J. D. Harrington
Headquarters, Washington
202-358-5241
j.d.harrington@nasa.gov
Lynn Chandler
NASA Goddard Space Flight Center,
Greenbelt, Md.
301-286-2806
lynn.chandler-1@nasa.gov
WASHINGTON -- An international team of
astronomers using data from NASA's Hubble Space Telescope has made an
unparalleled observation, detecting significant changes in the atmosphere of a
planet located beyond our solar system.
The scientists conclude 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," said lead
researcher Alain Lecavelier des Etangs at the Paris Institute of Astrophysics
(IAP), part of the French National Scientific Research Center located at Pierre
and Marie Curie University in Paris.
The exoplanet is HD 189733b, a gas giant
similar to Jupiter, but about 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 classify the planet as a
"hot Jupiter." Previous Hubble observations show that 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, and these events give astronomers an opportunity
to probe its atmosphere and environment. In a previous study, a group led by
Lecavelier des Etangs used Hubble to show that hydrogen gas was escaping 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 famous Dumbbell
Nebula. This makes HD 189733b an ideal target for studying the processes that
drive atmospheric escape.
"Astronomers have been debating the
details of atmospheric evaporation for years, and studying HD 189733b is our
best opportunity for understanding the process," said Vincent Bourrier, a
doctoral student at IAP and a team member on the new study.
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 they 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. The
hydrogen atoms were racing away at speeds greater than 300,000 mph. The
findings will appear in an upcoming issue of the journal Astronomy &
Astrophysics.
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 co-author 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.
Hubble is a project of international
cooperation between NASA and the European Space Agency. Swift is operated in
collaboration with several U.S. institutions and partners in the United
Kingdom, Italy, Germany and Japan. NASA's Goddard Space Flight Center in
Greenbelt, Md., manages both missions.
For images and video related to this
finding, visit http://go.nasa.gov/Osbvfi.
For more information about Swift, visit http://www.nasa.gov/swift.
For more information about Hubble, visit
http://www.nasa.gov/hubble.
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