Water on Extrasolar Planets

The detection of atmospheres on extrasolar planet is a very difficult task. 71% of the Earth is covered by oceans but up to now it is the only planet with water in liquid form on its surface. Venus might have had water on its early history, on Mars water may exist in a frozen state near the surface and climatic changes have occurred and formed river-like structures that are observed on its surface. There exists the possibility to find condensed water in the atmospheres of Jupiter and Saturn and in deeper layers of Uranus and Neptune. Subsurface oceans may exist on several satellites of the giant planets. But how can we detect water on extrasolar planets, how can we detect whether these objects have even an atmosphere ... 

HZ is defined as the orbital area around a star where a planet can sustain liquid water at the surface. Several studies have attempted to define the HZ as a function of stellar type (e.g. F, G, K, M dwarf stars) and time (see e.g. 46, 47, and references therein). The HZ is of interest because it is widely believed that liquid water is necessary for the genesis of (recognizable) life. The particular emphasis of the planned space missions is to search for signs of life on extrasolar Earth-like planets via spectroscopy. Atmospheric compounds such as O2, O3, N2O, CH4, and CH3CI are considered biomarkers, and their spectroscopic detection in a terrestrial-type atmosphere, particularly O2 or O3 found together with a reduced gas such as CH4, would suggest life (48, 49). Detection of CO2 would indicate that the planet is indeed a terrestrial-type planet... 

Water is one of the basic elements for life. It is even assumed that the evolution of life is only possible if there is liquid water present. A water molecule has some remarkable properties that make it quite unique in the universe. In the first chapter of this book we will review these basic properties of water and the role of water on Earth. All ancient civilizations realized the importance of water and their cities were constructed near great reservoirs of water. But is water unique on Earth Do we find water elsewhere in the solar system, on extrasolar planetary systems or in distant galaxies We will start the search for the presence of extraterrestrial water in our solar system. Surprisingly enough it seems that water in some form and sometimes in only minute quantities is found on any object in the solar system. Even on the planet nearest to the Sun, Mercury, there may be some water in the form of ice near its poles where never the light of Sun heats the surface. And there are objects in the solar system that are made up of a large quantity of water in terms of their mass, like comets and several satellites of the giant planets. 

Now, since the first extrasolar planetary systems have been detected, the search for water on such objects has just started. Because from observations it is very difficult to measure the spectroscopic signatures of the atmospheres of such planets, we have to wait for the newly planned observational facilities (both in space and on ground) some of them will be in operation very soon. 

Atmospheres of extraterrestrial planets can be directly studied during a transit. Two transit events with the Near Infrared Camera and Multi Object Spectrograph (NIC-MOS) camera on the Hubble Space Telescope (HST) were observed for the object GJ 436 b. In order to detect the atmosphere, high-cadence time series of prism spectra covering the 1.1-1.9 pm spectral range were analyzed (Pont et al., 2009 [266]). This object is an extrasolar hot Neptune. The authors measured a flat transmission spectrum at the level of a few parts per 10000 in flux, with no significant signal in the 1.4 pm water band. 

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