Now you see it, now you don’t: The case of the vanishing exoplanet

An artist’s impression of two asteroid-size bodies crashing together around the nearby star Fomalhaut. Astronomers initially believed they had spotted an exoplanet, but it now appears they actually saw a vast cloud of debris in the aftermath of the presumed collision. Image: ESA/NASA, M. Kornmesser

In 2008, astronomers using the Hubble Space Telescope announced the discovery of an exoplanet orbiting the star Fomalhaut a scant 25 light years away. First seen in 2004 and again in 2006, the presumed planet – Fomalhaut b – was brighter than would normally be expected and appeared to be following an unusual trajectory just inside a vast cloud of icy debris orbiting the star.

Then, in 2014, astronomers were stunned to find Fomalhaut b had disappeared. A search through archived data revealed it had slowly faded from view over several years.

Astronomers now think Fomalhaut b was never a fully evolved planet in the first place. Instead, the data suggest the bright object Hubble originally spotted was, in fact, a huge cloud of expanding dust in the aftermath of a collision between two asteroid-size bodies.

The photo on the left shows a Hubble Space Telescope image of vast ring of debris orbiting the star Fomalhaut (the star’s bright light has been blocked by an occulting disc). The image on the right simulates the appearance and motion of a presumed exoplanet now thought to be a cloud of debris that formed in the wake of two asteroids colliding. Image: NASA, ESA and A. Gáspár and G. Rieke (University of Arizona)

“These collisions are exceedingly rare and so this is a big deal that we actually get to see one,” said András Gáspár of the University of Arizona. “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, George Rieke of the University of Arizona’s Steward Observatory and a team of collaborators believe the apparent collision occurred shortly before the first images of Fomalhaut b were collected in 2004. By 2014, the object had disappeared from view after fading over several years.

“Clearly, Fomalhaut b was doing things a bona fide planet should not be doing,” said Gáspár.

Along with its slow fading, Fomalhaut b is likely on an escape trajectory, not in a planet-like elliptical orbit, a natural result of the host star’s influence on a massive, expanding dust cloud.

“The Fomalhaut system is the ultimate test lab for all of our ideas about how exoplanets and star systems evolve,” Rieke said. “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.”

A video posted by NASA’s (credit: A. Gáspár and G. Rieke) illustrates the behaviour of Fomalhaut b: