In 2008, astronomers spotted the first exoplanet in visual light orbiting a distant star. But now, a new study suggests that Fomalhaut b may simply be a swirl of space dust.
The Hubble Space Telescope first spotted a possible planet circling the star Fomalhaut about 25 light-years away in the Southern Fish constellation, Piscis Australis.
The star is surrounded by a debris ring that stretches some 21.5 billion miles across and bears an uncanny visual resemblance to the Eye of Sauron, from the Lord of the Rings films.
The cloud's distinctive cat's-eye shape, astronomers say, is evidence that at least one small planet is orbiting Fomalhaut.
Though it can't be seen, researchers suspected the planet is there because of its gravitational calling card: The oval shape and sharp inner edge of Fomalhaut's halo are signs that a planet is "sweeping" through the dust and gas.
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Now, new data from the Spitzer Space Telescope suggest that Fomalhaut b may be a dust cloud resulting from a collision between comets or asteroids, according to study leader Markus Janson, an astrophysicist at Princeton University.
That could explain why the potential object's light appears very blue in visible light but is nearly invisible in infrared—which would be true of an object with little mass, such as a dust cloud.
Paul Kalas, one of Fomalhaut b's original discoverers, said his team originally considered the same collision theory.
But Kalas, an astronomer at the University of California, Berkeley, believes such collisions are rather rare—and the odds of observing one even more unlikely.
"It's not excluded, but the bottom line is that scientists don't favor arguments that depend on fortunate observations," Kalas said. "You'd have to be quite lucky to observe such a collision between objects."
Janson, leader of the new study, doesn't think such cosmic smashups are all that uncommon in that galactic neighborhood.
"These kinds of collisions must happen quite frequently in this system, because they are the cause of the massive ring of dust that we see," said Janson, whose study will be published in an upcoming issue of the Astrophysical Journal.
Planet ... Or Not?
Likewise, if a planet existed at that location it would be young and hot, and thus glow with light visible to the infrared eye of the Spitzer Space Telescope, Janson noted.
"It should emit much more light at near-infrared wavelengths than it does at visible wavelengths," he said. "And that's the opposite of what's been observed."
But co-discoverer Kalas countered that the infrared observations simply aren't sensitive enough to detect a planet that's less than the mass of Jupiter.
"If you look at Saturn, it has held onto a ring system for 4.5 billion years," Kalas said.
"That means a collection of dust surrounding a planet can be long-lived, whereas a dust cloud produced by two coments colliding has an extremely short lifetime, and you would be lucky to observe it. That's why planetary rings are more plausible."
But Princeton's Janson finds the ring hypothesis less convincing. For instance, the Hubble data show more variations in brightness than you'd expect from a ring that circles the planet, he said.
Exciting Time for New Planets
Whatever Fomalhaut b's true identity, it's an exciting time to study exoplanets—planets outside our solar system, which currently number more than 700, and counting, Kalas noted.
(Related: "Fifty New Planets Found—Largest Haul Yet.")
"Our understanding of these exoplanets is constantly evolving," he said.
"There are many surprises—and fewer hard answers."