Aliens on our doorstep

Could life be closer than we think? Ben Evans surveys the Solar System's likeliest hideouts - and what might live there

- Ben Evans

As far as we know, life in our Solar System emerged only here on Earth - the Sun's 'Goldilocks' planet - under conditions that were just right for liquid water to exist. Only here did life's ingredients take root. Only here did water, nutrients, an energy source, a clement atmosphere, survivable temperatures and pressures, and benign radiation fall happily into equilibrium.

Not long after its formation, the earliest life stirred. Multilayered 'microbial mats', fossilised 'stromatolites' – sediments created by photosynthetic organisms and precipitates from hydrothermal vents have been found that are at least 3.5 billion years old. Life, it seems, favoured Earth as enthusiastically as the fairytale Goldilocks favoured the baby bear's perfect porridge. But there are tantalising signs elsewhere in the Solar System that point to alien life having also taken root. Let's look at the most likely contenders.

Microbes on Mars

Our next-door planet, cinnamon-red Mars, is a desiccated, subfreezing, radiation-drenched place with a thin carbon dioxide atmosphere, no global magnetic field and surface pressures about 100 times weaker than Earth's.

Yet 3.7 billion years ago, Mars may have been quite different. Girdled by a thicker atmosphere, warmed by a moderate greenhouse effect and shielded from solar radiation by a stable magnetic field, it could have had higher surface temperatures - and even life.