Hubble Telescope Reveals the Evolution and Collision of Exoplanets

A planet believed to be outside of the solar system now appears to have disappeared from sight. Now, according to astronomers, a fully developed planet had actually never existed.

Illustration from the Hubble Space Telescope’s observations of Fomalhaut b’s expanding dust cloud from 2004 to 2013. The cloud was produced in a collision between two large bodies orbiting the bright nearby star Fomalhaut. This is the first time such a catastrophic event around another star has been imaged. Image Credit: NASA, ESA, and A. Gáspár and G. Rieke (University of Arizona).

Instead, the NASA/ESA Hubble Space Telescope had visualized a growing cloud of extremely fine dust particles induced by a massive collision between a pair of icy asteroid-sized bodies that revolve around Fomalhaut—a bright star located approximately 25 light-years from Earth.

The Fomalhaut system is the ultimate test lab for all of our ideas about how exoplanets and star systems evolve. We do have evidence of such collisions in other systems, but none of this magnitude has ever been observed. This is a blueprint for how planets destroy each other.

George Rieke, Steward Observatory, University of Arizona

Earlier, it was assumed that the object was a planet, known as Fomalhaut b, and was initially reported in 2008 on the basis of the data collected in 2004 and then in 2006. Many years of Hubble observations clearly showed that this planet is a moving speck.

Different from other exoplanets that were imaged directly, the Fomalhaut b planet continued to puzzle the astronomers from the very outset. While the object was extraordinarily bright in visible light, it lacked any perceptible infrared heat signature.

Astronomers suggested that the superfluous brightness emerged from a large ring or shell of dust orbiting the object that could have been associated with a collision. Moreover, Hubble observations made in the past indicated that the object might not be following an elliptical orbit, as often done by planets.

These collisions are exceedingly rare and so this is a big deal that we actually get to see one. We believe that we were at the right place at the right time to have witnessed such an unlikely event with the Hubble Space Telescope.

András Gáspár, Assistant Astronomer, Steward Observatory, University of Arizona

Gáspár continued, “Our study, which analysed all available archival Hubble data on Fomalhaut b, including the most recent images taken by Hubble, revealed several characteristics that together paint a picture that the planet-sized object may never have existed in the first place.”

But to the astronomers’ surprise, the Hubble images taken in 2014 revealed that the object had disappeared completely. This mystery became more complicated when previous images revealed that the object continued to fade over time.

Clearly, Fomalhaut b was doing things a bona fide planet should not be doing,” added Gáspár.

The ensuing understanding is that Fomalhaut b is not a planet, but rather a gradually growing cloud that exploded into space because of a collision between a pair of massive bodies. According to the scientists, that collision took place soon before the initial observations taken earlier in 2004.

Currently, the debris cloud, comprising dust particles measuring about 1 µm (that is, 1/50th the diameter of a single strand of human hair), is below the detection limit of the Hubble Space Telescope. It has been estimated that the dust cloud has now grown to a size that is bigger than the Earth’s orbit around the Sun.

Another aspect that was equally confusing is that the object is not following an elliptical orbit, as predicted for planets, but on a hyperbolic path or an escape trajectory.

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

András Gáspár, Assistant Astronomer, Steward Observatory, University of Arizona

Since the Fomalhaut b object is currently within a large ring of icy debris orbiting the star, the colliding bodies could probably be a combination of dust and ice, similar to the comet-like bodies existing in the Kuiper belt on the external border of the solar system.

Both Gáspár and Rieke have estimated that all these comet-like bodies have a diameter of approximately 200 km. The duo also proposed that the Fomalhaut system may go through one of these collision events once in 200 000 years.

Using the forthcoming NASA/ESA/CSA James Webb Space Telescope, Gáspár, Rieke, and other astronomers will also be visualizing the Fomalhaut system. The James Webb Space Telescope is set to be launched in 2021.


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