An international team of astronomers using NASA’s Chandra X-ray Observatory has observed six transits of an exoplanet called HD 189733b, the closest hot Jupiter to our Solar System.
This artist’s impression shows HD 189733b, a gas giant that goes around its star once every 2.2 days. The inset shows HD 189733, its faint red companion and HD 189733b in X-rays (Illustration: NASA / CXC / M.Weiss; X-ray: NASA / CXC / SAO / K.Poppenhaeger et al)
Its parent star, HD 189733, is located in the constellation of Vulpecula approximately 63 light-years away. The planet is a huge gas giant. It is similar in size to our Jupiter but in very close orbit around the star – 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. Earlier this month, another team for the first time determined the true color of HD 189733b.
“Thousands of planet candidates have been seen to transit in only optical light. Finally being able to study one in X-rays is important because it reveals new information about the properties of an exoplanets,” said Dr Katja Poppenhaeger from Harvard-Smithsonian Center for Astrophysics, lead author of a paper accepted for publication in the Astrophysical Journal (arXiv.org preprint).
The study reveals clues to the size of the planet’s atmosphere. The team detected 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. However, we need more data to confirm this idea,” explained study co-author Dr Jurgen Schmitt of Hamburger Sternwarte.
The team also is learning about how the planet and the star can affect one another. Astronomers have known for about a decade ultraviolet and X-ray radiation from the main star in HD 189733 are evaporating the planet’s atmosphere over time. They estimate it is losing 100 million to 600 million kilograms of mass per second. The planet’s atmosphere appears to be thinning 25 percent to 65 percent faster than it would be if the 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 study co-author Dr Scott Wolk, also from Harvard-Smithsonian Center for Astrophysics.
The main star in HD 189733 also has a faint red companion, detected for the first time in X-rays with Chandra. The stars likely formed at the same time, but the main star appears to be 3 billion to 3 1/2 billion years younger than its companion star because it rotates faster, displays higher levels of magnetic activity and is about 30 times brighter in X-rays than its companion.
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Bibliographic information: Poppenhaeger K et al. Transit observations of the Hot Jupiter HD 189733b at X-ray wavelengths. ApJ, accepted for publication; arXiv: 1306.2311
