'Planet' disappears from sight prompting surprise and suspicions that it never actually existed

What was thought to be a planet beyond our solar system appears to have disappeared, astronomers say.

And the unexpected behaviour has led them to conclude that what they thought was a distant world never actually existed at all.

Instead, astronomers think that the phantom planet was actually a cloud of dust – left in the aftermath of a cosmic collision – that has since billowed out so much that it is now invisible.

When the exoplanet known as Fomalhaut b was first discovered, in the mid 2000s, it made history as one of the first worlds outside of our solar system to be seen directly. Researchers had previously mostly inferred the existence of planets through their effects on stars or their shadows as they moved in front of stars, but this suspected planet was seen through numerous years of observations from Nasa's Hubble Space Telescope.

But astronomers now think that those observations were actually the result of two icy bodies smashing into each other. When they did, they left behind a cloud of fine dust particles that was visible through space – and it was that which was mistaken for another planet by astronomers on Earth, 25 light years away.

Though the discovery means that the original discovery appears to have been mistaken, it also opens up an entirely new – and perhaps more surprising – kind of discovery, in seeing such a crash. The Hubble images might not show a planet but do show the aftermath of the suspect collision, allowing researchers a new phenomenon to research.

"These collisions are exceedingly rare and so this is a big deal that we actually get to see evidence of one," said Andras Gaspar, an assistant astronomer at the University of Arizona's Steward Observatory and lead author of the new paper. "We believe that we were at the right place at the right time to have witnessed such an unlikely event with NASA's Hubble Space Telescope."

The discovery might help shine a light not on where planets exist – but how they stop existing, when they destroy each other.

"The Fomalhaut star system is the ultimate test lab for all of our ideas about how exoplanets and star systems evolve," said George Rieke, a Regents Professor of Astronomy at Steward Observatory. "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."

The scientists – whose research is published in the Proceedings of the National Academy of Sciences – came to the conclusion after spotting a variety of unusual characteristics of the supposed exoplanet. The fact it could be seen in visible light was strange, given that any exoplanet should be too small to reflect enough light to be seen; on the other hand, its heat could not be seen in infrared, but researchers expected any planet would be warm enough to shine that way.

Initially, those supportive of the existence of the planet argued that it could have a shell or ring of dust surrounding it from a crash. But new images helped argue that the planet did not exist at all.

Hubble images from 2014 showed the planet had disappeared entirely. Before that, it had faded away.

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

Instead, the new paper proposes that the planet was actually the slowly expanding dust from a long ago collision that sent dust billowing into space. It gradually disappeared as the debris cloud expanded to be so large that it could no longer be seen, and scientists now suspect that it would be a size larger than the orbit of Earth around the Sun.