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.

Nasa's groundbreaking decade of space exploration: In pictures Show all 10 1 /10 Nasa's groundbreaking decade of space exploration: In pictures Nasa's groundbreaking decade of space exploration: In pictures Mystic Mountain, a pillar of gas and dust standing at three-light-years tall, bursting with jets of gas from fledgling stars buried within, was captured by Nasa's Hubble Space Telescope in February 2010 Nasa/ESA/STScI Nasa's groundbreaking decade of space exploration: In pictures The first ever selfie taken on an alien planet, captured by Nasa's Curiosity Rover in the early days of its mission to explore Mars in 2012 Nasa/JPL-Caltech/MSSS Nasa's groundbreaking decade of space exploration: In pictures Death of a star: This image from Nasa's Chandra X-ray telescope shows the supernova of Tycho, a star in our Milky Way galaxy Nasa Nasa's groundbreaking decade of space exploration: In pictures Arrokoth, the most distant object ever explored, pictured here on 1 January 2019 by a camera on Nasa's New Horizons spaceraft at a distance of 4.1 billion miles from Earth Getty Nasa's groundbreaking decade of space exploration: In pictures An image of the Large Magellanic Cloud galaxy seen in infrared light by the Herschel Space Observatory in January 2012. Regions of space such as this are where new stars are born from a mixture of elements and cosmic dust Nasa Nasa's groundbreaking decade of space exploration: In pictures The first ever image of a black hole, captured by the Event Horizon telescope, as part of a global collaboration involving Nasa, and released on 10 April 2019. The image reveals the black hole at the centre of Messier 87, a massive galaxy in the nearby Virgo galaxy cluster. This black hole resides about 54 million light-years from Earth Getty Nasa's groundbreaking decade of space exploration: In pictures Pluto, as pictured by Nasa's New Horizons spacecraft as it flew over the dwarf planet for the first time ever in July 2015 Nasa/APL/SwRI Nasa's groundbreaking decade of space exploration: In pictures A coronal mass ejection as seen by the Chandra Observatory in 2019. This is the first time that Chandra has detected this phenomenon from a star other than the Sun Nasa Nasa's groundbreaking decade of space exploration: In pictures Dark, narrow, 100 meter-long streaks running downhill on the surface Mars were believed to be evidence of contemporary flowing water. It has since been suggested that they may instead be formed by flowing sand Nasa/JPL/University of Arizona Nasa's groundbreaking decade of space exploration: In pictures Morning Aurora: Nasa astronaut Scott Kelly captured this photograph of the green lights of the aurora from the International Space Station in October 2015 Nasa/Scott Kelly

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.