More than a decade ago, the Hubble Space Telescope spotted a Jupiter-scale planet orbiting Fomalhaut, a star about 25 light years away from our solar system. The discovery marked the first time that an exoplanet, a world that orbits another star system, had ever been directly detected in visible light wavelengths, as opposed to the typical “transit” method that scans for a dip in light as a planet passes in front of its star.
But when András Gáspár, an assistant astronomer at the University of Arizona’s Steward Observatory, examined more recent images of the star system, captured by Hubble in 2013 and 2014, he was astonished to find that the exoplanet, known as Fomalhaut b, had vanished.
“To my surprise, it was not present on the latest images,” recalled Gáspár in an email. “So, I went through all the data and started to analyze it and noticed a pattern: it was fading.”
As it turns out, Fomalhaut b, the first optically imaged exoplanet, is not a planet at all. It is likely the explosive fallout of a crash between two icy space rocks, each at least 100 kilometers in radius, according to a study published on Monday in Proceedings of the National Academy of Sciences.
While cataclysmic collisions are common in the universe, especially in young star systems like Fomalhaut, it is extremely rare to capture imagery of them from Earth. “We know of many other directly imaged exoplanets now,” said Gáspár, who led the new study. “We have never seen anything like this before!”
The discovery was “pure luck,” he added, as he did not specifically set out to observe Fomalhaut b. Gáspár is helping to develop instruments for the James Webb Space Telescope (JWST), NASA’s next-generation observatory, which will be about 100 times more powerful than Hubble. He was scanning archival Hubble data to calibrate JWST’s optical observations, when he noticed the exoplanet’s absence.
The aftermath of this enormous collision is a literal dust-up. Some kilometer-scale space rocks probably remain in the mix, but the crash also ejected an expansive shell of “smoke-like dust” that has cast a surreal shimmer over the entire system, Gáspár said.
“These smallest dust particles scatter the light of the central star, much like dusty air sparkles in a sunbeam,” he explained. “The HST works at optical wavelengths (the same domain of the electromagnetic spectrum that we see with our eyes), therefore we see these smallest dust particles in the HST images.”
“While the larger fragments are likely continuing on an elliptical orbit around Fomalhaut, the trajectory of these smallest particles is greatly influenced by the radiative forces of the central star, resulting in them being pushed out of the system, whilst also expanding as a shell from the collisional energy,” he continued.
The crash serendipitously occurred right before Hubble originally observed the erstwhile exoplanet in 2004, which explains why it once looked like a flashy gas giant from our perspective. By now, the dust cloud is probably too evanescent to be observed by Hubble, as the team estimates it has expanded to an area larger than Earth’s orbit around the Sun.
That said, astronomers expect to examine the system in infrared wavelengths using the JWST, currently slated for launch in 2021. The observatory could probe the Fomalhaut system in far greater detail, and perhaps pinpoint some bonafide exoplanets orbiting this star. Gáspár and his colleagues are confident that these worlds exist, as “dynamical instability” from their migration is “the most likely explanation for events like Fomalhaut b,” according to the study.
While these crashes seem completely apocalyptic, they also played a crucial role in the emergence of life on Earth, and perhaps on other planets, too.
“As destructive as collisions are, they also end up building worlds!” Gáspár emphasized. “Think of the massive collision that formed the Earth-Moon system. Without this massive collision in the early history of the solar system, we would likely not be here to discuss all this. The Moon is vital to life on Earth.”
“So understanding collisions is in fact an important part of understanding how planetary systems form and evolve,” he concluded.