Astronomers have for the first time discovered an exoplanet passing in front of its parent star.
An advantageous alignment of a planet and its parent star in the system HD 189733, which is 63 light-years
from Earth, enabled NASA's Chandra X-ray Observatory and the European Space Agency's XMM Newton
Observatory to observe a dip in X-ray intensity as the planet transited the star.
"Thousands of planet candidates have been seen to transit in only optical light," said Katja Poppenhaeger
of Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Mass., who led a new study.
Click on image to enlarge
This graphic depicts HD 189733b, the first exoplanet caught passing in front of its parent star in X-rays.
Image Credit: X-ray: NASA/CXC/SAO/K.Poppenhaeger et al; Illustration: NASA
"Finally being able to study one in X-rays is important because it reveals new information about the properties
of an exoplanet."
The team used Chandra to observe six transits and data from XMM Newton observations of one.
The planet, known as HD 189733b, is a hot Jupiter, meaning it is similar in size to Jupiter in our
solar system but in very close orbit around its star. HD 189733b is more than 30 times closer to its
star than Earth is to the sun. It orbits the star once every 2.2 days.
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.
They have used NASA's Kepler space telescope to study it at optical wavelengths, and NASA's Hubble Space
Telescope to confirm it is blue in color as a result of the preferential scattering of blue light by silicate
particles in its atmosphere.
The study with Chandra and XMM Newton has revealed clues to the size of the planet's atmosphere. The
spacecraft saw light decreasing during the transits. The decrease in X-ray light was three times greater
than the corresponding decrease in optical light.
"The X-ray data suggest there are extended layers of the planet's atmosphere that are transparent to optical
light but opaque to X-rays," said co-author Jurgen Schmitt of Hamburger Sternwarte in Hamburg, Germany.
"However, we need more data to confirm this idea."
The researchers estimate it is losing 100 million to 600 million kilograms of mass per second.
HD 189733b's atmosphere appears to be thinning 25 percent to 65 percent faster than it would be if
the planet's atmosphere were smaller.
"The extended atmosphere of this planet makes it a bigger target for high-energy radiation from its star,
so more evaporation occurs," said co-author Scott Wolk, also of CfA.
"This star is not acting its age, and having a big planet as a companion may be the explanation,"
"It's possible this hot Jupiter is keeping the star's rotation and magnetic activity high because of
tidal forces, making it behave in some ways like a much younger star."
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