Yahoo Sports Astronomers discover 'planet that never was'
The first planet to be discovered outside our solar system, a Saturn-like world said to be lying 25 light years from Earth, never existed at all, new research has found.
But what the astronomers spotted was almost as amazing – a collision between two comets, an event so rare it occurs once every 200,000 years.
The supposed world was named Fomalhaut b, and made headlines around the world in 2008.
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But the “planet” that was photographed by the Hubble space telescope was actually a cloud of dust.
Hubble had captured the aftermath of a collision between two icy comets about 125 miles wide.
The colliding comets left an expanding cloud of very fine dust particles – and it was this that was captured by the space telescope.
Lead author Dr Andras Gaspar, of the University of Arizona's Steward Observatory, said: “These collisions are exceedingly rare and so this is a big deal that we actually get to see evidence of one.
“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.”
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Such an event is estimated to happen about once every 200,000 years and sheds fresh light on planet evolution, the US team said.
Professor George Rieke, also of the Steward Observatory, said: “The Fomalhaut star system is the ultimate test lab for all of our ideas about 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. 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 prior to the first observations taken in 2004.
By now, the debris cloud – consisting of dust particles a 50th the diameter of a human hair – is below Hubble's detection limit.
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It is estimated to have expanded by now to a size larger than the orbit of Earth around the sun.
Fomalhaut b was first announced in 2008. It 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 conjectured that the added brightness came from a huge shell or ring of dust encircling the planet that may have been collision-related.
