• Astronomers Discover Planet that Never Was (U of Arizona)
• The Hubble Space Telescope offers insight into the nature of exoplanet Fomalhaut b (Hubble Space Telescope)

Today’s news is a bit gassy. We’re not quite sure how this happened. While I knew a whole lot of science focused on the parts of the universe that aren’t solid or plasma, I hadn’t expected the solid stuff to become gassy stuff.

As described in no less than three press releases and one conference proceeding, the planet we thought we’d found around the star Fomalhaut has gone away, dissipating into a cloud of gas and dust. First discovered in 2004, the not-still-a-planet Fomalhaut b was directly imaged by the Hubble Space Telescope as a small, coherent, planet looking thing. It was observed repeatedly, and continued to shine bright in 2006, but starting in 2008, this world seemed to be expanding – and expansion is not something we expect of our planets. By the 2014 observations, this world was gone.

The observing team believes that shortly before its discovery images were taken in 2004, this world had a bad collision. According to the spacetelescope.org press release, this object always looked a little bit off. Here is how they describe it, “The object was unusually bright in visible light, but did not have any detectable infrared heat signature. Astronomers proposed that the added brightness came from a huge shell or ring of dust encircling the object that may have been collision-related.” While the object labeled “planet” could be explained with a certain amount of hand-waving, what was harder to explain was its path – which wasn’t an elliptical orbit so much as a trajectory of escape. 

Now… several years of observing later, scientists have realized it wasn’t so much a cloud around a planet as just a cloud. While it may have been vaguely planet-sized to begin with, over time this cloud has expanded to be about the size of the Earth’s orbit, and at this point, the light it reflects is so spread out that it isn’t detectable even to the Hubble Space Telescope. 

Remarkably, the objects that collided to create this debris cloud didn’t have to be that big. The star it orbited, Fomalhaut, is about 25 light years away and surrounded by a vast ring of icy debris. It’s believed the colliding objects may have been icy bodies about 200 km across. The ices would have largely gone to gas from the energy of the collision, transforming two solid worlds into a cloud of expanding gas and dust on an escape path out of its solar system.