The Case of the Disappearing Exoplanet

Humanity’s growing tally of exoplanets — worlds seen orbiting other stars — stands at 4,151. Most were found indirectly, as they passed in front of their stars and cast a telltale shadow, or as they caused their star to wobble as they swung around it. Only 50 have been directly imaged through a telescope.

Directly imaging an exoplanet was first achieved in two discoveries announced simultaneously in 2008. Multiple worlds were seen around the star HR 8799 through ground-based telescopes, and a solitary planet dancing around the star Fomalhaut was spotted by the Hubble Space Telescope. Fomalhaut b, as the latter was named, appeared to be a colossal world, potentially as massive as three Jupiters, zipping along the inner edge of a giant doughnut of debris.

Perusing a decade of Hubble’s observations, some scientists now say that planet Fomalhaut b never existed.

András Gáspár, an astronomer at the University of Arizona, was looking at Hubble’s images of the Fomalhaut system taken up through 2014, on the off chance that someone missed something. To his surprise, Fomalhaut b was nowhere to be found in 2014. Starting with the original 2004 and 2006 Hubble shots that led to the exoplanet’s identification, he flicked forward in time and noticed that it appeared to expand and fade away.

Dr. Gáspár, along with George H. Rieke, a fellow University of Arizona astronomer, used computer models to simulate scenarios that could reproduce Hubble’s observations. In the Proceedings of the National Academy of Sciences on Monday, they offered the hypothesis that instead of a planet, Fomalhaut b is the cloud of debris left behind after two 120-mile-long asteroids slammed into each other. In the ensuing decade, the debris drifted apart.

“I’ll buy it, if I can get a three-year return policy,” says Paul Kalas, an astronomer at the University of California, Berkeley, not involved with the work and one of the discoverers of Fomalhaut b. Future observations from Hubble and the long-delayed James Webb Space Telescope are needed to confirm the new finding.

By far the brightest star in its corner of the night sky, Fomalhaut is sometimes called the Loneliest Star, an even more apt nickname if what Hubble saw was not the star’s sole exoplanet but the dusty ghosts of two asteroids.

The star is 25 light-years from our solar system. Although it is only twice the size of the sun, Fomalhaut is 16 times brighter. That made Fomalhaut b, a billion times fainter than its star, remarkably difficult to spot — “one of the most difficult detections in the history of exoplanet science,” Dr. Kalas said.

While Hubble spotted something interesting around that bright star, questions lingered about its status as an exoplanet. A sizable and (at 400 million years old) young world like Fomalhaut b should be emitting heat and glimmering in infrared. But the infrared camera of another observatory in space, NASA’s Spitzer Space Telescope, failed to find any incandescence there.

The idea that Fomalhaut b may be a dust cloud of some kind had been proposed. This new paper’s detailed evidence makes a stronger case for a collisional cloud, says Bruce Macintosh, an astronomer at Stanford University who helped discover the exoplanets around HR 8799 and was not involved with the Fomalhaut study.

Scientists have seen plenty of planets orbiting other stars, but “we have never seen collisions between such massive objects,” Dr. Gáspár said. Such direct observations of asteroid-on-asteroid smash-ups are exceedingly rare even in our own solar system. Witnessing this tremendous display of annihilation, he said, is an excellent way to better understand how planetary systems evolve.

But that rarity gives other astronomers pause. Such monumental meetings of asteroids that could manufacture this sort of dust cloud are not believed to occur frequently, and when they do, simulations suggest, the debris would only be visible a decade afterward. The study’s observations and simulations suggest that the collision took place in or just before 2004, the year Hubble first spotted signs of Fomalhaut b. Such serendipity is implausible, Dr. Macintosh said.

“Was I really the luckiest astronomer in the world when I pointed the Hubble Space Telescope at Fomalhaut back in 2004?” Dr. Kalas said.

It will take more observations before astronomers can definitively describe the object as an exoplanet or an expanding cloud of asteroid wreckage. “It’s also quite possible it’s something no one has thought of,” Dr. Macintosh said. He added that Fomalhaut b, “definitely hasn’t gotten any less weird as it’s been studied more.”