The planet that never existed

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This artist’s illustration depicts the collision of two 125-mile-wide icy, dusty bodies orbiting the bright star Fomalhaut, located 25 light-years away. Astronomers believe that Hubble Space Telescope observations, spanning several years, provide observational evidence for the first-ever detection of such a titanic collision in another star system. What was first thought to be a planet […]

Planets are hard to lose, unless you’re Obi-Wan Kenobi in “Star Wars: Episode II – Attack of the Clones.” But astronomers believe that what was once among the first observed exoplanet discoveries, actually never existed.

Instead, NASA’s Hubble Space Telescope likely observed the leftover bright cloud of an explosion after two icy objects collided, according to a new study. The telescope just happened to look in time to catch the aftermath, rather than the explosion itself, as the fine dust particles expanded.

“These collisions are exceedingly rare and so this is a big deal that we actually get to see evidence of one,” said Andras Gaspar, lead study author and an assistant astronomer at the University of Arizona’s Steward Observatory, 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.”

The study published Monday in the Proceedings of the National Academy of Sciences.

Previously, the exoplanet was likely believed to be orbiting a star named Fomalhaut, which is 25 light-years from Earth. The planet was dubbed Fomalhaut b and announced in 2008 after data gathered in 2004 and 2006 suggested the presence of a planet around the star.

This was one of the first exoplanet discoveries found using direct imaging. Hubble showed the clearly observable moving dot.

But the astronomers did have questions about it. The planet showed up brightly in the visible light spectrum of Hubble’s observations — the light we can see. This isn’t normally the case for exoplanets, which are too small to reflect the light of their host star and be seen so clearly at such a distance from us.

And there was no heat signature. Exoplanets, especially young ones, usually radiate heat in the form of detectable infrared light.

But the researchers thought that perhaps a ring of dust orbited the planet.

And in images taken by Hubble in 2014, the planet simply isn’t there any longer. And images leading up to that one showed the object’s brightness fading.

Lost a planet these astronomers have, as Yoda would say.

“Our study, which analyzed all available archival Hubble data on Fomalhaut, revealed several characteristics that together paint a picture that the planet-sized object may never have existed in the first place,” Gaspar said. “Clearly, Fomalhaut b was doing things a bona fide planet should not be doing.”

So how did this bright dust cloud, which isn’t a planet, disappear?

As the dust cloud expanded over time, which was first observed in 2004, the dusty debris cloud is likely too faint to be seen by Hubble even though it’s likely larger than Earth’s orbit around the sun.

And the dust cloud isn’t in a typical orbit around its star. Instead, it’s on an “escape path.”

“A recently created massive dust cloud, experiencing considerable radiative forces from the central star Fomalhaut, would be placed on such a trajectory,” Gaspar said. “Our model is naturally able to explain all independent observable parameters of the system: its expansion rate, its fading and its trajectory.”

Fomalhaut b, or what’s left of it, is trapped in a large ring of debris around the star. This ring includes ice and dust and frozen objects, like comets and those that exist on the outskirts of our solar system in the Kuiper Belt.

The researchers believe that two icy objects similar to comets, each about 125 miles across, collided. This created the bright cloud that was confused for an exoplanet.

And based on their modeling, the researchers believe that an event like this may occur in the Fomalhaut system every 200,000 years. And they were lucky to witness the aftermath of this one.

NASA’s James Webb Space Telescope, launching next year, is an infrared observatory that will be able to directly image the warmest parts of the Fomalhaut system and capture information about its asteroid belt. This is the first time astronomers will be able to get such information about a solar system outside of our own.

And, of course, they’ll use the telescope to see if there are actually any planets orbiting the star.

“The Fomalhaut star system is the ultimate test lab for all of our ideas about how exoplanets and star systems evolve,” said George Rieke, study co-author and a Regents Professor of Astronomy at the Steward Observatory, in a statement. “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.”

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