All heavenly bodies have physics and chemistry, and most have geology, but not all have biology  the other great science which we study. By biology, I mean life-supporting, or at least potentially life-supporting. 

The overwhelming majority of objects in the Solar System are clearly not habitable. Most  the asteroids and the smaller moons  are lumps of rock which lack atmosphere or almost anything else conducive to life. Others are positively hostile excessively cold, boiling hot, subject to intense pressures, or bathed in poisonous gases. 

Saturn's largest moon Titan (imaged below) has a hydrocarbon-based atmosphere similar to the primordial atmosphere of our planet  an atmosphere with chemicals essential for the creation of amino acids and other organic molecules. These gases are poisonous to us, but maybe not to all possible life forms. Some scientists now seriously consider that methane-based, hydrogen-breathing life might conceiveably exist on Titan.

Several other moons of the gas planets are known to possess surface water ice, and possibly liquid water beneath the ice. These include Jupiter's moons Ganymede and Callisto, and Saturn's moon Enceladus. Some may even have thin atmospheres containing oxygen. Foremost among these intriguing worlds is probably Jupiter's moon Europa. 

Fly-by missions reveal an icy surface. Astronomers have strong reason to believe that below this seemingly immensely thick ice sheet, there may be an even deeper moon-wide ocean of water, heated by geothermal forces. On our planet complex life can exist far from sunlight around deep sea volcanic vents - it seems the presence of water and heat may be all that is required to sustain life. One can therefore fantasize with some legitimacy that moons such as Europa may embrace macrobiology. 

The implications are exciting, and missions are planned to land and probe deep beneath the ice, though it must be stressed that a lot of speculation is involved here - we do not as yet know anything certain about what lies beneath Europa's surface. 

Mars is almost hospitable. Well, not really, but it's known history of one-time rivers of water, active volcanoes and dense atmosphere certainly make it the prime candidate for life in times gone by. Micro-organisms could possibly have evolved on Mars in its early days, and on our planet microbes are known to have an extraordinary ability to cling on in adverse circumstances. 

If life once existed on Mars, then it is at least conceivable that life may still cling on in places such as deep subterranean soils, or encased in Martian rocks safe from the solar radiation to which the planetary surface is now exposed.

But of course this is one category of 'wonder' in which there is no contest. The only heavenly body known to have any life at all is a planet with millions of species of plants and animals. Inhabiting the so-called Goldilocks zone of the Solar System (not too hot, not too cold - just right). This planet is EARTH.


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