Fomalhaut b was one of the first planets reported orbiting another star, after being directly imaged using the Hubble Space Telescope. A decade later, its disappearance has cast doubt on its existence, and two scientists argue Hubble actually picked up something much rarer – the aftermath of a catastrophic collision between two objects in the gray zone between asteroids and comets.
Today, databases of exoplanets (those orbiting other stars) have thousands of entries, but a little over a decade ago, the numbers were sparse, known from tiny wobbles their gravity induced in the star they circled.
Fomalhaut b was something different, the first case of an exoplanet detected directly from light in the visible part of the spectrum, or so we thought when it was announced in 2008. Fomalhaut is much hotter than the Sun and far too young to support advanced life. Nevertheless, the discovery was exciting both because it suggested a new way to find exoplanets was possible and because Fomalhaut is among the brightest stars in the sky. Anyone not living a long way north of the equator could walk outside, even in an area with quite polluted skies, and say, “There is a planet around that star.” It was considered so significant, it was one of the few exoplanets given an official name, Dagon, after a Middle-Eastern god.
Except, Dr Andras Gaspar of the University of Arizona says there isn't a planet. Doubts arose as astronomers struggled to find similar objects around other stars. There just don't seem to be other examples of exoplanets this bright at wavelengths humans can see. Infrared telescopes can't find Fomalhaut b, despite operating at wavelengths at which planets are usually more visible, and it also doesn't appear to be gravitationally affecting the system's mighty dust ring.
In Proceedings of the National Academy of Sciences, Gaspar and Professor George Rieke report that when Hubble photographed Fomalhaut again in 2014, there was no sign of a planetary companion. Intervening images show the object fading but also growing larger.
"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," Gaspar said in a statement.
Instead, the authors think two super-comets around 200 kilometers (125 miles) across collided shortly before Hubble took its first images, producing a cloud of debris lit up by the bright light from the young star. (An object half this size hitting a much larger one is also possible). We know when our Sun was a similar age, the Solar System was a dangerous place, with impacts that would have looked similar to this one. Nevertheless, the authors expect events like this to happen once every 200,000 years in a system of this age, making catching the aftermath a remarkable piece of luck.
"The Fomalhaut star system is the ultimate test lab for all of our ideas about how exoplanets and star systems evolve," Rieke said. At just 25 light-years away, it offers us one of our best opportunities to observe a planetary system in formation.
The James Webb Space Telescope, if it finally launches successfully, has Fomalhaut on its priority observation list. With much greater power than Hubble, it should be able to check if Gaspar and Rieke's theory and models are correct.