both with NIRCam and MIRI,” the astronomer explains. “We may end up finding a bona fide planet in the system that can explain both the increased dynamical activity and its eccentric debris ring.
“For planetary system evolution in general, it is always exciting to witness the large scale collisions we know statistically have to happen to produce the debris discs we observe.”
Special thanks to Andras Gáspár.
Gáspár. A, Rieke. G. H, “New HST data and modelling reveal a massive planetesimal collision around Fomalhaut,” Proceedings of the National Academy of Sciences, 
Rob is freelance science journalist from the UK, specialising in physics, astronomy, cosmology, quantum mechanics and obscure comic books.
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