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15 years ago the Hubble Space Telescope captured a planet orbiting a star 25 light-years away. However, he was absent from the photos taken 10 years later. Now astronomers have solved the puzzle of the missing exoplanet.
In 2004 and 2006, the Hubble Space Telescope captured a planet orbiting a star called Fomalhaut 25 light years away. The planet was well detectable in visible light, which was extremely rare because exoplanets are often too small and weak to be seen.
Anyway, the object, named Fomalhaut b or Dagon, was announced in 2008, and confirmed in 2012. It was detected to be a gas giant in a highly elliptical 1,700-year orbit around its host star.
Examining the unreleased Hubble images taken in 2014, the scientists were highly surprised, because Dagon disappeared entirely.
Dagon, a troublesome exoplanet
From the start, identifying Dagon as an exoplanet posed a problem.
Fomalhaut is a very young star, about 440 million years old, and is still surrounded by an icy ring of dust and gas, the remains of a circumstellar disk. That means that any planet orbiting the star should also be quite young, and therefore warm, emitting infrared radiation.
However, no infrared radiation was detected issued by Dagon.
Furthermore, it was unusually bright at blue optical wavelengths, which is not consistent with our planet formation models.
To explain these peculiarities, astronomers decided that the planet was covered by a huge ring or cloud of dust perhaps as a result of collisions with other objects. Some proposed that Dagon could be a neutron star.
However, none of these explanations was satisfactory. Plus there was another problem: Dagon’s orbit crossed the ring of debris around the star without disturbing it gravitationally, like a planet should do.
The planet that never existed
Astronomers from the University of Arizona (USA) analyzed all available Hubble data on Dagon.
“Our study revealed several features that together paint a picture that the planetary-sized object may never have existed in the first place,” said astronomer Andras Gaspar.
When they discovered the astonishing absence of Dagon in the 2014 Hubble data, Gaspar and his colleague, astronomer George Rieke, reexamined the above observations.
The researchers found that the object appears to have vanished over time.
“Clearly, Fomalhaut b was doing things that a planet in good faith should not be doing,” said Gaspar.
Astronomers concluded that the bright spot visible in the first Hubble images was a sequel to a collision between two celestial bodies the size of an asteroid.
“These collisions are extremely rare and therefore it is quite difficult for us to see one,” Gaspar said.
Based on their analysis of the data, Gaspar and Rieke believe that the collision occurred not long before the first Hubble image in 2004. The two objects would each have been around 200 kilometers in diameter, and would likely have been composed of rock and ice, like comets in the solar system.
The two objects came together in a colossal collision that was briefly visible, but has expanded and dissipated over time. The particles are now too small to be collected by Hubble.
The study was published in PNAS.
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