'Disappearing' Exoplanet Might Not Have Been a Planet After All

In 2008, NASA’s Hubble Space Telescope delivered humanity its first direct images of exoplanets—far away worlds orbiting foreign stars. One of those distant planets circled a star some 25 light-years away called Fomalhaut, and astronomers creatively named the exoplanet Fomalhaut b. The planet looked huge, perhaps triple the mass of our solar system’s biggest planet, Jupiter.

But now some scientists are saying Fomalhaut b is the exoplanet that never was, reports Robin George Andrews for the New York Times.

Looking back over a decade’s worth of Hubble’s images of Fomalhaut in search of anything overlooked, astronomer András Gáspár of the University of Arizona saw something was missing from the 2014 images—namely, Fomalhaut b.

In the images from 2004 and 2006 that led to the exoplanet’s discovery, there it was. But as Gáspár flipped through the years, he saw Fomalhaut b expand and then fade to almost nothing.

To figure out what might be going on, Gáspár and George H. Rieke, also a University of Arizona astronomer, devised computer simulations of various scenarios capable of creating visuals similar to those in the Hubble photographs. Using this technique to narrow the field of potential explanations, Gáspár and Rieke hypothesize that Fomalhaut b was never a planet at all, much less a disappearing one.

Instead, the phenomenon astronomers named Fomalhaut b may have just been a cloud of asteroid debris that drifted apart over time, they report this week in the journal the Proceedings of the National Academy of Sciences.

“I’ll buy it, if I can get a three-year return policy,” Paul Kalas, an astronomer at the University of California, Berkeley who was not involved with the work and was one of the discoverers of Fomalhaut b, tells the New York Times. Kalas’s hedging refers to the new finding’s needing to be confirmed by additional observations from Hubble as well as the James Webb Space Telescope, which may finally launch into orbit in 2021.

If Gáspár and Rieke are right, then the plume of debris formerly known as Fomalhaut b is actually the leavings of an exceedingly rare celestial event: the collision of two massive 125-mile-long asteroids. That debris then drifted apart over the course of the ten years that followed.

"These collisions are exceedingly rare and so this is a big deal that we actually get to see one," says Gáspár in a statement. "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."

Gáspár and Rieke are not the first to suggest that Fomalhaut b is just a cloud of dust, but the serendipity of observing something so fleeting and rare has some other astronomers wondering if the explanation is too incredible to be true. “Was I really the luckiest astronomer in the world when I pointed the Hubble Space Telescope at Fomalhaut back in 2004?” Kalas tells the New York Times.

“Previous calculations have suggested that the frequency of collisions between objects that are about 100 kilometers (62 miles) in size is very, very low, fewer than one in the age of the system,” Grant Kennedy, an astronomer at the University of Warwick who was not involved in the research, tells Eos. Given that the Fomalhaut system is roughly 400 million years old, it’s exceedingly unlikely that we’d be witness to such an event, according to Kennedy.

Will additional observation of this planet or dust cloud hold more twists and turns? “It’s also quite possible it’s something no one has thought of,” Bruce Macintosh, an astronomer at Stanford University who was not involved with the new study, tells the New York Times. He added that Fomalhaut b, “definitely hasn’t gotten any less weird as it’s been studied more.”

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