Of all 160 or so planets and moons in the solar system, Mars has so-far received most interest from astrobiologists. There are many signs that very early in its history Mars was much like Earth, with large areas of liquid water, a thick atmosphere, ample sources of energy for life and a good inventory of organic molecules.

But the surface of Mars is today a bitterly cold desert, it’s atmosphere largely stripped away. Did life ever get a foot-hold on the planet, could cells be eking out a living deep underground even today, or has it all fallen extinct leaving only scattered biosignatures as traces of its prior existence? The only way we’ll ever know is by sending probes and eventually human explorers to find out…

We are unlikely to ever find hints whether Venus hosted life before the ancient catastrophe of run-away global warming, but there is a chance life continues high up in the clouds banks. Here the atmosphere is cool enough for droplets of liquid water, and there is the possibility they support bugs soaking up the sun’s rays.

Another piece of real estate in the solar system that astrobiologists are getting more and more excited about is Europa, the second moon of Jupiter. There is good evidence that this enigmatic moon hides a great ocean of liquid water beneath its icy surface. One problem facing life on Europa, however, is that sources of energy might be severely limited. Perhaps Europan microbes thrive off gases bubbling out of the crust, or hydrothermal vents like on Earth’s sea floor, or even nutrients created in the surface ice by ionising particles and subducted down into the ocean – ecosystems living not off sunlight like on Earth but indirectly eating space radiation.

A fourth possible habitat for extraterrestrial life is Titan, the huge moon of Saturn. Titan is the only moon in the solar system with a thick atmosphere, but it is still far from the sun and too cold for liquid water on the surface. Instead, hard-frozen water builds the hills and rocks seen by the Huygens probe, and clouds, lakes and rivers appear to be made up of methane. Could chemistry become complex enough for life in such a frigid environment, wet with methane instead of water? Another possibility is microbes within pools of water at geothermal hotspots - volcanoes of molten water rather than silicate rock as on the hotter Earth.