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Fomalhaut b … smashed to pieces?
NASA / ESA / A. Gaspar / G. Rieke / University of Arizona
Fomalhaut b is a ghost with a complicated story.
In 2004 Paul Kalas, an astronomer at the University of California, Berkeley, made a surprising finding. Using NASA’s powerful Hubble Telescope, Kalas and colleagues saw direct evidence of a planet moving around the star Fomalhaut, which is about 25 light-years from Earth. They published their findings in the journal Science, describing the huge, young planet as three times the mass of Jupiter.
It was, and still is, a rare feat for astronomers to see a planet with optical light from outside our solar system directly, they are generally obscured by the light from their stars and so far away that they don’t flicker to us like a star power. So planet hunters use indirect methods to detect exoplanets how to see 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 spot of light on the images. It was one of the first times that an exoplanet was photographed.
“Fomalhaut b is one of the most intriguing discoveries ever made with the Hubble Space Telescope,” says Kalas.
But Fomalhaut b disappeared. Missing. New research, published in the journal Proceedings of the National Academy of Sciences on Monday, reveals that Fomalhaut b may not be a planet at all. Instead, it may be the lingering light of a giant collision between two huge asteroids.
As luck would have it
An animation that simulates the collision of two large asteroids. The debris ring surrounding the star Fomalhaut is shown in yellow, while in the box, a simulation shows the fading signal from 2004 to 2015.
NASA / ESA / A. Gaspar / G. Rieke / University of Arizona
The notion of Fomalhaut b may not be an exoplanet has been raised since its discovery by Kalas in 2004. Although visible in optical light, the researchers were unable to find the infrared signature that a planet of 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’ original 2008 article. It has been suggested that Fomalhaut b is a cloud of dust or material captured from the huge debris disk surrounding Fomalhaut, the star.
“Astronomers have had trouble categorizing Fomalhaut b,” says András Gáspár, an astronomer at the University of Arizona and co-author of the new article. “That alone makes it an interesting object.”
Gáspár is part of the scientific team at the University of Arizona that has access to NASA James Webb Space Telescope, a successor to Hubble slated to launch in 2018 but plagued with development delays. The team has already scheduled nearly 50 days of observation time for Fomalhaut b when James Webb takes off in 2021.
In preparation, Gáspár downloaded the archive data from the Hubble telescope and began searching for things that other researchers might have missed in a pair of unpublished data sets.
He noticed something strange with Fomalhaut b: his light was fading.
“This is how it all started, pure luck,” he says.
Gáspár and his colleague George Rieke studied the Hubble data and noted that Fomalhaut b faded over time. Since a pinprick of light in the 2004 data, Fomalhaut b became the ghost of a planet, the light that appeared in the Hubble data began to dissipate and expand before disappearing in 2014.
Archival data from Hubble in 2014 prompted Gáspár to investigate further.
Gáspár has modeled debris disks and collisions in the past and investigated the fading signal with computer modeling. By connecting the collision of two gigantic pre-planets, about 200 kilometers wide, to their system, the team discovered that the features seen by Hubble combined perfectly.
“Our model shows that the observed characteristics are consistent with a model of an expanding dust cloud produced in a massive collision,” says Gáspár. The model explains all the strangeness observed with Fomalhaut b during its observation history, from the discovery of Kalas to some of the last observations seven years ago.
Collide-o-scope
Gáspár and Rieke are not the first to propose that Fomalhaut b be no a planet. Previous analyzes of the Hubble data suggested that Fomalhaut b was just an unfortunate dual deletion. But the latest study is the first to show a model showing two large space rocks (slightly smaller than the “dwarf planet” Hygiea) colliding with 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 that a collision that could cause such a dust cloud would only occur “once every 100,000 years” and the resulting cloud would remain only for a decade. Such odds have seemed to him to be fighting his own good fortune.
“Was he really the luckiest astronomer in the world when I pointed the Hubble Space Telescope at Fomalhaut in 2004?” he asks. “If I had tried a few years before or a few years later, I would never have discovered it.”
Gáspár and Rieke’s observations suggest that he may be even more fortunate, as their calculations show that such a collision could occur once every 200,000 years. That would mean that collisions have only happened twice in human history. Gáspár says that it is “really 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 disappear. Gáspár doubts that we will ever see this object again now that it has disappeared.
“As far as I’m concerned, we can put a period at the end of the sentence that describes Fomalhaut b,” he says.
But our understanding of the cosmos constantly evolves with new observations. In fact, the latest study shows the scientific method in action: the discoveries are analyzed and, with new evidence, the hypotheses change.
And Kalas will continue to examine Fomalhaut, a system he has been studying since he was a student in the 1990s. He has asked him to use the Hubble Space Telescope to re-observe Fomalhaut in the coming year. This, he says, could validate the collision hypothesis.
The mystery of Fomalhaut b seems largely solved, but researchers are now awaiting the launch of the James Webb telescope in 2021. Kalas says the telescope “will probably take the next big leap in understanding the planetary system around Fomalhaut.” The imaging instruments in that space telescope can discover planets in good faith, and Gáspár points out that any new discoveries will contribute to our understanding of how planetary systems, such as our solar system, evolve over time.
“The Fomalhaut system, with its huge disk of planetary debris, still has many mysteries to uncover,” he says.
