Fomalhaut b

Fomalhaut is one of the brightest stars in the night sky, a white star about twice the Sun’s mass, 25 light-years away and only about 440 million years old. Since 1983 astronomers have known that it is surrounded by a ring of cold dust, the debris left over from making planets. In November 2008 Paul Kalas and his team published Hubble images that seemed to show one of those planets: a faint dot moving slowly around the star, just inside the ring. It was called Fomalhaut b, and it was hailed as one of the first exoplanets ever photographed in visible light.

A planet that did not behave

The ring itself pointed to a planet. It is narrow and sharp-edged, and its centre is offset from the star, as if something were shepherding it. Fomalhaut b sat about 120 AU from the star, roughly where such a shepherd should be, and models of the ring limited its mass to no more than three Jupiters.

But the dot was too bright in visible light, and its brightness changed. A young giant planet should glow in infrared, yet the Spitzer Space Telescope found nothing there in 2012, and later images with the James Webb Space Telescope found nothing either. A 2013 study showed that its orbit was extremely elongated, which no longer fitted the idea of a planet tending the ring. Then, in Hubble images from 2014, Fomalhaut b was gone.

A cloud of dust

In 2020 András Gáspár and George Rieke went back through all the Hubble images. The dot had been growing fainter and more spread out, and it seemed to be moving outward on a path that could take it out of the system. Their conclusion was that Fomalhaut b had never been a planet. It was a cloud of fine dust, grains about a micron across, thrown out when two icy bodies collided, each perhaps about 200 kilometres across. The cloud spread until it was larger than Earth’s orbit and too thin for Hubble to see.

It happened again

In September 2023 Hubble caught a new point of light near the ring. In a paper published in December 2025, Kalas and colleagues described it as a second collision, which they named Fomalhaut cs2, and renamed the original object cs1. The bodies that collided this time were estimated at about 60 kilometres across. Theory had suggested that a collision like this should be seen around Fomalhaut perhaps once in 100 000 years; astronomers have now seen two in about twenty.

Not every cloud near Fomalhaut has been real. A “Great Dust Cloud” reported in Webb’s images in 2023 turned out to be a distant background object, already visible in older images at the same place in the sky and not moving with the star. And the nature of the first dot is still formally debated: a few studies have argued for dust around a small planet instead.