Saturn-like ‘planet’ may have been a giant dust cloud caused by asteroid collision

The first planet to be discovered outside our solar system was never a planet and may be a giant dust cloud created by the collision of two icy asteroids, a study finds.

Astronomers spotted what was thought to be a Saturn like planet lying 25 light years from Earth in the star system Fomalhaut 12 years ago and called it Fomalhaut b.  

Researchers from the University of Arizona found the visible and infrared images taken by Hubble showing the ‘planet’ were actually of a cosmic collision.

The team studied the images in more detail and found they were showing the aftermath of two 125 mile wide icy comets crashing into each other.

The comets left an expanding cloud of very fine dust particles that were photographed by the Hubble space telescope – just missing the smash.

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Such an event is estimated to happen about once every 200,000 years – and sheds fresh light on the way planets evolve, say the US team. 

Lead author Dr Andras Gaspar, of the University of Arizona’s Steward Observatory, said they are ‘exceedingly rare’ and so this is a major discovery. 

‘We believe we were at the right place at the right time to have witnessed such an unlikely event with NASA’s Hubble Space Telescope,’ Gaspar said.

The collision is thought to have occurred about 11 billion miles away from the star Fomalhaut in the constellation Piscis Austrinus.

Fomalhaut is much hotter than our Sun, 15 times as bright and is blazing through hydrogen at such a furious rate that it will burn out in only one billion years.

George Rieke, of the Steward Observatory, said it’s a star system that has proved to be the ultimate lab test for ideas on how exoplanets and star systems evolve.

‘We do have evidence of such collisions in other systems, but none of this magnitude has been observed in our solar system,’ Rieke said.

‘This is a blueprint of how planets destroy each other.’ 

Taking into account all available data, Gaspar and Rieke think the collision occurred not too long before the first observations taken in 2004.

The debris can’t be seen by Hubble anymore as the debris cloud is made up of minute dust particles a 50th of the diameter of a human hair.  

The cloud has been growing since the first observation and is thought to now be larger than the orbit of the Earth around the Sun – more than 93 million miles.

Fomalhaut b was first announced in 2008 and was clearly visible in several years of Hubble observations – and seemed to have a massive ring round it.

Until then, evidence for exoplanets had only been inferred indirectly through subtle stellar ‘wobbles’ and shadows from planets passing in front of their stars.

But puzzles arose with Fomalhaut b. The object was bright in visible light – highly unusual for an exoplanet that should be too small to be seen from Earth.

At the same time, it did not have any detectable infrared heat signature – a planet should be warm enough to shine, especially a young one like Fomalhaut b.

Astronomers say that the added brightness came from a huge shell or ring of dust encircling the planet that may have been linked to a collision.

‘Our study, which analysed 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,’ said Dr Gaspar.

The final nail in the coffin for it as a planet came when their data analysis of Hubble images taken in 2014 showed the object had vanished.

Earlier images also appeared to show the object continuously fade over time, say the researchers, something unlikely to happen if it were a planet.

‘Fomalhaut b was doing things a bona fide planet should not be doing,’ Gaspar said.

It is slowly expanding from the smashup that blasted a dissipating dust cloud into space, explained the researchers.

The findings were published in the journal Proceedings of the National Academy of Sciences.