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Scientists Catch An X-Ray Eclipse Of A Planet Where It Rains Glass

Earlier this month, astronomers using the Hubble Space Telescope studied exoplanet HD 189733b and determined that it was a deep blue color, thanks to silicates in its atmosphere that can create storms akin to raining glass.
Now astronomers using NASA’s Chandra X-ray Observatory and the European Space Agency’s XMM Newton Observatory have managed to catch sight of the planet as it eclipsed its sun using X-rays. This marks the first time that a planet has ever been observed in this manner.
This is actually similar to how HD 189733b was discovered in the first place. In 2005, French astronomers observed the planet transit in front of its star using the normal optical wavelengths that our eyes use. Indeed, most planets outside of our solar system have been discovered in this manner. When a planet transits, it causes part of the star’s brightness to dim as the planet passes in front of the star. Similarly, astronomers at Chandra and Newton viewed the planet pass in front of the star using X-ray wavelengths, noting a visible “dip” in the X-ray intensity of the planet’s star where HD 189733b passed in front of it.
Located about 63 light years from Earth, HD 189733b is about the size of Jupiter, but is much closer to its star – so close that a “year” on the planet is only about 2.2 days. Surface temperatures on the side facing its star reach 2,000 degrees and scientists in 2007 estimated that its winds probably reach up to 4,500 miles per hour.
By observing the transit of the planet using X-rays, the astronomers were able to learn even more about the composition for the planet. First, they discovered that its atmosphere is most likely much larger than originally estimated. They could tell that because the atmosphere of a planet blocks X-rays, but not normal light. In this case, the planet blocked about 2-3 times as big an area of X-rays as it did optical light, meaning the atmosphere extends out quite further. They also ascertained that as a result, the planet is losing its atmosphere at a very fast rate over time.
“That means that the planet’s atmosphere blocks X-rays at altitudes of more than 60,000 km above its optical radius – a 75% larger radius in X-rays!” wrote lead author Katja Poppenhaeger in a blog post. “That is pretty big, and I calculated that the outer atmosphere has to be heated up to about 20,000 K to sustain itself at such high altitudes. The bigger X-ray radius also means that the planet loses its atmosphere about 40% faster than thought before.”
The astronomers’ paper is published in the current issue of the Astrophysical Journal, but they’re not done with X-ray transit observations by a long shot.
“I’d like to know if the X-ray radius of other hot exoplanets is extended as well – and if yes, does the extent depend on the heating of the planet by the host star, or more on things like the overall density of the planet?” she wrote in her blog post. “Together with my former colleagues from Hamburg Observatory, we will test this for the even hotter Jupiter CoRoT-2b, using XMM-Newton. Personally, I’d like it best if the CoRoT-2b experiment had a completely surprising outcome as well – we’ll know next year!”

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