The team leaded by Alain Lecavelier des Etangs (IAP, CNRS-UPMC) and financially supported by CNES used the Hubble telescope to observe the atmosphere of the exoplanet HD 189733b . The planet had already been observed twice when passing in front of its host star like Venus in front of the Sun: first time in April 2010 and then in September 2011. During these transits, the planet's atmosphere put its signature in the star light which was analysed by the Hubble telescope's spectrographs . The astronomers can thus measure the atmosphere's specifications. The observations were performed in order to confirm the team's first observations in another planetary system: evaporation of an exoplanet's atmosphere (heic0303).
This artist view shows the exoplanet HD189733b transiting in front of its host star. The Hubble Space Telescope observed this transit in April 2010 and September 2011. The observation of September 2011 was performed just after a strong stellar eruption (see image) observed by the Swift's X-ray Telescope. After this eruption, Hubble determined that at least 1,000 tones of gas was leaving the planet's atmosphere every second. (On this image, the star surface is inspired from the observations of the Sun performed by the Solar Dynamics Observatory probe).
Discovery of alterations in an exoplanet's atmosphere
The first series of observations obtained in April 2010 did not show any trace of the planet's atmosphere. But when Hubble performed the second series of observations in September 2011, the signature of the atmosphere evaporating clearly appeared.
The spectrum obtained by Hubble showed a plume of gas streaming away from the exoplanet at a rate of not less than 1,000 tons of hydrogen every second. This result was the first detailed observation of temporal variations in the atmosphere of an exoplanet: exo-meteorology application. Double impact: this confirms that giant planets' atmospheres near their host stars can evaporate and that the physical conditions in these evaporating atmospheres vary over time.
Why this alteration?
The observed evaporation was triggered by the energy brought by the star in the form of X-ray and UV radiations. This scenario had been reinforced by the simultaneous observations performed by the NASA's satellite observatory: the Swift's X-ray Telescope . A few hours before Hubble was scheduled to perform the second observation, Swift monitored a blast of X-rays coming from the star surface. The energy brought to the planet by this eruption very likely drove the escape which had been seen a few hours later by Hubble. Even if it is less spectacular, a similar process occurs when a solar flare hits the Earth affecting our radio communications.
If the eruption observed in the X-rays range is most likely to have triggered these alterations of the planet HD 189733b's atmosphere, others explanations are also possible: For example, the X-ray emissions flux of the star may have steadily increased between 2010 and 2011 in a similar way to the Sun's 11-year cycle. Another reason could be that, simultaneously with the stellar eruption, the proton wind coming from the star increased and triggered an acceleration of the outer layers of the planet's atmosphere, making it easier to detect. The pending questions might be answered by the next simultaneous observations of both Hubble and the ESA's X-ray observatory, XMM-Newton. However, a stellar eruption did hit the planet and the physical conditions of the planet's upper atmosphere drastically changed at such point that the escape of the atmosphere's uppest layers could be observed by Hubble.
More generally, this research bought important information not only on the giant planets orbiting their stars but also on the smaller planets like the Earth. In fact, the rocky "super-Earths" discovered near their host stars such as CoRoT-7b or Kepler-10b could be remnants of giant planets' evaporation like HD189733b. These planets, initially giant gaseous planets, would have been too close from their stars to withstand a catastrophic evaporation and would have lost the major part of their gaseous envelope, only leaving their solid central nuclei .
 The planet HD 189733b is an "hot Jupiter" exoplanet. It is orbiting HD 189733, located at approximately 60 light years away from Earth. The hot Jupiters are giant gaseous planets orbiting very close from their parent stars. HD 189733b is very close from its host star, about 30 times closer than Earth's distance from the Sun and that is why the temperature is exceeding 1,000°C. Even Mercury, the closest planet from the Sun, is 10 times farer. HD189733b circles its host star in 53 hours. It is approximately 10% more massive and bigger than Jupiter. The atmosphere of HD189733b is cloudy and mainly composed of hydrogen. The observations performed in the visible range showed that, like on Earth, the atmosphere of this planet diffuses more in the short wavelengths and so the sky is blue.
 This observation method for exoplanets is called the transit method because it takes advantage of the fact that observed from Earth, the planet transits in front of its parent star's disk. Only a few exoplanets can be studied using the transit method because their orbits have to be viewed exactly from the side. Nevertheless, for these planets, the transit observation is an extremely powerful tool to characterize the exoplanets.
The observations were performed with the Hubble Space Telescope Imaging Spectrograph (STIS), an instrument which divides the light into its primary colours, like a prism. The brightnesses of the different wavelengths are analysed like fingerprints of the chemical compositions. The light keeps the signature of the physical characteristics and even motions of the gas it crosses. In that case, the observations performed with the Hubble telescope consisted in observing the hydrogen which was streaming away from the exoplanet's atmosphere.
 The Swift satellite is an international observatory designed by NASA, the UK Science and Technology Facilies Council and the Italian space agency (ASI). Its main goal was to detect and study the gamma bursts but its X-ray and UV telescopes are also used for other astronomic observations.
 The so called "Super-Earths" are rocky exoplanets with masses of several times the Earth's. The Super-Earths in the habitable area of their stars (where the temperature allows water to be liquid) are considered to be good candidates for life seeking. The exoplanets Kepler-10b and CoRoT-7b are considered to be super-Earths even if they are much too close from their stars to host liquid water. They are probably planet's remnants (rocky nuclei) similar to HD 189733b the atmospheres of which were entirely evaporated.
A. Lecavelier des Etangs12, V. Bourrier12, P.J. Wheatley3, H. Dupuy12, D. Ehrenreich4, A. Vidal-Madjar12, G. Hébrard12, G.E. Ballester5, J.-M. Désert6, R. Ferlet12, D.K. Sing7, (2012), Temporal variations in the evaporating atmosphere of the exoplanet HD 189733b, A&A Letters, Earth and Planetary Astrophysics.
1 CNRS, Institut d'Astrophysique de Paris, 98 bis boulevard Arago, Paris, France
2 Université Paris 6, Institut d'Astrophysique de Paris, 98 bis boulevard Arago, Paris, France
3 Department of Physics, University of Warwick, Coventry CV4 7AL, UK
4 UJF-Grenoble, Institut de Planétologie et d'Astrophysique de Grenoble (IPAG), Grenoble, France
5 Lunar and Planetary Laboratory, university of Arizona, 1541 E Université Blvd, Tucson, AZ 85721-0063, USA
6 Harvard-Smithsoninan Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
7 Astrophysics Group, School of Physics, University of Exeter, Stocker Road, Exeter EX4 4QL, UK
- Scientific contact : Alain Lecavelier des Etangs
- Astrophysics program scientist at CNES: Olivier La Marle