Direct images of Fomalhaut b or Dagon reveal that what we thought was an exoplanet is actually a cloud of scattered dust that was produced by a catastrophic collision between two large planetesimals in an extrasolar planetary system; according to a team of astronomers from the Steward Observatory of the University of Arizona (USA) in the journal PNAS.
In case you don't remember, Fomalhaut b was one of the first planets on display that orbited another star, after being photographed directly with the Hubble Space Telescope. The discovery at that time was so significant that it was one of the few exoplanets that received an official name, Dagon, after a Middle Eastern god.
Its disappearance in 2014, according to Hubble's images and the fact that no similar objects were found around other stars, cast doubt on its existence. It wasn't that the orbit had changed unexpectedly but more so that it completely disappeared. The new study points to the consequences of a catastrophic collision between two objects in the grey zone between asteroids and comets as an explanation.
Thus, that bright spot visible in those early Hubble images was the aftermath of a collision between two asteroid-sized planetesimals that was briefly visible and not the first case of an exoplanet detected directly from the light in the visible part of the spectrum, which was what we thought when it was announced in 2008.
"These collisions are extremely rare, so it's a big problem that we can actually see one," said University of Arizona astronomer András Gáspár. "We think we were in the right place at the right time to have witnessed such an unlikely event with the Hubble Space Telescope".
The star Fomalhaut is located only 25 light-years away from Earth in the constellation of Pisces Austrinus. It is twice as big as the Sun and 20 times brighter and is surrounded by a ring of dust and debris.
"The Fomalhaut system is the ultimate testing laboratory for all our ideas about how exoplanets and star systems evolve," says George Rieke, co-author of the study.
"Our study, which analyzed all available Hubble archive data on Fomalhaut b, including the most recent images taken by Hubble, revealed several features that together paint a picture that the planet-sized object could never have existed in the first place," the authors explain.
Thus, the authors believe that two super comets about 200 kilometres in diameter collided shortly before Hubble took its first images, producing a cloud of debris illuminated by the young star's bright light.
The James Webb Space Telescope, if finally launched successfully - as its launch has been delayed again to 2021 - has Fomalhaut on its priority observation list. With a much greater power than Hubble (which is already unable to pick up particles so small that they must remain), it should be able to verify whether these astronomers' theory and models are correct.
Referencee: 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