CXC: HD 189733: Chandra Sees Eclipsing Planet in X-rays

[bystander]

HD 189733: NASA's Chandra Sees Eclipsing Planet in X-rays for First Time
NASA | Marshall | SAO | Chandra X-ray Observatory | 2013 Jul 29

Exoplanet HD 189733b (NASA/CXC)
X-ray: NASA/CXC/SAO/K.Poppenhaeger et al; Illustration: NASA/CXC/M.Weiss

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For the first time since exoplanets, or planets around stars other than the sun, were discovered almost 20 years ago, X-ray observations have detected 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 to be published in the Aug. 10 edition of The Astrophysical Journal. "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 also are 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 atmosphere of HD 189733b over time. The authors 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.

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.

"This star is not acting its age, and having a big planet as a companion may be the explanation," said Poppenhaeger. "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."

Transit observations of the Hot Jupiter HD 189733b at X-ray wavelengths - K. Poppenhaeger, J.H.M.M. Schmitt, S.J. Wolk

• Astrophysical Journal 773(1)62 (2013 Aug 10) DOI: 10.1088/0004-637X/773/1/62
  arXiv.org > astro-ph > arXiv:1306.2311 > 10 Jun 2013

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[bystander]

The First Detection of a Transiting Exoplanet in the X-rays
Smithsonian Astrophysical Observatory
Weekly Science Update | 2013 Aug 23

About 940 confirmed exoplanets (planets around stars other than our Sun) have been discovered since the first one was detected over ten years ago. Researchers have been working steadily ever since to characterize and model the physical properties of these bodies. Since many of these exoplanets are found close to their host stars (unlike planets in the solar system), they are subject to strong stellar irradiation that can affect their atmospheres -- if they have one. Models predict that in some cases the incident stellar flux can deposit enough energy into the planet's atmosphere to swell it in size, or even cause it to evaporate. The process is thought to be driven by X-ray and extreme-UV radiation, although astronomers disagree on the details.

A so-called "hot Jupiter" is an exoplanet that is about the size of Jupiter and orbits its host star at a very much closer distance than Mercury is to the Sun in our solar system. Not surprisingly, then, the first observational evidence for extended planetary atmospheres -- and their possible evaporation -- was found for hot Jupiters. One hot Jupiter, HD 189733b, is a prime candidate for studying exoplanet atmospheres because it is relatively close to us, only sixty-four light-years away. With a mass of about 1.138 Jupiters, it orbits its star once every 2.22 days.

CfA astronomers Katja Poppenhaeger and Scott Wolk, and a colleague, used the Chandra X-ray Observatory and the XMM satellite to monitor the transits of HD 189733b at X-ray wavelengths. In the optical, the transit of this exoplanet across the face of its host star causes a dip in the starlight of 2.41%. To their surprise, the astronomers found that in the X-ray band the transit caused a dip in the light three to four times larger than this. It is possible that the cause of this large decrease is that the host star has an X-ray emitting hot spot that is smaller than the optical disk, but the scientists, after exploring this and other possibilities, conclude that the alternative is more likely. The atmosphere of the exoplanet is swollen due to the intense radiation; although it is transparent in the optical and plays little role in the optical transit depth, it is opaque at X-rays and so blocks relatively more of the star's X-ray emission. The discovery improves our understanding of how exoplanetary atmospheres are heated, and paves the way for future X-ray research into the ways in which a planet's host star can affect its atmosphere.

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[Ann]

So blue planet HD 189733b sparkles in X-rays, too! How appropriate. This is a short-wave planet!

Ann

[Beyond]

Short wave

[Ann]

It's a royal planet, then! Kings and queens and people like that usually make small waves!

Click to play embedded YouTube video.

Ann