NASA/ESA/A. Gaspar/G. Rieke/University of Arizona

Fomalhaut b is a ghost with a complicated history.

In 2004, Paul Kalas, an astronomer at the University of California, Berkeley, made a surprising find. Using NASA's powerful Hubble telescope, Kalas and his colleagues saw direct evidence of a planet moving around the star Fomalhaut, which is about 25 light-years away from Earth. They published their findings in the journal Science, describing the the massive, young planet as three times the mass of Jupiter.

It was, and still is, a rare feat for astronomers to see a planet in optical light from outside our solar system directly -- usually they are obscured by the light from their stars and so far away they don't blink at us like a star might. So planet hunters use indirect methods to detect exoplanets like seeing how much a star wobbles due to the gravitational effects of a planet. But in 2004, Kalas used Hubble to look at Fomalhaut and noticed a speckle of light in the images. It was one of the first times an exoplanet had ever been imaged.

"Fomalhaut b is one of the most intriguing discoveries ever made with the Hubble Space Telescope," Kalas says.

But Fomalhaut b vanished. Disappeared. New research, published in the journal Proceedings of the National Academy of Sciences on Monday, reveals Fomalhaut b may not be a planet at all. Instead, it may be the lingering light from a giant collision between two huge asteroids.

As luck would have it

NASA/ESA/A. Gaspar/G. Rieke/University of Arizona

The notion Fomalhaut b may not be an exoplanet has been raised since its discovery by Kalas in 2004. Although visible in optical light, researchers couldn't find the infrared signature a planet that size should create. As a result, Fomalhaut b's true identity has remained enigmatic.

Alternative hypotheses have been suggested in the past, including in Kalas's original 2008 paper. There have been suggestions Fomalhaut b is a dust-cloud or material captured from the huge disk of debris surrounding Fomalhaut, the star.

"Astronomers have struggled categorizing Fomalhaut b," says András Gáspár, an astronomer at the University of Arizona and co-author on the new paper. "That alone makes it an interesting object."

Gáspár is part of the science team at the University of Arizona which has access to NASA's James Webb Space Telescope, a Hubble successor scheduled to launch in 2018 but plagued with development delays. The team has already scheduled in almost 50 days of observation time for Fomalhaut b when James Webb gets off the ground in 2021.

In preparation, Gáspár downloaded archival Hubble telescope data and started looking for things other researchers may have missed in a couple of unpublished datasets.

He noticed something weird with Fomalhaut b: Its light was fading.

"That's how it started, pure luck," he says.

Gáspár and colleague George Rieke studied the Hubble data and noticed Fomalhaut b was vanishing over time. From a pinprick of light in 2004 data, Fomalhaut b became a ghost of a planet, the light that appeared in Hubble data began to dissipate and expand before disappearing in 2014.

The archival Hubble data, from 2014, led Gáspár to investigate further.

Gáspár has modelled debris disks and collisions in the past and investigated the fading signal with computer modelling. Plugging the collision of two gargantuan pre-planets, around 125 miles wide (200 kilometers), into their system, the team discovered the characteristics seen by Hubble matched up neatly.

"Our modeling shows the observed characteristics agree with a model of an expanding dust cloud produced in a massive collision," says Gáspár. The model accounts for all of the strangeness seen with Fomalhaut b during its observation history, from Kalas's discovery to some of the last observations seven years ago.

Collide-o-scope

Gáspár and Rieke aren't the first to propose Fomalhaut b is not a planet. Previous analyses of the Hubble data suggested Fomalhaut b was just an unlucky dual wipeout. But the latest study is the first to show a model demonstrating two big space rocks (a little smaller than the "dwarf planet" Hygiea) smashing into each other as a definitive explanation. And that's pretty phenomenal: When Kalas pointed the Hubble Space Telescope at Fomalhaut in 2004, he saw something incredibly rare.

Kalas says a collision that could cause such a dust cloud would only happen "once every 100,000 years" and the resulting cloud would linger for just a decade. Such odds have seem him wrestle with his own good fortune.

"Was I really the luckiest astronomer in the world when I pointed the Hubble Space Telescope at Fomalhaut back in 2004?" he asks. "If I had tried just a few years earlier or a few years later, I never would have discovered it."

Gáspár and Rieke's observations suggest he may be even luckier -- with their calculations showing such a collision might occur once every 200,000 years. That would mean the collisions have only occurred twice in the history of humanity. Gáspár says it's "genuinely exciting" to be able to measure and analyze such an event.

Is there still hope for the planet hypothesis? It seems less and less likely. Planets don't just vanish. Gáspár doubts we will ever see this object again now that it has disappeared.

"As far as I am concerned, we can put a period at the end of the sentence describing Fomalhaut b," he says.

But our understanding of the cosmos is constantly evolving with new observations. Indeed, the latest study shows the scientific method in action: Discoveries are scrutinized and, with new evidence, hypotheses change.

And Kalas will continue to examine Fomalhaut, a system he's been studying since he was a student in the 1990s. He's asked for some time using the Hubble Space Telescope to re-observe Fomalhaut in the next year. This, he says, could validate the collision hypothesis.

The mystery of Fomalhaut b seems mostly settled, but now researchers await the launch of the James Webb Telescope in 2021. Kalas says the telescope will "probably give the next big leap in understanding the planetary system around Fomalhaut." Imaging instruments on that space telescope may discover bona fide planets and Gáspár notes any new discoveries will contribute to our understanding of how planetary systems -- like our solar system -- evolve over time.

"The Fomalhaut system, with its massive planetary debris disk, still holds many mysteries to uncover," he says.