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Disappearing Planet

November 20, 2020

A planet that appeared to orbit the star Fomalhaut has blown away like dust on a stellar wind. Instead of a planet, the object may be debris from a collision.

Fomalhaut is low in the south as night falls, far to the lower left of the Moon. It’s the only bright star in its region of the sky, so it stands out.

The star is bigger, brighter, and heavier than the Sun. And it’s encircled by wide rings of dust. Gaps between rings suggest they’re being sculpted by the gravity of hidden planets.

In 2008, images of the system from Hubble Space Telescope appeared to show a planet — the first exoplanet ever photographed at visible wavelengths. A few years later, astronomers gave it a formal name: Dagon.

Some other telescopes, though, suggested Dagon wasn’t really a planet at all. And more-recent Hubble observations agree. They show that the “planet” is expanding and getting fainter. That suggests that the object is the aftermath of a collision between two comet-like bodies. Each of them was about 125 miles in diameter.

The collision probably took place not long before Hubble’s first pictures of the object. It could have been caused by the gravitational influence of one or more hidden planets.

After the impact, the debris spread out. And Fomalhaut’s wind of charged particles started blowing the cloud of dust out of the system. Today, it’s too spread out for Hubble to see it at all — a non-planet carried away on a stellar wind.

Script by Damond Benningfield

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