The Exoplanet That Never Was

The Hubble telescope captured what astronomers thought was an exoplanet 25 light-years from Earth. Now they think Hubble captured the aftermath of an interstellar collision between two icy bodies.

This view shows a new picture of the dust ring around the bright star Fomalhaut from the Atacama Large Millimeter/submillimeter Array (ALMA). The underlying blue picture shows an earlier picture obtained by the NASA/ESA Hubble Space Telescope. (ESO / NAOJ / NRAO)

(CN) — A planet astronomers thought was just beyond the solar system has now seemingly vanished from their sights, leading them to believe one of the first exoplanets ever discovered with direct imaging may have never existed at all.

A study published Monday in the journal Proceedings of the National Academy of Sciences details the scientists’ discovery.

Instead of an exoplanet, two astronomers from the University of Arizona concluded that NASA’s Hubble Space Telescope was in reality looking at an expanding cloud of very fine dust particles from two icy bodies that smashed into each other.

They suspect the telescope came along after the event, missing the collision and only capturing the aftermath. The last sighting of this missing-in-action planet before its disappearance was located orbiting the star Fomalhaut, 25 light years away.

“These collisions are exceedingly rare and so this is a big deal that we actually get to see evidence of one,” said lead author Andras Gaspar, an assistant astronomer at the University of Arizona’s Steward Observatory. “We believe that we were at the right place at the right time to have witnessed such an unlikely event.”

“The Fomalhaut star system is the ultimate test lab for all of our ideas about how exoplanets and star systems evolve,” added George Rieke, a regents professor of astronomy at Steward Observatory. “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.”

The previously suspected exoplanet was named Fomalhaut b, first announced in 2008 after having been discovered based on data from 2004 and 2006. It was clearly visible in several years of Hubble’s observations, which confirms that it was a moving object.

Until this announcement, the evidence for exoplanets had mostly been inferred through indirect detection methods, including subtle back-and-forth stellar wobbles and shadows from planets passing in front of their stars.

Fomalhaut b posed quite a puzzle for the astronomers, however, unlike other directly imaged exoplanets for a couple of reasons. First, the object shone bright in visible light, which is a highly unusual trait for an exoplanet as they are simply much too small to reflect enough light from their host star to be seen from Earth.

Secondly, it did not emit any sort of detectable infrared heat signature, also unusual as a planet — especially a young one — should be warm enough to shine in the infrared. The astronomers have concluded that the brightness must have been caused from a huge shell or ring of dust that may have been related to a collision.

“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.

The research team added that the final nail in the coffin confirming the true identity of Fomalhaut b was when their data analysis of Hubble images taken in 2014 showed the object had vanished. Although it came as a shock, they had noticed that earlier images of this enigma of a planet showed the object to continuously fade over time, they say.

“Clearly, Fomalhaut b was doing things a bona fide planet should not be doing,” Gaspar said.

Gaspar and Rieke said they believe that Fomalhaut b is slowly expanding from the smash-up and suspect the collision occurred not too long prior to the first observations taken in 2004, based on all the available data.

Currently, the debris cloud — consisting of dust particles around 1 micron in size, or about 1/50th the diameter of a human hair — is now below Hubble’s detection limit. It’s estimated to have expanded by now to a size larger than the orbit of Earth around the sun.

Furthermore, the team reports they were again fascinated to find that the object is more likely on an escape path, rather than on an elliptical orbit as expected for planets, based on their later observations to the trajectory plots from earlier data.

“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 presently exists inside a vast ring of icy debris encircling the star Fomalhaut, meaning that colliding bodies would likely be a mixture of ice and dust, like the comets in the Kuiper Belt on the outer fringe of our solar system.

Gaspar and Rieke estimate that each one of these comet-like bodies measures to about 125 miles across, which is roughly half the size of the Vesta asteroid, one of the largest asteroids in our solar system.

The authors add that their model explains all the observed characteristics of Fomalhaut b, and contains sophisticated modeling of how dust moves over time. The model, done on a cluster of computers at the University of Arizona, shows that such a model quantitatively fits all the observations.

According to the findings, the Fomalhaut system may only experience one of these events every 200,000 years.

Moving forward, Gaspar and Rieke’s team will also be observing the Fomalhaut system with NASA’s new James Webb Space Telescope in its first year of science operations.

The team plans on directly imaging the inner warm regions of the system, and for the first time in a star system other than our own they will obtain detailed information about the architecture of Fomalhaut’s elusive asteroid belt. Additionally, the team will be searching for undiscovered planets orbiting Fomalhaut.

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