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Hot planet grows a tail
Posted: 16 July 2010

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A planet with a tail like a comet is slowly evaporating in the face of a wind of radiation from its parent star, according to brand new observations from the Hubble Space Telescope that are published in the latest issue of The Astrophysical Journal.

An artist’s impression of HD 209458b and its tail. Image: Alfred Vidal-Madjar (Institut d’Astrophysique de Paris)/NASA.

The gas giant known as HD 209458b, which is 153 light years from us in the constellation Pegasus, was the first ever planet to be discovered by the transit method back in 1999, when astronomers spotted its star’s light dimming as the planet passed in front of it. The planet, which is only seven million kilometres from its star and orbits once every three-and-a-half days, was then observed in 2003 by the Hubble Space Telescope’s imaging spectrograph (STIS), which saw a bloated hydrogen atmosphere that extends over 200,000 kilometres. Last month, it was announced that a giant storm with winds racing from the dayside to the night-side at speeds reaching up to 10,000 kilometres per hour had been discovered (read our news story here). Now, using the higher resolution Cosmic Origins Spectrograph on Hubble, it has been found that this scorched planet is even more interesting with the news that its atmosphere isn’t merely puffed up, but is being ripped away by its star’s stellar wind of charged particles, much like a comet’s tail.

An animation showing the ‘comet-planet’ in orbit. Video: ESA/NASA.

“Since 2003 scientists have theorised that the lost mass [from the atmosphere] is being pushed back into a tail, and they have even calculated what it looks like,” says Jeffrey Linsky of the University of Colorado, Boulder, who led the new Hubble study. “We have measured gas coming off the planet at specific speeds.”

As much as 10,000 tonnes of gas could be escaping every second, some of it rushing away from the planet at 9.8 kilometres per second, or 35,400 kilometres per hour. Even at this rate it would take a trillion years for the planet, which has almost two-thirds of the mass of Jupiter, to evaporate says Linsky.

The location of the star HD 209458, which is a seventh magnitude star in Pegasus that hosts the ‘comet-planet’. Image: ESA/NASA/Digitised Sky Survey.

Ordinarily the planet would block 1.5 percent of its star’s light as it transits, but with the bloated atmosphere and tail this increases to eight percent. As HD 209458b passes in front of its star, starlight flickers through the more tenuous regions of its atmosphere,being absorbed by chemical elements and molecules within the atmosphere. In the past oxygen, carbon, sodium and even water molecules have been detected, and Hubble has now even found silicon atoms in the 1,090 degrees Celsius atmosphere. As the star heats the atmosphere and bloats it, it dredges up material from deep within the planet. Hence it provides a unique way to probe not only the upper atmosphere of this swollen world, but what materials also lurk deeper within it.