We present a study of plasma conditions in the atmospheres of the Hot Jupiters HD 209458b and HD 189733b and for an HD 209458b-like planet at orbit locations between 0.2-1 AU around a Sun-like star. We discuss how these conditions influence the radio emission we expect from their magnetospheres. We find that the environmental conditions are such that the cyclotron maser instability (CMI), the process responsible for the generation of radio waves at magnetic planets in the solar system, most likely will not operate at Hot Jupiters. Hydrodynamically expanding atmospheres possess extended ionospheres whose plasma densities within the magnetosphere are so large that the plasma frequency is much higher than the cyclotron frequency, which contradicts the condition for the production of radio emission and prevents the escape of radio waves from close-in exoplanets at distances <0.05 AU from a Sun-like host star. The upper atmosphere structure of gas giants around stars similar to the Sun changes between 0.2 and 0.5 AU from the hydrodynamic to a hydrostatic regime and this results in conditions similar to solar system planets with a region of depleted plasma between the exobase and the magnetopause where the plasma frequency can be lower than the cyclotron frequency. In such an environment, a beam of highly energetic electrons accelerated along the field lines towards the planet can produce radio emission. However, even if the CMI could operate the extended ionospheres of Hot Jupiters are too dense to let the radio emission escape from the planets.

We study the origin and escape of catastrophically outgassed volatiles (H2O, CO2) from exomoons with Earth-like densities and masses of 0.1, 0.5 and 1 M⊕ orbiting an extra-solar gas giant inside the habitable zone of a young active solar-like star. We apply a radiation absorption and hydrodynamic upper atmosphere model to the three studied exomoon cases. We model the escape of hydrogen and dragged dissociation products O and C during the activity saturation phase of the young host star. Because the soft X-ray and EUV radiation of the young host star may be up to ~100 times higher compared to today’s solar value during the first 100 Myr after the system’s origin, an exomoon with a mass < 0.25 M⊕ located in the HZ may not be able to keep an atmosphere because of its low gravity. Depending on the spectral type and XUV activity evolution of the host star, exomoons with masses between ~0.25 and 0.5 M⊕ may evolve to Mars-like habitats. More massive bodies with masses >0.5 M⊕, however, may evolve to habitats that are a mixture of Mars-like and Earth-analogue habitats, so that life may originate and evolve at the exomoon’s surface.

We investigate under which conditions supermassive hot Jupiters can sustain source regions for radio emission, and whether this emission could propagate to an observer outside the system. We study Tau Bootis b-like planets (a supermassive hot Jupiter with 5.84 Jupiter masses and 1.06 Jupiter radii), but located at different orbital distances (between its actual orbit of 0.046 AU and 0.2 AU). Due to the strong gravity of such planets and efficient radiative cooling, the upper atmosphere is (almost) hydrostatic and the exobase remains very close to the planet, which makes it a good candidate for radio observations. We expect similar conditions as for Jupiter, i.e. a region between the exobase and the magnetopause that is filled with a depleted plasma density compared with cases where the whole magnetosphere cavity is filled with hydrodynamically outward flowing ionospheric plasma. Thus, unlike classical hot Jupiters like the previously studied planets HD 209458b and HD 189733b, supermassive hot Jupiters should be in general better targets for radio observations.

We investigate the atmospheric and magnetospheric conditions of the massive, close-in exoplanet υ Andromedae b (hereafter ups And b). In particular, we explore whether radio emission can be produced by the Cyclotron Maser Instability (CMI), and whether this emission can escape from its source region. For this, we compare the local cyclotron frequency to the local plasma frequency. The planetary mass has a decisive impact on both of these frequencies: the cyclotron frequency depends on the (mass-dependent) estimate of the planetary magnetic moment, and the plasma frequency is determined by the (gravity-dependent) atmospheric profile. For this reason, the planetary mass is one of the decisive parameters determining whether the CMI can operate efficiently. As the precise planetary mass is unknown in the case of ups And b, we compare the plasma conditions for a range of hypothetical masses of the planet in order to determine at which mass the atmosphere becomes ‘compact’, i.e. is not strongly extended, and thus provides favourable conditions for the CMI. In the case of detected planetary radio emission, this approach can provide a new way to constrain the mass of an exoplanet for which only a minimum mass is known.

Abstract. The magnetospheric phenomenon of non-thermal radio emission is known since the serendipitous discovery of Jupiter as radio planet in 1955, opening the new field of "Planetary Radio Astronomy". Continuous ground-based observations and, in particular, space-borne measurements have meanwhile produced a comprehensive picture of a fascinating research area. Space missions as the Voyagers to the Giant Planets, specifically Voyager 2 further to Uranus and Neptune, Galileo orbiting Jupiter, and now Cassini in orbit around Saturn since July 2004, provide a huge amount of radio data, well embedded in other experiments monitoring space plasmas and magnetic fields. The present paper as a condensation of a presentation at the Kleinheubacher Tagung 2013 in honour of the 100th anniversary of Prof. Karl Rawer, provides an introduction into the generation mechanism of non-thermal planetary radio waves and highlights some new features of planetary radio emission detected in the recent past. As one of the most sophisticated spacecraft, Cassini, now in space for more than 16 years and still in excellent health, enabled for the first time a seasonal overview of the magnetospheric variations and their implications for the generation of radio emission. Presently most puzzling is the seasonally variable rotational modulation of Saturn kilometric radio emission (SKR) as seen by Cassini, compared with early Voyager observations. The cyclotron maser instability is the fundamental mechanism under which generation and sufficient amplification of non-thermal radio emission is most likely. Considering these physical processes, further theoretical investigations have been started to investigate the conditions and possibilities of non-thermal radio emission from exoplanets, from potential radio planets in extrasolar systems.

A study of the plasma conditions in the atmosphere and ionosphere of the Hot Jupiter HD 209458b and for an HD 209458b-like planet at orbit locations of 0.2-1 AU around a Sun-like star is presented. It is discussed how these conditions influence the radio emission expected from the planet's magnetosphere. We find that the cyclotron maser instability (CMI) most likely will not operate at Hot Jupiters. It is found that close-in gas giants possess hydrodynamically expanding atmospheres and extended ionospheres with too high plasma densities within their magnetospheres, i.e. the plasma frequency is much higher than the cyclotron frequency, which is a contradiction to the necessary condition for the production of radio emission and also prevents the escape of radio waves for close-in extrasolar planets at distances <0.05 AU from a Sun-like host star. The structure of the upper atmosphere of Hot Jupiters around stars similar to the Sun changes for orbital distances between 0.2 and 0.5 AU from the hydrodynamic to a hydrostatic regime. This results in conditions where the plasma frequency can be lower than the cyclotron frequency, because a region of depleted plasma between the exobase and magnetopause can form. Like for e.g. Earth, in such an environment a beam of highly energetic electrons can propagate and be accelerated along the field lines towards the planet to produce radio emission. We also investigate the possible radio emission of the Hot Jupiter Tau Bootis b by placing it at different orbital distances from the host star, i.e. 0.046, 0.1 and 0.2 AU. It is checked if the atmosphere of Tau Bootis b