Organic molecules contain carbon, which is a basic building block for life as we know it. Observations of the Interstellar Medium show that organic molecules, such as precursors to simple amino acids, are present in space. Organic molecules, including an amino acid, have also been found in comets and meteorites. It is very likely that such molecules were already present in the gas and dust from which our Solar System was formed.

While most life on Earth is sensitive to environmental conditions, some organisms, the extremophiles, have been found to survive in extreme conditions showing that life can exist where it is least expected. These organisms can be very resistant to a wide range of temperatures, pressures, pH and radiation exposure. Some of them live in places such as deserts, poles, deep inside the ocean, inside the crust or even in volcanoes. One of the most resilient organisms known can survive in vacuum conditions. These facts are cautious grounds for optimism when it comes to the possibility of life on other planets or moons, which often present comparatively harsh environmental conditions.

Liquid water is a key factor for the development of life as we know it. For this reason, the search for liquid water on other planets and their moons has been an important goal in the search for extraterrestrial life. Over the years, potential traces of liquid water were found on the surface of Mars, adding to the long-standing debate about its existence on this planet. Although evidence for the current presence of liquid water on Mars is strongly debated, potential traces provide support to the idea that simple life forms may have existed. If there is currently liquid water deep beneath the surface of Mars, there is the potential for life to exist.

Among the many moons orbiting the giant planets of the Solar System, some share characteristics with terrestrial planets, such as dense atmospheres and volcanic activity. Europa, one of the largest moons of Jupiter, has a frozen surface that might cover a liquid ocean. Scientists believe that this ocean might provide the right conditions for simple life forms to exist. Another candidate to host simple life is Titan, the largest moon of Saturn. Titan is rich in complex organic compounds, has a dense atmosphere, liquid methane on the surface, and has been hypothesised to feature a subsurface water ocean.

Since the discovery of the first planet orbiting another star, thousands of planets orbiting stars other than the Sun, called exoplanets, have been detected. The number of discovered exoplanets continues to increase at an accelerating pace, and we are now able to characterize the population of exoplanets in the solar neighbourhood.

Exoplanets show a wide range of physical and orbital properties. With masses ranging from that of Mercury to several times that of Jupiter, exoplanets can have a radius of hundreds of kilometres to several times the radius of Jupiter. Exoplanet orbital periods can be as short as a few hours, and their eccentricities can be as high as those of a Solar System comet. Most exoplanets tend to be found in systems, comprised of several planets orbiting the same star.

By pushing the precision of the detection methods, we are now able to find planets with a mass as low as one Earth mass and a size of about the radius of Earth. Our search so far, limited as it is, has shown that the Solar neighbourhood is teeming with planets. Some of these planets even orbit within the so-called habitable zone around the host star. Based on definition, a planet orbiting within the habitable zone receives just the right amount of radiation from its star as to allow for the existence of liquid water on its surface.