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.

This video simulates what astronomers, studying Hubble Space Telescope observations, consider the evidence for the first-ever detection of the aftermath of a titanic planetary collision in another star system. The colour-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. Image credit: NASA, ESA, A. Gáspár and G. Rieke (University of Arizona)

“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.

Top: Composition of the 2010–2014 images of the Fomalhaut system The green circles with crosses highlight the then-current positions of Fomalhaut b, with 3σ astrometric error radii, while the smaller cyan colour circles show the previous positions, to highlight the spatial motion of the source. For the 2014 image, we show the two locations predicted by the two independent trajectory fits. The bright spot “near” the predicted locations is too far to be considered associated with Fomalhaut b. Image credit: Gáspár. A, Rieke. G. H

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.