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.
"Transit observations of the hot Jupiter HD 189733b at x-ray wavelengths," K. Poppenhaeger, J. H. M. M. Schmitt, and S. J. Wolk, ApJ 773, 62, 2013.