NASA's Hubble detects an "evaporating" exoplanet

  • An international team of astronomers have positively identified significant changes in the atmosphere of a planet located beyond our solar system. 

    Based on data from NASA's Hubble space telescope, the scientists hypothesize the atmospheric variations occurred in response to a powerful eruption on the planet's host star, an event observed by NASA's Swift satellite.

    "The multiwavelength coverage by Hubble and Swift has given us an unprecedented view of the interaction between a flare on an active star and the atmosphere of a giant planet," explained Alain Lecavelier des Etangs of the Paris Institute of Astrophysics (IAP).

    The above-mentioned exoplanet - dubbed HD 189733b - is a gas giant similar to Jupiter, but approximately 14 percent larger and more massive. The planet circles its star at a distance of only 3 million miles, or about 30 times closer than Earth's distance from the sun, and completes an orbit every 2.2 days. Its star, named HD 189733A, is about 80 percent the size and mass of our sun.

    Astronomers currently classify the planet as a "hot Jupiter." Previous Hubble observations indicate the planet's deep atmosphere reaches a temperature of about 1,900 degrees Fahrenheit (1,030 C).

    HD 189733b periodically passes across, or transits, its parent star, giving astronomers an opportunity to probe its atmosphere and environment. In a previous study, a group led by Lecavelier des Etangs used data captured by Hubble to illustrate how hydrogen gas escaped from the planet's upper atmosphere. The finding made HD 189733b only the second-known "evaporating" exoplanet at the time.

    The system is just 63 light-years away, so close that its star can be seen with binoculars near the Dumbbell Nebula. This makes HD 189733b an ideal target for studying the processes that drive atmospheric escape. Indeed, when HD 189733b transits its star, some of the star's light passes through the planet's atmosphere. This interaction imprints information on the composition and motion of the planet's atmosphere into the star's light.

    In April 2010, the researchers observed a single transit using Hubble's Space Telescope Imaging Spectrograph (STIS), but detected no trace of the planet's atmosphere. Follow-up STIS observations in September 2011 showed a surprising reversal, with striking evidence that a plume of gas was streaming away from the exoplanet.

    The researchers determined that at least 1,000 tons of gas was leaving the planet's atmosphere every second, with hydrogen atoms racing away at speeds greater than 300,000 mph.

    Because X-rays and extreme ultraviolet starlight heat the planet's atmosphere and likely drive its escape, the team also monitored the star with Swift's X-ray Telescope (XRT). On Sept. 7, 2011, just eight hours before Hubble was scheduled to observe the transit, Swift was monitoring the star when it unleashed a powerful flare. It brightened by 3.6 times in X-rays, a spike occurring atop emission levels that already were greater than the sun's.

    "The planet's close proximity to the star means it was struck by a blast of X-rays tens of thousands of times stronger than the Earth suffers even during an X-class solar flare, the strongest category," said  Peter Wheatley, a physicist at the University of Warwick in England.

    After accounting for the planet's enormous size, the team notes that HD 189733b encountered about 3 million times as many X-rays as Earth receives from a solar flare at the threshold of the X class.

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