Life in the Solar System

The three most likely candidates for life in the solar system (besides Earth) are the planet Mars, the Jovian moon Europa, and Saturn's moon, Titan.[38][39][40][41][42] This speculation is primarily based on the fact that (in the case of Mars and Europa) the planetary bodies may have liquid water, a molecule essential for life as we know it for its use as a solvent in cells.[43] Water on Mars is found in its polar ice caps, and newly-carved gullies recently observed on Mars suggest that liquid water may exist, at least transiently, on the planet's surface,[44] [45] and possibly in subsurface environments such as hydrothermal springs as well. At the Martian temperatures and pressures, such liquid water is likely to be highly saline.[46] As for Europa, liquid water likely exists beneath the moon's icy outer crust.[47] This water may be warmed to a liquid state by volcanic vents on the ocean floor (an especially intriguing theory considering the various types of extremophiles that live near Earth's volcanic vents), but the primary source of heat is probably tidal heating.[48][49]

Another planetary body that could potentially sustain extraterrestrial life is Saturn's largest moon, Titan.[42] Titan has been described as having conditions similar to those of early Earth; according to bbc.co.uk, "The atmosphere on Titan could be identical to that of the early Earth when life began".[50] On Titan, scientists have discovered the first liquid lakes outside of Earth, but they are made of ethane and methane, not water.[51] Additionally, Saturn's moon Enceladus may have an ocean below its icy surface.[52]

Rare Earth hypothesis

In the book Rare Earth: Why Complex Life is Uncommon in the Universe, Peter Ward, a geologist and paleontologist, and Donald Brownlee, an astronomer and astrobiologist, propose that life as we know it is rare in the universe.[19][20] They suggest that microbial life, however, is probably common in the universe, because of recently discovered extremophiles.[21] The book argues that the chances of all the conditions that occurred to create the Earth occurring again would be rare; thus intelligent life would be rare. One important factor focused on in the book is planetary habitability (see section below).

Peter Ward, one of the authors, said the following:[1]

How do we define life as we do know it? Life on Earth has DNA, a specific genetic code. It also uses only 20, and the same 20, amino acids. Life is always cellular according to some people, but I think not. I personally define a virus as alive. As for other life, what could it be? Could there be non-DNA life? If such life does exist, what does chemistry permit? Certainly chemistry permits certain types of life on our planet and others not. But once we move out in the solar system, especially in the vast realm of cold, chemistry changes. There could be different information systems, different solvents, different membranes. And as we go from hotter to colder, when we go to Venus, out to Mars, to Europa, and to Titan, we really should expect radically different chemistries.

Rare Earth hypothesis

In the book Rare Earth: Why Complex Life is Uncommon in the Universe, Peter Ward, a geologist and paleontologist, and Donald Brownlee, an astronomer and astrobiologist, propose that life as we know it is rare in the universe.[19][20] They suggest that microbial life, however, is probably common in the universe, because of recently discovered extremophiles.[21] The book argues that the chances of all the conditions that occurred to create the Earth occurring again would be rare; thus intelligent life would be rare. One important factor focused on in the book is planetary habitability (see section below).

Peter Ward, one of the authors, said the following:[1]

How do we define life as we do know it? Life on Earth has DNA, a specific genetic code. It also uses only 20, and the same 20, amino acids. Life is always cellular according to some people, but I think not. I personally define a virus as alive. As for other life, what could it be? Could there be non-DNA life? If such life does exist, what does chemistry permit? Certainly chemistry permits certain types of life on our planet and others not. But once we move out in the solar system, especially in the vast realm of cold, chemistry changes. There could be different information systems, different solvents, different membranes. And as we go from hotter to colder, when we go to Venus, out to Mars, to Europa, and to Titan, we really should expect radically different chemistries.