Exoplanet HD 189733b
This graphic depicts HD 189733b, the first exoplanet caught passing in front
of its parent star in X-rays. As
described in our press release , NASA’s Chandra X-ray Observatory and the
European Space Agency’s XMM Newton Observatory have been used to observe a dip
in X-ray intensity as HD 189733b transits its parent star.
The main figure is an artist’s impression showing the HD 189733 system,
containing a Sun-like star orbited by HD 189733b, an exoplanet about the size of
Jupiter. This “hot Jupiter” is over 30 times closer to its star than Earth is to
the Sun and goes around the star once every 2.2 days, as determined from
previous observations. Also in the illustration is a faint red companion star,
which was detected for the first time in X-rays with these observations. This
star orbits the main star about once every 3,200 years.
The inset contains the Chandra image of HD 189733. The source in the middle
is the main star and the source in the lower right is the faint companion star.
The source at the bottom of the image is a background object not contained in
the HD 189733 system.
The exoplanet itself cannot be seen in the Chandra image, as the transits
involve measuring small decreases in X-ray emission from the main star. The
authors estimate that the percentage decrease in X-ray light during the transits
is about three times greater than the corresponding decrease in optical light.
This tells them that the region blocking X-rays from the star is substantially
larger than the region blocking optical light from the star, helping to
determine the size of the planet's atmosphere. The extended atmosphere implied
by these results is shown by the light blue color around the planet. Recent
observations of HD 189733b with the Hubble Space Telescope have confirmed that
the lower atmosphere of the planet has a deep blue color, due to the
preferential scattering of blue light by silicate particles in its
atmosphere.
For about a decade astronomers have known that ultraviolet and X-ray
radiation from the main star in HD 189733 are evaporating the atmosphere of its
closely orbiting planet over time. The authors of the new study estimate that HD
189733b is losing between 100 million and 600 million kilograms per second. This
rate is about 25% to 65% higher than it would be if the planet's atmosphere were
not extended.
At a distance of just 63 light years, HD 189733b is the closest hot Jupiter
to Earth, which makes it a prime target for astronomers who want to learn more
about this type of exoplanet and the atmosphere around it.
Chandra was used to make observations of six transits by HD 189733b and the
team also used archival data from XMM-Newton for one transit. These results are
available online and will appear in
the August 10th issue of The Astrophysical Journal.
Credit: X-ray: NASA/CXC/SAO/K. Poppenhaeger et al;
Illustration: NASA/CXC/M. Weiss
› View large image:
http://chandra.harvard.edu/photo/2013/hd189733/
› Chandra on Flickr:
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NASA
Guillermo Gonzalo Sánchez Achutegui
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