By Dr Valerie Jeffries FLS.

Exploration of the Solar System looking for evidence of habitability is about ‘primitive life’, with no expectation of anything beyond tiny bacterial-style cells, possibly with viruses. Single-celled life forms ruled Earth for 1.5 billion years before the evolution of bigger modern cells, and in terms of total biomass they still do. That step ‘up’ to the first plant, fungal, and animal cells was a fundamental breakthrough, and probably intrinsically rare. Early Earth provided benign conditions for long enough for this extraordinary lucky event to happen, and evolution once started hasn’t stopped yet. Any life form visible without a microscope needs to be an organised body with thousands of cells, and for intelligence size and complexity have to reach a stage permitting bodies big enough to house an intricate brain.

Gas Giants

Among the moons of the great cold outer planets, Jupiter’s Enceledus and Europa look the best bets for water in underground oceans, but Saturn’s Titan, even further from the Sun, appears the most earth-like in its landscape. Cliffs, lakes, oceans, even flowing rivers, under dense orange clouds of heavy organic gases.   The British space scientist John Zarnecki led the design of the Huygens mission to Titan, which landed in 2005. The surface turned out to be not too hard, nor too soft, for Huygens, but something Professor Zarnecki described “in those heady hours” following arrival of the first data, as crème brûlée.

But Titan’s rivers are liquid methane and ethane, and the cliffs are granite-hard ice. Anything recognisable as life would have to be using ‘chemistry’ that’s so contrary to our knowledge as to be just fantasy.  Potential ingredients for life may well be there, but the necessary reactions do not happen at minus 180 degrees C.

Pluto is the smallest and furthest planet, now unkindly demoted to ‘dwarf planet’ for controversial reasons. It’s far beyond the four gas-giant planets, has a rocky core, very unexpected atmosphere, and a thrillingly unfamiliar landscape. Pluto has organic substances and near-fantasy chemistry on its surface. But again, while potential ingredients for life may well be there, the necessary reactions do not happen at minus 230 degrees C.

Not where, but when.

Back to “Goldilocks” Earth, so-called not only for its benign habitability today, but because it has remained  habitable for at least 3.5 billion years (fossil evidence) and probably longer (biochemical, genetic, and geological evidence).  Mars may well have started similarly, but Mars has not held on to benign conditions long enough for complex life to evolve: no current oxygen, so evidently no plants making it; and no animals are possible without it. Venus is a boiling choking horror covered with volcanoes that could be active, and a faint hint of possible life solely in its clouds.  Venus maybe began by resembling its neighbours Mars and especially Earth, but its surface bears no resemblance now.  Mercury and the Moon lost any habitability long ago.

The stage is ours for a few billion more years. Let’s take care of Earth, the one place in the Universe where we know intelligent life can exist.”

We can see that to be supportive of life prolonged stability is necessary, as well as benign conditions. To assess a planet or moon’s chances of life now, and particularly of complex life, we need to know its history.  Habitability is transient. People have speculated about the far future of life on Earth, and whether Humanity will find means of interstellar travel, to unimaginably distant worlds offering a second chance.  Otherwise Life here will end when the ageing Sun expands in a few billion years, roasting and swallowing up the inner four planets, including our own.

But then the outer planets will be warm: water might be released from its chemical bondage on Pluto. Titan’s ice will melt, its methane lakes will be replaced by water, erosion by rivers will dissolve minerals and warm little ponds may generate a new dawn of life in our Solar System.  A second beginning would suggest that there is life, perhaps complex life, emerging commonly among billions of habitable planets, with the possibility of new brains and societies. When we look for evidence of habitability on far off worlds, we shall only find it on those which are still within their envelope of Goldilocks time, not on worlds where life flourished but has perished, nor on worlds which are not yet the cradles of life that they’ll become.

The stage is ours for a few billion more years.  Let’s take care of Earth, the one place in the Universe where we know intelligent life can exist.

By Dr Valerie Jeffries FLS.