The Incredible Evaporating Planet

There is a star so close to its parent star that its heated atmosphere is expanding away into space. Some astronomers studying this distant planetary system now believe they have detected water vapor among the gases being liberated.  Planet HD 209458b may be evaporating.

In 2003–4, astronomers used the Hubble Space Telescope Imaging Spectrograph to discover an enormous ellipsoidal envelope of hydrogen, carbon and oxygen around the planet. The hydrogen exosphere extended far out into space giving rise to a significant ‘tail’ of atoms moving at speeds greater than the escape velocity. The planet is estimated to be losing about 100–500 million (1–5×108) kg of hydrogen per second.

Analysis of the starlight passing through the envelope shows that the heavier carbon and oxygen atoms are being blown from the planet by the extreme “hydrodynamic drag” created by its evaporating hydrogen atmosphere. The hydrogen tail streaming from the planet is approximately 200,000 kilometres long, which is roughly equivalent to its diameter.

It is thought that this type of atmosphere loss may be common to all planets orbiting Sun-like stars closer than around 0.1 astronomical units (a measure of distance equal to the distance between our Sun and the Earth). HD 209458 b will not evaporate entirely, although it may have lost up to about 7% of its mass over its estimated lifetime of 5 billion years. It may be possible that the planet’s magnetic field may prevent this loss, as the exosphere would become ionized by the star, and the magnetic field would contain the ions from loss.

This controversial claim, if true, would mark the first instance of planetary water beyond our solar system, and indicate anew that life might be sustainable elsewhere in the universe. Although spectroscopic observations from the Hubble Space Telescope are the basis for the water detection claim, the planetary system is too small and faint to image. The image is an artist’s concept of the HD 209458b system.

Image Credit: NASA, European Space Agency, Alfred Vidal-Madjar (Institut d’Astrophysique de Paris, CNRS)

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